Answer some questions from the assigned chapters

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The Puerperium & Lactation

Parturition

Fetal Attachment & Gestation

Early Embryogenesis & Maternal Recognition of Pregnancy

Ovulation & Fertilization

Cyclicity

Regulation of Reproduction

Tract Function

Puberty

Prenatal Development

.. "' \

, ..

Spermatogenesis

Regulation of Reproduction

Tract Function

Puberty

Prenatal Development

Take Home Message The luteal phase consists of three major processes. They are: 1) luteinization (th e

transformation of follicular cells into luteal cells after ovulation), 2) synthesis and secretion (growth and development of the co1p11s luteum accompanied by increasing quantities of progesterone) aml3) luteolysis (destruction of the c01pus luteum) accompanied by rapidly declining blood progesterone that results in a subsequent follicular phase. Regression of the corpus luteum is brought about by prostaglandin F1a that is synthesized and secreted by the uterine endometrium in most mammals and by the ovmy in women. The negative feedback exerted by progesterone on the hypothalamus is removed and the f emale enters a new follicular phase because the pulse frequency and amplitude ofGnRH increases thus allowing FSH and LH to increase. In women, luteolysis causes the initiation ofmenstma- tion that is follo wed by another follicular phase.

The luteal phase lasts fro m the time of ovula- tion untilluteolysis of the corpus luteurn (CL) near the end of the estrous cycle. It includes metestrus and diestrus (See Figure 9- 1 ). The dominant ovar ian hormone during the luteal phase is progesterone.

The luteal phase consists of: •luteinization (formation ofthe CL) • synthesis and secretion of large quantities of progesterone

• luteolysis

When the fo ll icle ruptures at ovulation, blood vessels within the foll icul ar wall also rupture . This vascular breakage results in a structure with a " bloody" clot-li ke appearance . Th is structure is ca ll ed the corpus hemorrhagicum because of its hemorrhagic (bl oody) appearance when viewed fro m the surface of the ovary. Corpora hemo!Thagica can be observed from the time of ovulation until about day 1 to 3 of the estrous cycle (See Figures 9-3 through 9-6). Imme- diately after ovulation, corpora hemo1Thagica appear as small, pimple-like struch1res on the surface of the ovmy. At about day 3 to 5, the corpus luteum (CL) begins to increase in size and lose its hemorrhagic ap- pearance. It increases in mass until the m iddle of the cycle, when its size is maximal and coinc ides with the maximum secretion of progesterone duri ng diestrus. Near the end of the luteal phase, luteolysis occurs and the CL loses its functional integrity and decreases in size. Luteolysis results in an irreversible struchrral degradation of the corpus luteum. A regressed corpus luteum will become a corp us albi cans (white body).

Figure 9-1. The Luteal Phase

Luteal Phase METESTRUS

p,, p roduct io n

0 I 2 3 4 5 6 7 8 9 10 I I 12 13 14 15 16 17 18 19 20 2 1 0

Day of the estrous cycle

The luteal phase beg ins immediately afte r ovulation. During the early luteal phase, the corpus luteum (CL) develops (luteinization ) and progesterone increases. Duri ng the mid- luteal phase (diestrus) the corpus luteum is fully functional and progesterone (P 4 ) plateaus. During the last 2-3 days of the luteal phase , destru ction of the corpus luteum occurs (lute- olysis) and the luteal phase terminates. Fol- lowing luteol ysis , pro estrus is initiated.

V et B oo ks .ir

The Puerperium & Lactation

Parturition

Fetal Attachment & Gestation

Early Embryogenesis & Maternal Recognition of Pregnancy

Ovulation & Fertilization

Cyclicity

Regulation of Reproduction

Tract Function

Puberty

Prenatal Development

.. "' \

, ..

Spermatogenesis

Regulation of Reproduction

Tract Function

Puberty

Prenatal Development

Take Home Message The luteal phase consists of three major processes. They are: 1) luteinization (th e

The luteal phase lasts fro m the time of ovula- tion untilluteolysis of the corpus luteurn (CL) near the end of the estrous cycle. It includes metestrus and diestrus (See Figure 9- 1 ). The dominant ovarian hormone during the luteal phase is progesterone.

The luteal phase consists of: •luteinization (formation ofthe CL) • synthesis and secretion of large quantities of progesterone

• luteolysis

When the fo ll icle ruptures at ovulation, blood vessels within the foll icul ar wall also rupture . This vascular breakage results in a structure with a " bloody" clot-li ke appearance . Th is structure is ca ll ed the corpus hemorrhagicum because of its hemorrhagic (bl oody) appearance when viewed fro m the surface of the ovary. Corpora hemo!Thagica can be observed from the time of ovulation until about day 1 to 3 of the estrous cycle (See Figures 9-3 through 9-6). Imme- diately after ovulation, corpora hemo1Thagica appear as small, pimple-like struch1res on the surface of the ovmy. At about day 3 to 5, the corpus luteum (CL) begins to increase in size and lose its hemorrhagic ap- pearance. It increases in mass until the m iddle of the cycle, when its size is maximal and coinc ides with the maximum secretion of progesterone duri ng diestrus. Near the end of the luteal phase, luteolysis occurs and the CL loses its functional integr ity and decreases in size. Luteolysis results in an irreversible struchrral degradation of the corpus luteum. A regressed corpus luteum will become a corp us albi cans (white body).

Figure 9-1. The Luteal Phase

Luteal Phase METESTRUS

p,, p roduct io n

0 I 2 3 4 5 6 7 8 9 10 I I 12 13 14 15 16 17 18 19 20 2 1 0

Day of the estrous cycle

V et B oo ks .ir

182 The Luteal Phase

In general , a corpus albicans can be observed for a substantial period oftime (several estrous cycles) after luteolysis. The corpus albicans appears as a white scar- like structure because the connective tissue remains after the glandular tissue disappears.

The corpus lutemn originates from an ovulatory follicle.

After ovulation the theca interna and the granulosa! cells of the follicle undergo a dramatic transfom1ation known as luteinization. Luteinization is the process whereby cells of the ovulatory follicle are transformed into luteal tissue. This transforma- tion is governed by LH. Shortly before ovulation the basement membrane of the follicle undergoes partial disintegration and the physical separation of the thecal and granulosa! cells disappears (See Figure 9-2). Dur- ing ovulation, follicular fluid leaks from the follicle. At the same time, the wall of the follicle collapses fann- ing many folds (See Figure 9-2). These folds begin to interdigitate, allowing thecal cells and the granulosa! cells to mix, thus forming a gland consisting of con- nective tissue cells, thecal cells and granulosa! cells. In general, the cells of thecal origin and the cells of granulosa! origin mix unifom1ly with one another (See Figure 9-2). An exception to this is found in the corpora Jutea of the woman and other primates, where thecal and granulosa! cells are clustered into distinct "islets". It is easy to distinguish microscopically between luteal cells that originate from the granulosa! cells and those that originate from the thecal cells. Large luteal cells are derived from granulosa! cells while small luteal cells are derived from thecal cells. Portions of the basement membrane that separated the thecal cells from the granulosa! cells remain and constitute the connective tissue network of the corpus Juteum (See Figure 9-2).

Luteal tissue consists of large and small luteal cells: • large cells originate from the granulosal cells

• small cells originate from the cells oftlze theca interna

Large luteal cells (sometimes called granu- losal-lutein cells) va1y in diameter from 20-70 microm- eters (!lm), depending on species. In some species (nuninants ), there are a large number of dense secretory granules close to the plasma membrane. These secre-

tory granules contain oxytocin in the corpus luteum of the cycle and are believed to contain relaxin in the corpus luteum of pregnancy.

Small luteal cells (sometimes called thecal- lutein cells) are less than 20 in diameter, have an irregular shape and possess numerous lipid droplets in their cytoplasm. They do not contain secretory gran- ules as do the large luteal cells. Both small and large luteal cells are steroidogenic (possessing the ability to produce steroids), in this case progesterone.

In general, the corpus luteum increases in size until about midway through the luteal phase (See Fig- ures 9-3 through 9-6). For example, a skilled examiner can almost always determine whether a corpus luteum is present or absent in cows. In mares , it is almost impossible to ascertain the presence or absence of the corpus luteum because it does not protrude from the surface of the ovary.

In the cow, palpation cannot accurately predict the functional status of the corpus luteum. In four separate studies, cows were transrectally palpated by experienced diagnosticians. Corpora lutea were classi- fied as func tional (secreting high quantities of proges- terone) or nonfunctional (regressing or secreting low levels of progesterone) by the diagnosticians. Using measurements of blood proge sterone as the indi cator of corpus luteum function, it was found that 25% to 39% of cows classified as having a functional corpus luteum were not secreting high quantities of progester- one. Furthermore, 15% to 21% of cows classified as having a nonfunctional corpus luteum had high blood progesterone. Clearly, the use of transrectal palpation to assess the functional status of the corpus luteum has limitations. From a practical reproductive manage- ment perspective, this problem lim its the effectiveness of treating animals with luteolytic agents to induce estrus and ovulation. In other words, administering luteolytic agents (prostaglandin F 2a) on the basis of transrectal palpation of the ovaries alone will provide suboptimal results.

The use of real-time ultrasonography has proven effective for the examination of corpora lutea, as well as ovarian follicles . In cattle, progesterone concentration in blood is correlated with the diameter of the corpus luteum as measured by ultrasonography.

L arge luteal cells rarely multiply after ovula- tion. Therefore, the total number of granulosa! cells " donated" by the follicle detem1ines the number of large luteal cells in the newly-formed CL. Luteal function may in-part be related to the vigor (as judged by the number of granulosa! cells) of the follicle prior to ovulation. In the ewe (and presumably other spe- cies), an increase in corpus luteum size and weight is due to a threefo ld increase in volume of large luteal cells coupled with a fivefold increase in the number

The Luteal Phase 183

Figure 9-2. Formation of the Corpus Luteum

Preovulatory Follicle The preovulatory follicle consists of granu- losa! cells that line the antrum. The base- ment membrane, separating the granulosa! cells from the cells of the theca interna begins to deteriorate prior to ovulation be- cause of the action of collagenase . Com- plete separation between the granulosa! cells and the th eca intern a no longer exists and the cells can begin to intermingle.

Corpus Hemorrhagicum (CH) During ovulation, many small blood vessels rupture causing local hemorrhage. This hemorrhage appears as a blood clot on the surface of the ovary that sometimes penetrates into the center of the foll icle after ovulation (See Figures 9-3, 1 A and

9-4, 1A and B). Followi ng evacua- of the follicular fluid and oocyte, the

foll icle collapses into folds. The cells of the theca interna and the granulosa begin to mix. The basement mem brane fo rms the connective tissue substructure of the corpus luteum.

Functional Corpus Luteum (CL) The corpus luteum is now a mixture of large luteal cells, LLC (formerly granulosa! cells) and many small luteal cells, SLC (fo rmerly thecal cells). In some cases, there is a remna nt of the follicu lar antrum th at forms a small cavity in the center of the corpus luteum (See Figures 9-3, 3B and 9-4, 2B; 9-6, 3B).

V et B oo ks .ir

182 The Luteal Phase

The corpus lutemn originates from an ovulatory follicle.

Luteal tissue consists of large and small luteal cells: • large cells originate from the granulosal cells

• small cells originate from the cells oftlze theca interna

tory granules contain oxytocin in the corpus luteum of the cycle and are believed to contain relaxin in the corpus luteum of pregnancy.

The Luteal Phase 183

Figure 9-2. Formation of the Corpus Luteum

9-4, 1A and B). Followi ng evacua- of the follicular fluid and oocyte, the

foll icle collapses into folds. The cells of the theca interna and the granulosa begin to mix. The basement mem brane fo rms the connective tissue substructure of the corpus luteum.

V et B oo ks .ir

F igure 9-3.

Luteal A natom

y in R elation to P

rogesterone S ecretion D

uring the E strous C

ycle in the C ow

The area designated by the circle repre- sents a developing corpus luteum

. 28

T he corpus luteum

has been sliced in half.

N otice the increase in size w

hen com

pared to that show n in 1 B.

E arly M

etestrus

Late M etestrus

D iestrus

10 - b.O

c:::: -"'CCV

c:::: 0

0 -:.... alcv

C ircled area is a corpus hem

or- rhagicum

. N

otice the bloody ap- o..

pearance at the apex. 1

The corpus hem orrhagicum

has been sliced in half.

N otice the

rem nant of the follicular lum

en that is filled w

ith a blood clot (arrow ).

8 6 4 2

0 + O

vulation

5 10

D ay of C

ycle (C ow

)

A large corpus luteum

(circle) at peak progesterone secretion.

3 8

large m ass of orange tissue can be

seen w hen the C

L is sliced in half. The orange color reflects the high content of P

-carotene. The central cavity (arrow ) is a

rem nant of the follicular antrum

. A central

cavity does not exist in every C L.

+ O

vulation

The circle indicates the approxim ate

area ofthe regressing corpus luteum .

4 8

The corpus luteum

has changed in color and in size.

The secre- tory com

pon ent of th

e tissue has decreased significantly as a result of luteolysis. A

rrow designates re-

gressing C L from

a previous cycle.

Figure 9-4. Luteal A natom

y in R elation to P

rogesterone S ecretion D

uring the E strous C

ycle in the E w

C ircles A

and B indicate deve

loping cor- pora lut ea

. 28

C orpus luteum

B has been sliced in half.

N otice the developing luteal tissue (circle)

that surro unds a sm

all cavity (arrow ) that

is the rem nant of the

follicular antrum .

N otice that the hem

orrhagic appearance is no longer present.

E arly M

etestrus

- -E

- b.O

c:::: -"'CCV

c:::: 0

0 -:.... m

c v

C ircles indicate corpora hem

or- b.O

h .

0 r ag1ca

. :....

1 8

ircled area show s the corpus

hem orrhagicum

sliced in half. The clot is indicated by the ar- row

0..

Late M etestrus

D iestrus

0 t O

vulation

2 4

6 8

10 12

D ay of C

ycle (Ew e)

A corpus luteum

(circle) during the peak luteal phase.

3 8

e luteal tissue (sliced in half) is a rela- tively large m

ass of secretory tissue.

16 1 O

vulation

Th e circle indicates the

surface of a reg ressing

co rpus luteum

. 48

T h

e co rpu

s luteum has

becom e pale and th

e se- cretory tissue m

ass has decreased in size.

0 )

::.. :;! m r-c: m -Q

) en m

:;! m r-c: Cil' Q

)

-m

0 )

0 1

VetBooks.ir

F igure 9-3.

Luteal A natom

y in R elation to P

rogesterone S ecretion D

uring the E strous C

ycle in the C ow

The area designated by the circle repre- sents a developing corpus luteum

. 28

T he corpus luteum

has been sliced in half.

N otice the increase in size w

hen com

pared to that show n in 1 B.

E arly M

etestrus

Late M etestrus

D iestrus

10 - b.O

c:::: -"'CCV

c:::: 0

0 -:.... alcv

C ircled area is a corpus hem

or- rhagicum

. N

otice the bloody ap- o..

pearance at the apex. 1

The corpus hem orrhagicum

has been sliced in half.

N otice the

rem nant of the follicular lum

en that is filled w

ith a blood clot (arrow ).

8 6 4 2

0 + O

vulation

5 10

D ay of C

ycle (C ow

)

A large corpus luteum

(circle) at peak progesterone secretion.

3 8

large m ass of orange tissue can be

seen w hen the C

L is sliced in half. The orange color reflects the high content of P

-carotene. The central cavity (arrow ) is a

rem nant of the follicular antrum

. A central

cavity does not exist in every C L.

+ O

vulation

The circle indicates the approxim ate

area ofthe regressing corpus luteum .

4 8

The corpus luteum

has changed in color and in size.

The secre- tory com

pon ent of th

e tissue has decreased significantly as a result of luteolysis. A

rrow designates re-

gressing C L from

a previous cycle.

Figure 9-4. Luteal A natom

y in R elation to P

rogesterone S ecretion D

uring the E strous C

ycle in the E w

C ircles A

and B indicate deve

loping cor- pora lut ea

. 28

C orpus luteum

B has been sliced in half.

N otice the developing luteal tissue (circle)

that surro unds a sm

all cavity (arrow ) that

is the rem nant of the

follicular antrum .

N otice that the hem

orrhagic appearance is no longer present.

E arly M

etestrus

- -E

- b.O

c:::: -"'CCV

c:::: 0

0 -:.... m

c v

C ircles indicate corpora hem

or- b.O

h .

0 r ag1ca

. :....

1 8

ircled area show s the corpus

hem orrhagicum

sliced in half. The clot is indicated by the ar- row

0..

Late M etestrus

D iestrus

0 t O

vulation

2 4

6 8

10 12

D ay of C

ycle (Ew e)

A corpus luteum

(circle) during the peak luteal phase.

3 8

e luteal tissue (sliced in half) is a rela- tively large m

ass of secretory tissue.

16 1 O

vulation

Th e circle indicates the

surface of a reg ressing

co rpus luteum

. 48

T h

e co rpu

s luteum has

becom e pale and th

e se- cretory tissue m

ass has decreased in size.

0 )

::.. :;! m r-c: m -Q

) en m

:;! m r-c: Cil' Q

)

-m

0 )

0 1

VetBooks.ir

- -

F igure 9-5. Luteal A

natom y in R

elation to P rogesterone S

ecretion D uring the E

strous C ycle in the S

1 A

and 1B

D eveloping corpora lutea betw

een days 3 and 6.

B ecause of variation in length of

the cycle and the tim e of ovulation relative

to the stage of the cycle, precise age of these corpora lutea is difficult to estab- lish

. N otice that all structures still have a

hem orrhagic appearance and som

e have a visible stigm

a (arrow s) indicating the

point at w hich ovulation occurred.

- -E

-- 1:).0 c ._. -ccu

c 0

0 - : I . .

£ C

C 1

) ..., en C1) 1:).0 0 :I.. I:L

0 i Ovulation E

arly to Late M etestrus

D iestrus

5 I 0

I 5

D ay of C

ycle (Sow )

2 A

a n

d 2B

um bers designate six corpora lutea dur-

ing high secretory activity. C orpora lutea

4, 5 and 6 have been sliced in half. N otice

that corpus luteum 5 has an antrum

. A lso,

notice that P 4 is m

uch higher in the sow

than in the cow , ew

e and m are.

2 1

i Ovulation

P roestrus

3 A

a n

d 3B

egressing corpora lute a

. N

otice the pale color. The in- terval from

luteolysis to estrus is long er than rum

inants.

Figure 9-6. Luteal A

natom y in R

elation to P rogesterone S

ecretion D uring the E

strous C ycle in the M

are

2 A

rea desig nated by th

e arrow is th

e developing corpus luteum

. N ote: th

e C L

does not protrude from the surface and

is not yellow as in oth

er species. 2B

The corpus luteum

seen in 2A has been

sliced in half. A rrow

indicates a follicle sliced in half.

E arly M

etestrus

--E

10 - b.O

c -"CC1

J 0

c 0

0 -

:r...

1 A

a::IC L

J ....... U

l C1J

T he corp

us hem orrhag

icum is

w ithin the circle.

It is not highly visible from

the exterior as in other species. 1B

rrow indicates the hem

orrhag ic

tissue w ithin the w

all of the new ly

ovulated follicle th at has been

sliced in half.

b.O

0 :r... D

8 6 4 2

Late M etestrus

D iestrus

5 10

O vulation

D ay of C

ycle (M are)

3 A

a n

d 3B

The structures are sliced in half to expose the inner tissue m

ass. Tw o distinct types of corpora

lutea can be seen during the peak luteal phase. In som

e, there is a heterogeneous m ass of tissue

w ithout a central cavity (3A

), w hile in others a

hom ogenous m

ass of tissue w ith a central cavity

(arrow ) exists (3B). Both types ae norm

al and secrete adequate quant ities of progesterone. In alm

ost all cases, corpora lutea in the m are

are "buried" w ithin the ovarian cortex and are

not palpable per rectum . Ultrasonography easily

identifies a corpus luteum in the m

are

2 1

t O

vulation

P roestrus

Tw o exam

ples of reg ressing

co rpo

ra lutea .

S pecim

ens w

ere sliced in half. N otice that

the size has decreased. A rrow

in 4B

indicates a residual blood cl ot w

ithin the corpus luteum .

0 0

(j')

:;! (!) r-t: (i) Q) -Q) (/) (!) :;! (!) r-t: iD Q) -Q) (/) (!) 00 ........

VetBooks.ir

- -

F igure 9-5. Luteal A

natom y in R

elation to P rogesterone S

ecretion D uring the E

strous C ycle in the S

1 A

and 1B

D eveloping corpora lutea betw

een days 3 and 6.

B ecause of variation in length of

the cycle and the tim e of ovulation relative

to the stage of the cycle, precise age of these corpora lutea is difficult to estab- lish

. N otice that all structures still have a

hem orrhagic appearance and som

e have a visible stigm

a (arrow s) indicating the

point at w hich ovulation occurred.

- -E

-- 1:).0 c ._. -ccu

c 0

0 - : I . .

£ C

C 1

) ..., en C1) 1:).0 0 :I.. I:L

0 i Ovulation E

arly to Late M etestrus

D iestrus

5 I 0

I 5

D ay of C

ycle (Sow )

2 A

a n

d 2B

um bers designate six corpora lutea dur-

ing high secretory activity. C orpora lutea

4, 5 and 6 have been sliced in half. N otice

that corpus luteum 5 has an antrum

. A lso,

notice that P 4 is m

uch higher in the sow

than in the cow , ew

e and m are.

2 1

i Ovulation

P roestrus

3 A

a n

d 3B

egressing corpora lute a

. N

otice the pale color. The in- terval from

luteolysis to estrus is long er than rum

inants.

Figure 9-6. Luteal A

natom y in R

elation to P rogesterone S

ecretion D uring the E

strous C ycle in the M

are

2 A

rea desig nated by th

e arrow is th

e developing corpus luteum

. N ote: th

e C L

does not protrude from the surface and

is not yellow as in oth

er species. 2B

The corpus luteum

seen in 2A has been

sliced in half. A rrow

indicates a follicle sliced in half.

E arly M

etestrus

--E

10 - b.O

c -"CC1

J 0

c 0

0 -

:r...

1 A

a::IC L

J ....... U

l C1J

T he corp

us hem orrhag

icum is

w ithin the circle.

It is not highly visible from

the exterior as in other species. 1B

rrow indicates the hem

orrhag ic

tissue w ithin the w

all of the new ly

ovulated follicle th at has been

sliced in half.

b.O

0 :r... D

8 6 4 2

Late M etestrus

D iestrus

5 10

O vulation

D ay of C

ycle (M are)

3 A

a n

d 3B

The structures are sliced in half to expose the inner tissue m

ass. Tw o distinct types of corpora

lutea can be seen during the peak luteal phase. In som

e, there is a heterogeneous m ass of tissue

w ithout a central cavity (3A

), w hile in others a

hom ogenous m

ass of tissue w ith a central cavity

(arrow ) exists (3B). Both types ae norm

al and secrete adequate quant ities of progesterone. In alm

ost all cases, corpora lutea in the m are

are "buried" w ithin the ovarian cortex and are

not palpable per rectum . Ultrasonography easily

identifies a corpus luteum in the m

are

2 1

t O

vulation

P roestrus

Tw o exam

ples of reg ressing

co rpo

ra lutea .

S pecim

ens w

ere sliced in half. N otice that

the size has decreased. A rrow

in 4B

indicates a residual blood cl ot w

ithin the corpus luteum .

0 0

(j')

:;! (!) r-t: (i) Q) -Q) (/) (!) :;! (!) r-t: iD Q) -Q) (/) (!) 00 ........

VetBooks.ir

[]] I

188 The Luteal Phase

of small luteal cells. Thus, large luteal cells undergo hypertrophy (increase in size), while small luteal cells undergo hyperplasia (increase in cell numbers) as the CL develops. In addition to changes in steroidogenic cells, non-steroidogenic cells (fibroblasts, capillary cells and eosinophils) increase in number during the estrous cycle. The net effect of these cellular changes is a marked enlargement ofthe corpus luteum.

The "vigor" of the corpus luteum probably depends on:

• the number of luteal cells • the degree to which the CL becomes vascularized

The functional capability (ability to secrete pro- gesterone) of the newly developed corpus luteum may also depend on the degree of vascularity in the cellular layers of the follicle. The ability of the corpus luteum to vascularize may relate to its ability to synthesize and deliver hom1ones. As presented in the previous chapter, follicular fluid contains angiogenic factors. The degree to which these angiogenic factors promote vascularization of the corpus luteum is probably related to the quantity of angiogenic factors present in the fol- licular tissue.

Insufficient luteal function (poor progesterone synthesis and secretion) is believed to be a possible contributor to reproductive failure in mammals. A corpus luteum secreting suboptimal concentrations of progesterone probably results in the inability of the dam's uterus to suppmi development of the early embryo.

The primary target organs for progesterone are the hypothalamus, the uterus and the mammary gland (See Figure 9-7). The uterus has two target components: 1) the glandular endometrium and 2) the muscular myometrium. Progesterone stimulates maximal secretion by the endometrial glands. Secre- tory products from the endometrial glands contribute to an environment that supports the development of the "free-floating" conceptus after it enters the uterine lumen. An important inhibitory role of pro- gesterone is to reduce the motility (contractions) of the myometrium. Such a role causes a "uterine quies- cence" effect on the myometrium in the cow, pig and ewe. In the mare, myometrial motility is not inhibited to the same degree so that the conceptus is transported around the uterus but not expelled. Myometrial inhi- bition is thought to be important because it provides "calming" conditions for attachment of the conceph1s to the uterine endometrium. In the mare, the conceptus

is transported about in the uterine lumen by contrac- tions of the myometrium. This phenomenon will be discussed in more detail in Chapter 13. Progesterone causes final alveolar development of the mammary gland during pregnancy, thereby allowing initiation of lactation.

Progesterone Synthesis Requires Cholesterol and LH

The presence of basal (tonic) LH and cho- lesterol is necessary for progesterone to be secreted by luteal cells. The mechanism whereby LH causes secretion of progesterone in luteal cells is illustrated in Figure 9-8. In order to fillly understand progesterone synthesis, you should carefully read the explanation of each step in the boxes provided in Figure 9-8.

Progesterone is of major importance in the en- docrine control of reproduction because it exerts a strong negative feedbacl\: on the hypothalamus (See Figure 9-7). Elevated progesterone reduces the pulse frequency of GnRH by the tonic GnRH center in the hypothala- mus. However, the amplih1de of the LH pulses is still relatively high. Such a pattern of LH secretion along with tonic FSH secretion allows follicles to develop during the luteal phase. These follicles do not reach preovulatory stah1s until progesterone decreases and the frequency ofLH pulses increases. H igh progester- one therefore prevents development of steroidogenic preovulatory follicles, secretion of estradiol, behavioral estrus and the p reovulatory surge of GnRH and LH.

Progesterone is an inhibitor because it: • reduces GnRH pulse frequency • prevents behavioral estrus • stops the preovulat01y LH surge • reduces myometrial tone (except in the mare)

Progesterone almost totally inhibits estrual behavior. In general, females under the influence of progesterone do not display estrus and will not copulate with the male. However, as pointed out in Chapter 7, progesterone exerts a positive priming effect on the brain to enhance the behavioral effects of estradiol after progesterone is reduced. For example, if females are ovariectomized (removal of ovaries) and treated with estradiol, they will display behavioral characteristics of estrus. These traits will be amplified in both intensity and duration if cows are treated with progesterone for about 5 to 7 days before they receive estradiol.

Lysis of the Corpus Luteum Must Occur Before the Female Can Enter

the Follicular Phase Luteolysis is the loss of progesterone secre-

tion by the CL followed by loss of luteal tissue m ass . It occurs dming a one-to-three day period at the end of the luteal phase. Luteolysis is a process whereby the corpus luteum undergoes irreversible degeneration characterized by a dramatic drop in blood concentra- tions of progesterone (See Figures 9- 1,9-3 through 9-6). The hormone inducing luteolysis is PGF2a secreted by

The Luteal Phase 189

the uterine endometrium. Communication between the and the ute1ine endometrium is necessary

m order to bnng about successfhl luteolysis. The utems f1mctions as an endocrine organ and is responsible for secreting PGF2a that causes luteolysis. Ifluteolysis does not occur, the animal remains in a sustained luteal phase because progesterone inhibits gonadotropin secreti on (See Figure 9-7). The importance of the uterus in con- trolling the life-span of the corpus luteum is illustrated in Figure 9-9. In mammals , other than p rimates, com- plete removal ofthe utems (uterectomy) after ovulation causes the corpus luteum to be maintained j ust as if the

Figure 9-7. Progesterone (P4 ) has Many Physiological Effects

Corpus luteum ( ovory)

P4 produced by the CL exerts a nega- tive (-) feedback on the GnRH neu- rons of the hypothalamus. Therefore, GnRH, LH and FSH are suppressed and little estrogen is secreted. Pro- gesterone is thought to decrease the number of GnRH receptors on the anterior pituitary.

U t e rine t issu e (ute ru s)

P4 promotes alveo- lar developme nt in t he mamma ry gla nd , es pecially during pregnancy.

G la ndular se crctlo ti s

P 4 exerts a strong positive ( +) influ- ence on the endometrium of the uterus. Under the influence of P4, the uterine glands secrete materials into the uterine lumen. Progester- one inhibits the myometrium and th us reduces its con tractility and tone.

V et B oo ks .ir

[]] I

188 The Luteal Phase

The "vigor" of the corpus luteum probably depends on:

• the number of luteal cells • the degree to which the CL becomes vascularized

Progesterone Synthesis Requires Cholesterol and LH

Progesterone is an inhibitor because it: • reduces GnRH pulse frequency • prevents behavioral estrus • stops the preovulat01y LH surge • reduces myometrial tone (except in the mare)

Lysis of the Corpus Luteum Must Occur Before the Female Can Enter

the Follicular Phase Luteolysis is the loss of progesterone secre-

The Luteal Phase 189

the uterine endometrium. Communication between the and the ute1ine endometrium is necessary

Figure 9-7. Progesterone (P4 ) has Many Physiological Effects

Corpus luteum ( ovory)

U t e rine t issu e (ute ru s)

P4 promotes alveo- lar developme nt in t he mamma ry gla nd , es pecially during pregnancy.

G la ndular se crctlo ti s

V et B oo ks .ir

190 The Luteal Phase

Figure 9-8. Mechanism of Progesterone Synthesis by Luteal Cells

Esterified cholesterol is delivered to the luteal cell primarily by way of low and high density lipoprotein (LDL and HDL). The blood-borne lipoprotein-cholesterol complex binds to specific receptors on the outside of the plasma membrane. The LDL-cholesterol complex binds to specific receptors on the outside of the plasma membrane. The LDL-cholesterol receptor complex is internalized and cholesterol Is released from the receptor complex in the fo rm of cholesterol esters. After LDL-cholesterol is removed, the receptor is "recycled" and becomes available to transport another LDL-cholesteroi complex.

LH binds to specific LH receptors (LHR) on the plasma membrane.

Pregnenolone leaves the mitochondria and is converted enzymatically to progesterone (PROG) by the smooth endoplasmic reticulum. Progesterone leaves the cell and enters the blood, where it travels to target tissues.

0 ' CHOL \... ' ESTERASE ... ', • m

' ' ' \ ',0®

'' ' ' \ I .......... ,

The LH receptor complex activates a G-proteln (G) that activates membrane-bound adenylate cyclase (AC}.

Mitochondrial enzymes are res po nsible fo r converting cholest erol to p regneno- lone (PREG).

Cyclic AMP activates prot ein kinase enzymes. Protein kinases (a) acceler- ate LDL-cholesterol receptor internalizat ion, (b) activat e cholesterol- esterase that cleaves ch olesterol fro m its ester and (c) promote entry of cholesterol into mitochon- dria.

Ad enylate cyclase prom otes t he con version of ATP to cyclic AM P (cAMP}, t he second messenger.

female were pregnant. For example, in ewes with an intact uterus the life-span of the corpus luteum is identi- cal to that seen in the nonnal cycle ( 17 days). However, when the entire uterus is removed (total uterectomy), the life-span of the corpus luteum is prolonged for months and is similar to a nonnal gestation period (148 days). Clearly, removal of the entire uterus extends the life- span of the corpus luteum dramatically.

The uterus is required for successful luteolysis in many species.

When partial uterectomy is perfor med, a less dramatic effect can be seen. For example, when the uterine hom ipsilatera l (on the same side) to the corpus luteum is removed, the life-span of the corpus luteum is almost twice as long (about 35 days) as the nonnal cycle. In contrast, when the contral ateral (opposite side) uterine horn is removed, there is little, if any, ef- fect on th e life-span of the corpus luteum. The response to complete and partial uterectomy is summarized in Figure 9-9. Several important findings have emerged from the classic exper iments illustrated in Figure 9-9. First, the uterus is requ ired for lysis of the corpus luteum. Therefore, the uterus secretes a substance(s) that causes luteolysis. Second, removal of the utem s ipsilateral to the corpus luteum increases the life-span of the corpus luteum, while removal of the uterine hom contralateral to the corpus luteum does not. A local effect of the utems directly upon the ipsilateral ovary containing the corpus luteum is obvious. A local effect can be further supported by the fact that when the ovary is transplanted into the neck of the female, but the uterus remains intact, the corpus luteum life-span is prolonged by many weeks. Co llectively, what these experime nts have told us is: I ) the uterus is responsible fo r luteo lysis and 2) the uterus must be near the ovary.

You should now understand from the above discussion that the utems is required for luteolysis. Clearly then, the uterus must secrete a substance that causes destruction of the corpus luteum. After years of intensive and heavi ly focused research, it has been conclus ive ly demonstrated that prostag land in F2a is the luteolysin in domestic animals. Prostaglandin F2a is also the luteolytic agent in p rimates but is secreted by the corpus luteum. Among domestic animals, the uterectomized bitch cycles normally and has a luteal phase of nom1al duration suggesting that the uterus has little or no influence upon luteal fimction in canids.

The Luteal Phase 191

A vascular countercurren t transport system ensures that PGF2a will reach the

ovary in sufficient quantities to cause luteolysis in the ewe, cow and sow.

How does PGF2a get fro m the uterus to the ovary, where it causes luteolysis? Prostaglandin F2a from the utems is transported to the ipsilateral ovary through a vascular countercurrent exchange mecha- nism. A countercurrent exchange system involves two closely associated blood vessels in whi ch bl ood from one vessel fl ows in the oppos ite direction to that of the adjacent vessel. Low molecular weight substances in high concentrations in one vessel diffuse into the adjacent vessel, w here they are in low concentrations. The PGF2a secreted by the endometr ium enters the uterine vei n and the uterine lymph vessels, at relatively high concentrations. The ovari an artery lies in close association with the utero-ovarian vein (See Figure 9- 1 0). By countercuiTent exchange, transfer ofPGF2 a is accomplished by a prostaglandin transport protein that faci litates movement ofPGF2u across the wall of the uterine vein into the blood of the ovarian artery. This spec ial anatomical relationship ensures that a high prop ortion of the PGF2a secreted by the uterus will be transpor ted directly to the ovary and the corpus luteum without dilution by the systemic circ ulation. This mechanism is particularly important because much of PGF2a is denatured during one circul atory pass through the pulmonary system in the ewe and the cow (around 90%). In the sow, only about 40% of the PGF2u is de- natured in the pulmonary circulation. By entering the ovarian arte1y, PGF 2a can exert its lytic effect directly on the corpus luteum. The counterctment transport system is present in the cow, sow and ewe, but not in the mare. The mare does not metabolize PGF2a as rapidly as other species, so the need for a local transport specialization is not as important in the m are. In addition, the mare CL is thought to be m ore sensitive to PGF2a than the CL of the cow, sow and ewe.

Exogenous PGF2a causes luteolysis during about 60% of the cycle in most species. For example, it exerts its most potent effect after day six of the cycle and will almost always cause luteolysis if administered after this time in the cow. In contrast, PGFza has a negligib le effect during the first two to fou r days after ovulation. In the pig, the corpus luteum does not be- come responsive to the luteolytic action of a single dose of PGF2a until day 12 to 14 of the cycle. Prostaglandin Fza and its analogs are used widely to cause regression of the corpus luteum and thus synchronize estrus and ovulation, to induce abortion and sometimes to induce parhtr ition.

V et B oo ks .ir

190 The Luteal Phase

Figure 9-8. Mechanism of Progesterone Synthesis by Luteal Cells

LH binds to specific LH receptors (LHR) on the plasma membrane.

0 ' CHOL \... ' ESTERASE ... ', • m

' ' ' \ ',0®

'' ' ' \ I .......... ,

The LH receptor complex activates a G-proteln (G) that activates membrane-bound adenylate cyclase (AC}.

Mitochondrial enzymes are res po nsible fo r converting cholest erol to p regneno- lone (PREG).

Ad enylate cyclase prom otes t he con version of ATP to cyclic AM P (cAMP}, t he second messenger.

The uterus is required for successful luteolysis in many species.

The Luteal Phase 191

A vascular countercurren t transport system ensures that PGF2a will reach the

ovary in sufficient quantities to cause luteolysis in the ewe, cow and sow.

V et B oo ks .ir

192 The Luteal Phase

Figure 9-9. Effect of Uterectomy upon Estrous Cycle Duration in the Ewe

Intact uterus

G1 CL

\ n '"•' Ovary

In the intact uterus, the CL lifespan is the same as in a normal cycle (15-17 d).

Partial uterectomy (Contralateral to CL)

.. '. . ' '. '' . '

', ',

. ' ... _,' \

', ... ;

Ovary · . - G1 CL r /) ., ••. A partial uterectomy contralateral to the CL will yield a lifespan similar to a normal cycle (15-17 d).

The requirements for luteolysis (in subprimate mammals) are: • presence of oxytocin receptors on endometrial cells

• presence of a critical level of oJ..ytocin • PGF

2 a. synthesis by the endometrium

Total uterectorny .. --- ... _

" '' . . . ' . ' .. ' ' ' " .. _ .. ' \

Ovary \.-? ... '

' ' '' •: •' ' . ' . '' ' '

/ /

With a total uterectomy, the CL lifespan is similar to a normal gestation length (148 d).

Partial uterectomy (Ipsilateral to CL)

: :

Ovary

" " '' ' '' ''

A partial uterectomy ipsilateral to the CL will cause the CL to have a lifespan longer than normal (35 d).

What stimulates the secretion ofPGF2a during the late luteal phase? In addition to progesterone, large luteal cells synthesize and secrete oxytocin. In fact, in the cow and the ewe the corpus luteum contains very large quantities of oxytocin. Luteal oxytoci n is s tored in secretory granules analogous to those observed in the nerve terminals of the posterior pih1itary gland. When oxytocin is injected into ewes near the end of the luteal phase, PGF2a appears in the circulating blood in response to these injections.

During the first-half of the luteal phase, pros- taglandin secretion by the endometrium of the uterus is almost nonexistent. However, during the late luteal phase, secretion ofPGF2a. begins to occur in pulses (See Figure 9-11 ). The pulses increase in frequency and amplitude as the end of the luteal phase approaches.

The Luteal Phase 193

Figure 9-10. The Utero-Ovarian Vascular Countercurrent Transport System

Schematic illustration of the countercu rrent trans- port system in the cow, sow and ewe. A portion of uterine PG Fza is trans- ported directly from the ute ro-ovaria n ve in into the ovarian artery where it has a direct lytic effect on the corpus luteum.

In the two photographs, a blue latex medium was injected into the utero- ov arian v ein (UOV) a nd a red latex medium into the ovaria n artery (OA). The latex was allowed to polymerize and solidify. The tissue was then dissolved with repeated treatme nts of saturated sodium hydroxide followed by wash ings with water until all of the tissue was removed (Fro m Cody et al. 1999. Bioi. Reprod. 60(Suppl 1 ): 90). The dashed lines in the photo at left approximate the boundaries of the uterine horns (UH) and the ovary (0 ). Th e uterus secretes prostaglandin Fza that enters the venous drainage at high concentrations. In the photo below PGFza diffuses from the utero-ovarian vein into the ovarian artery and is tra nsported directly to the ovary (artery-arrows) where it causes luteolys is.

V et B oo ks .ir

192 The Luteal Phase

Figure 9-9. Effect of Uterectomy upon Estrous Cycle Duration in the Ewe

Intact uterus

G1 CL

\ n '"•' Ovary

In the intact uterus, the CL lifespan is the same as in a normal cycle (15-17 d).

Partial uterectomy (Contralateral to CL)

.. '. . ' '. '' . '

', ',

. ' ... _,' \

', ... ;

Ovary · . - G1 CL r /) ., ••. A partial uterectomy contralateral to the CL will yield a lifespan similar to a normal cycle (15-17 d).

The requirements for luteolysis (in subprimate mammals) are: • presence of oxytocin receptors on endometrial cells

• presence of a critical level of oJ..ytocin • PGF

2 a. synthesis by the endometrium

Total uterectorny .. --- ... _

" '' . . . ' . ' .. ' ' ' " .. _ .. ' \

Ovary \.-? ... '

' ' '' •: •' ' . ' . '' ' '

/ /

With a total uterectomy, the CL lifespan is similar to a normal gestation length (148 d).

Partial uterectomy (Ipsilateral to CL)

: :

Ovary

" " '' ' '' ''

A partial uterectomy ipsilateral to the CL will cause the CL to have a lifespan longer than normal (35 d).

The Luteal Phase 193

Figure 9-10. The Utero-Ovarian Vascular Countercurrent Transport System

V et B oo ks .ir

194 The Luteal Phase

Figure 9-11. Changes in PGF Metabolite (PGF-M) During Late Diestrus and Proestrus

PGF-M (brown line) is an accurate estimate of PGF2a· As the graph shows, PGF2a is low.

The amplitude and fre- quency of episodes of PGF2a secretion increase at about day 16. Abou t 5 pulses of PGFza in a 24 hour period are required to cause luteolysis and a dramatic drop in P4.

Episod ic secretion of PGF 2a remains high for about 2 days after luteolysis.

P4 Luteolysis

350 8 ::::-

::::- 300 E -b(l c. 250 -I: I

200 u. C) a. '"0 ISO 0 ..2 1:0

7 E b:o c -6 Cl) l

c 5 0 ,_ Cl) 4 Ill Cl)

b(l 0

3 ,_ c.

'"0 2 0 0

100 1:0

50 13 14 IS 16 17 18 19 20

Day of estrous cycle

A critical number of PGF 2a pulses within a given time- span are required to induce complete luteolysis. The exact number of pulses required has not been defined for all species. However, based on data from the ewe, about five pulses in a 24 hour period are required to induce complete luteolysis. Pulsatile release ofPGF2a is apparently not required under conditions of exogenous PGF2a administration. For example, one injection of PGF2a is sufficient to cause luteolysis.

The exact stimulus that initiates PGF2a secre- tion is controversial. One school of thought maintains that the uterus must be exposed to elevated progester- one for a period of days before it can synthesize and secrete PGF2a in sufficient quantities to cause luteolysis. During the first half of the estrous cycle, progesterone prevents secretion ofPGF2a by blocking the fonnation of oxytocin receptors in the uterus . After I 0 to 12 days progesterone loses its ability to block formation of oxytocin receptors, although it is not known how this

occurs. During the late luteal phase injections of exog- enous oxytocin cause secretion of PGF2a by the uterus. Injections ofPGF2a during the late luteal phase lead to a rapid release of ovarian oxytocin. Thus, oxytocin and PGF2a stimulate each other in a positive feedback manner. In the ewe, oxytocin episodes precede PGF2a episodes.

It should be emphasized that our understand- ing of the precise luteolytic mechanis m is not com- plete. Progesterone is believed to play a maj or role in regulating the timing of PGF 2a secretion . For example, as progesterone increases during the luteal phase , progesterone receptors decrease in the endometrium. The decrease in progesterone receptor numbers in the endometrium is fo llowed by episodes of PGF2a secretion by the endometrium later in the cycle. The exact interaction between progesterone concentrations, progesterone receptors, oxytocin secretion and PGFza secretion n eeds further clarification.

The Luteal Phase 195

Figure 9-12. Proposed Steps Resulting in the Loss of Progesterone Secretion from Luteal Cells

PGF2a binds to specific receptors on the plasma membrane of the luteal cells.

The PGF2a recept or complex is believed to open ca++ cha nn els so that Ca++ influ x occurs. High intracell ular Ca++ is thought to ca use apoptotic effects.

Luteoly sis results in:

• :e ... .

• cessation of progesterone seaetion • structural regression to form a c01pus albicans

• removal of negative feedback by pro- gesterone upon GnRH secretion resulting in a new follicular phase

The intracellular mechanis ms that cause luteolysis have been the subject of intense resear ch during the last 20 years. One of the original theories to explain the demise of the corpus luteum was that PGF2a caused reduction in blood flow to the corpus luteum by causing vasoconstriction (contraction) of arterioles supplying the luteal tissue. While blood flow to the corpus luteum does decrease during luteolysis, blood flow to the corpus luteum is still 5 to 20 times greater than to the sun ounding ovarian tissue. Thus, ischemia (reduced blood flo w) as a primary mode for luteolysis seems unlikely. It is known that capillaries in the corpus luteum undergo degeneration during luteo lysis. It is possible that this cap illary degenera- tion is more responsible for reducing blood flow than vasoconstriction associated w ith PGF2u . Neverthe- less, a degree of circulatory disruption is associated

Luteal Cell

T h e PGF2a rece pto r comp lex also activates protein kin ase-C (PK-C) wh ich inh ib its proges- terone synthesis.

with the luteo lytic process. However, it is unlikely that disruption to the luteal vasculature can totally account for luteolys is.

A second line of thinking is the theory that PGF2a binds to specific receptors on large luteal cells a nd trig- gers a cascade of events resulting in the death of these cells and thus , cessation of steroidogenesis. These events are presented in Figure 9-1 2 .

The Immune System May Be Involved in Regression of the Corpus Luteum

It is well -known that immune cells are present in the corpus luteum at the time of luteo lysis . These cells are capable of performing phagocytosis of lutea l cells. P hagocytic cells increase prior to the onset of lu- teo lysis . Lymphocytes secrete cytoldnes. C ytokines are non-antibody proteins secre ted by a variety of immune cells that activate macrophages that then phagocytize damaged dead luteal ce lls and cellular debris . Examples of cytokines are interferons , interleukins and tu mor necrosis factors (TNF). Cytokines have been shown to cause luteal cell death in vitro. They a lso inhibit proges- terone synthes is by luteal cells. While the mechanism involving the role s of cytokines in luteolysis is far from clear, it appears that normal morphologic and fu ncti onal integrity of the corpus luteum can be reduced when cytokines are present.

V et B oo ks .ir

194 The Luteal Phase

Figure 9-11. Changes in PGF Metabolite (PGF-M) During Late Diestrus and Proestrus

PGF-M (brown line) is an accurate estimate of PGF2a· As the graph shows, PGF2a is low.

The amplitude and fre- quency of episodes of PGF2a secretion increase at about day 16. Abou t 5 pulses of PGFza in a 24 hour period are required to cause luteolysis and a dramatic drop in P4.

Episod ic secretion of PGF 2a remains high for about 2 days after luteolysis.

P4 Luteolysis

350 8 ::::-

::::- 300 E -b(l c. 250 -I: I

200 u. C) a. '"0 ISO 0 ..2 1:0

7 E b:o c -6 Cl) l

c 5 0 ,_ Cl) 4 Ill Cl)

b(l 0

3 ,_ c.

'"0 2 0 0

100 1:0

50 13 14 IS 16 17 18 19 20

Day of estrous cycle

The Luteal Phase 195

Figure 9-12. Proposed Steps Resulting in the Loss of Progesterone Secretion from Luteal Cells

PGF2a binds to specific receptors on the plasma membrane of the luteal cells.

The PGF2a recept or complex is believed to open ca++ cha nn els so that Ca++ influ x occurs. High intracell ular Ca++ is thought to ca use apoptotic effects.

Luteoly sis results in:

• :e ... .

• cessation of progesterone seaetion • structural regression to form a c01pus albicans

• removal of negative feedback by pro- gesterone upon GnRH secretion resulting in a new follicular phase

Luteal Cell

T h e PGF2a rece pto r comp lex also activates protein kin ase-C (PK-C) wh ich inh ib its proges- terone synthesis.

with the luteo lytic process. However, it is unlikely that disruption to the luteal vasculature can totally account for luteolys is.

The Immune System May Be Involved in Regression of the Corpus Luteum

V et B oo ks .ir

196 The Luteal Phase

In addition to a direct effect on the luteal cell, cytokines may serve as triggering agents for a process called apoptosis. Apoptosis (pronounced "a-pa-toe- sis") is a phenomenon known as " programmed cell death" . It is quite nonnal for cells throughout the body to die on a daily basis. Cell death occurs by one of two processes. The first, cell necrosis, is brought about by pathologic damage. The second type of cell death, apoptosis, is an ordered biochemical process. This process involves distinct biochemical and morphologic changes in the cell. The process of apoptosis is probably the final step resulting in the death of the luteal cell. Final destruction and " clean-up" of the non-functional luteal cells is probably perfom1ed by macrophages that phagocytize damaged luteal cells. Over time the luteal cells disappear completely, leaving only connective tis- sue behind. Thus, the scar-like corpus albicans (white body) is formed.

Luteolysis in Women is an Intra-Ovarian Event. The Uterus is Not Required.

Uterectomy in the woman does not influence ovarian cyclicity. In other words, the normal pattern of folliculogenesis , luteal development and Juteolysis occurs in a rhythmic fashion about every 28 days after the removal of the uterus. A proposed mechanism for luteolysis in primates is presented in Figure 9-13. Even though traces of luteal oxytocin have been identified, it is thought that oxytocin from the posterior pituitary acts on ovarian oxytocin receptors to generate small amounts of intraovarian PGF2u. Luteolysis is thought to be a local effect and therefore only small amounts of PGF2u are required to lyse the CL. As a result of oxytocin receptors binding oxytocin, the synthetic pathway for PGF2uis activated and this causes luteolysis. Luteolysis therefore causes a marked reduction in progesterone that is thought to cause endometrial synthesis ofPGF2a· Endometrial PGF2u is important because it causes local vasoconstriction ofthe endometrial arterioles and initi- ates menstruation. This significant reduction in blood flow brought about by vasoconstriction in the luminal region of the endometrium causes necrosis and slough- ing of the endometrial tissue. A more detailed descrip- tion of the mechanism of menstruation is presented in Chapter 16.

Administration of Progesterone Results in Manipulation of the Estrous Cycle Now that you understand the mechanisms that

control progesterone synthesis, secretion and luteolysis, an understanding of how progesterone is used to control/ manipulate cyclicity will provide you with practical knowledge that is based on the physiologic principles.

Figure 9-13. Proposed Mechanism of Luteolysis in Primates

Hypothalamus

Endometrial synthesis of PGF2u (See Ch apt er 16)

Vasoconstriction of e ndometrial arterioles

(See Chapt er 16)

Endometrial necrosis and sloughing (menses)

(See Chapte r 16)

A s you know, progesterone provides negative feedback to the hypothalamus to suppress GnRH . This fact has been appl ied to the developme nt of many ap- plications designed to manipulate the repr oductive cycles in domestic animals. The administration of progesterone serves as an "artificial corpus luteum".

Exogenous progesterone suppresses estrus and ovulation. H owever, when the exogenous progesterone is removed or w ithdrawn, the animal will enter proestrus and estrus within two to t hree days after progesterone removal. This approach enables estrus to be synchronized in large groups offemales so that artific ial insemination can be accomplished within a few days. This appl ica- tion is intended to increase the conven ience of artificial insemination programs and to f aci litate ferti lity (higher pregnancy rates). In contrast, the use of exogenou s pro- gesterone in women is intended to b lock ovulation and minimize the likelihood of pregnancy. Mechanisms o f this application are presented in Chapter 16.

Intravaginal Progesterone is Effective at Synchronizing Estrus in Cattle The EAZI-BREED™ CIDR® C attle Insert is

an intravaginal progesterone-releasing device used for of estrus in beef cattle and dairy heifers.

CIDR is an acronym for " Controlled Internal Drug Re- lease" . T he product has also been approved for advanc- ing first estrus in anestr us postp artum beef cows and in prepubertal beef heifers. The CIDR® is inserted into the vag ina of the cow/heifer and remains there for 7 days.

While in the vagina, progesterone diffuses out of the CIDR® Insert, crosses the vagina l mucosa and enters the vasculature of the vagina . The blood profiles of progesterone in ovar iectom ized cows immediate ly follow ing insertion, during a 7 day administration and immediately after CID R® Insert removal are shown in Figure 9- 14.

For synchronization of estrus the CIDR® Insert is administered for 7 days w ith an inj ectio n of 5ml Lutalyse® Sterile Solution (25mg prostag landin F 2 u) on the sixth day. Progesterone from the CIDR® Insert sup- presses GnRH release, gonadotropin r elease, fo llicular development and ovulation in those cows and heifers that have corpora lutea that regress spon taneously during the 7 day adminis tration period. L utalyse® is adm inistered to initiate luteal regression in those cows and he ifers that have a functional corpus luteum at the end of the CIDR® Insert administration period. Upon removal of the CIDR® Insert and injection of Lutalyse®, cows and heifers w ill experience a rapid decline in the concen- tration of progesterone followed by elevated GnRH , elevated gonadotropins and folli cular deve lopment and will enter proestrus and estrus with in t\vo to three days (a synchronized estrus).

The Luteal Phase 197

Figure 9-14. Blood Progesterone Profiles After the CIDR® Insertion and Remova l

I Plasma Progesterone Absorption 1 Profile Following CIDR® Insertion , , into Cows (n = 8) '

0.00 1.00 2.00 3.00

t Time after insertion (hr) C ID R® inserted

Bars = Standard error of t he mean

I - --

: Plasma Progesterone Absorption

4.00

Profile for the Entire Admin istration '--- __ in Cows (n=8)

CII C'

5 Re moval of 5.€

';; !!- ! -.......__ C ID R® 4 • I

... 3 -!-!- t - i t 1 --f

21 • ., u I I i £s o+ I I I tt. T

0 2 4 6 8

t Time (days) CID R® inserted

Bars = Standard error of the mean

' - Plasma Progesterone Clearance

i Profile Following Removal of CIDR® I_ from Cows (n=8)

CIIC' 3.0 2.5 2.0 . 1.5

c...... • ns 'l: 1.0 '\

0.5 ns c - ........._ ii: 8 0.0 • •

0 5 10 IS 20 25

t Time after removal (hr) C ID R® removed

Bars = Stand ard error o f the mean

V et B oo ks .ir

196 The Luteal Phase

Luteolysis in Women is an Intra-Ovarian Event. The Uterus is Not Required.

Administration of Progesterone Results in Manipulation of the Estrous Cycle Now that you understand the mechanisms that

Figure 9-13. Proposed Mechanism of Luteolysis in Primates

Hypothalamus

Endometrial synthesis of PGF2u (See Ch apt er 16)

Vasoconstriction of e ndometrial arterioles

(See Chapt er 16)

Endometrial necrosis and sloughing (menses)

(See Chapte r 16)

Intravaginal Progesterone is Effective at Synchronizing Estrus in Cattle The EAZI-BREED™ CIDR® C attle Insert is

an intravaginal progesterone-releasing device used for of estrus in beef cattle and dairy heifers.

The Luteal Phase 197

Figure 9-14. Blood Progesterone Profiles After the CIDR® Insertion and Remova l

I Plasma Progesterone Absorption 1 Profile Following CIDR® Insertion , , into Cows (n = 8) '

0.00 1.00 2.00 3.00

t Time after insertion (hr) C ID R® inserted

Bars = Standard error of t he mean

I - --

: Plasma Progesterone Absorption

4.00

Profile for the Entire Admin istration '--- __ in Cows (n=8)

CII C'

5 Re moval of 5.€

';; !!- ! -.......__ C ID R® 4 • I

... 3 -!-!- t - i t 1 --f

21 • ., u I I i £s o+ I I I tt. T

0 2 4 6 8

t Time (days) CID R® inserted

Bars = Standard error of the mean

' - Plasma Progesterone Clearance

i Profile Following Removal of CIDR® I_ from Cows (n=8)

CIIC' 3.0 2.5 2.0 . 1.5

c...... • ns 'l: 1.0 '\

0.5 ns c - ........._ ii: 8 0.0 • •

0 5 10 IS 20 25

t Time after removal (hr) C ID R® removed

Bars = Stand ard error o f the mean

V et B oo ks .ir

198 The Luteal Phase

Another use of an exogenous progesterone- like compound is in mares. A material with the trade name Regu-Mate® is used to control cyclicity. The ac- tive ingredient in Regu-Mate® is a synthetic progestin called altrenogest. The physiologic action of altrenogest is the same as progesterone. It is used in mares for the following reasons: I) to induce regular cyclicity in mares making the transition from winter anestrus to the breeding season, 2) to suppress undesired estrous behavior and 3) allow for scheduled breeding during the breeding season .

Altrenogest is administered by placing the appropriate dose on the posterio-dorsal smface of the mare's tongue or is applied to the grain ration. It is given daily for 15 consecutive days. During the time that altrenogest is being administered Gn.RH is sup- pressed, and behavioral estrus does not occur. After cessation of the treatment, mares will display estrus four to five days later.

Exogenous Prostaglandin F za is a Potent Luteolysin and Can Synchronize Estrus

Following the discovery that PGF2o. was the luteolysin, a major research emphasis was placed on us- ing this hormone to shorten the estrous cycle and induce estr us in cattle. Injections ofPGF2u between day seven and day 18 will cause the cow to begin to show estrus in about three days (60-80 hours after the injection). Figure 9-15 illustrates the effect of prostaglandin for inducing estrus. It must be emphasized that the corpus luteum ofthe cow is not sensitive to PGFza between days one and six of the cycle. In other words, injecting the cow with PGF2a during this time will not have an effect (See Figure 9-15).

Reproductive physiologists at the University of Wisconsin and Michigan State University have devel- oped an innovative use of GnRH and PGF2a that syn- chronizes ovulation. This protocol is named Ovsynch (See Figure 9- I 6, green section). When Gn.RH and PGF2a are used together in the proper timed-sequence, visual detection of estrus can be eliminated and timed artificial insemination (TAI) can be performed. This program is being used routinely as a reproductive man- agement tool in the dairy industry. The Ovsynch innova- tion incorporates the mechanisms of follicular dynamics described in Chapter 8 and the mechanisms ofluteolysis (described earlier in this chapter) into a practical appli- cation of physiologic principles. A solid understanding of these mechanisms will translate into understanding of the Ovsynch protocol described later.

The basic strategy for the Ovsynch program is presented in the steps that follow. Step 1- GnRH is injected into cows that are eligible to be inseminated

(fully recovered from their last parturition). The GnRH injection causes one of two events to take place. First, if there is a dominant follicle on the ovary (a fol- licle that is greater than I Omm and has an adequate population of LH receptors) the cow will ovulate in response to GnRH. A CL will then form. Second, if the cow does not have a dominant follicle (an im- mature follicle that has few LH receptors), GnRH will promote continued follicular growth. In this case, there is a CL present from the previous ovulation; Step 2- An injection ofPGFza seven days after GnRH causes luteolysis and the cow will enter the follicular phase; Step 3- A second injection of GniUJ 48 hours later causes the cow to ovulate. Step 4- The cow can then be inseminated without detection of estrus I 6 hours after the second Gn.RH injection.

This strategy, when properly applied in commercial dairy herds has resulted in acceptable conception rates without detection of estrus in lactat- ing dairy cows. The Ovsynch strategy will enable almost l 00% of the cows to be inseminated after the designated postpartum waiting p eriod (typically 60 days and called the "voluntary wait period"). The first GnRH injection in the Ovsynch program is given at random (without knowledge of the specific day of the cycle). This can result in several problems. If cows are not cyclic, GnRH will not initiate cyclic- ity in all of them. Those that do ovulate in response to GnRH have reduced conception. Some GnRH- treated cows will recruit follicles from the second or third follicular wave and the follicle may not ovulate. Therefore, the PGFza injection is not totally effec- tive (because there is no CL present) in these cows.

In order to help minimize the above problems, a strategy has been developed that is called Presynch (See Figure 9-16, brown section). The Presynch pro- gram begins 26 days prior to the first GnRH injection. At random, all cows are given PGF20 . Fourteen days later a second PGF2a injection is given. Remember, the first PGF2o. will regress an existing corpus luteum if it is between days 7 and 17. Obviously, all cows will not fall into this range and the second PGF2o. regresses all corpora lutea that are present because they are in the " sensitive window" between days 7 and I 7. Twelve days after the prostaglandin injection, GnRH is injected. GnRH may cause a new follicle to ovulate, forming a new CL as per the original Ovsynch protocol. More detail about each method can be obtained from the Key References section at the end of the chapter.

The Luteal Phase 199

Figure 9-15. Influence of Prostaglandin F2a Upon Cycle Length in the Cow

........ 10 'E

8 .._, ] 6 0 0 ma

t: 4 Qj llO e 2 c.

Estrus

Normal Cycle - Estrus Every 21 Days

5 10 15

Day of cycle (cow)

Estrus

PGFza Injections - Day 0 to Day 6 - No Effect

........ 10 'E

8 ....... ] 6 0 0

t: 4 Qj llO e 2 c.

Estrus

........ 10 'E

8 ....... -g 6 0 0 iiit

t: 4 Qj llO e 2 c.

Estrus

5 10 IS

Day of cycle (cow)

5 7 10 IS

Day of cycle (cow)

' ' I •

Estrus

In the normal cyclic cow estrus and ovula- tion occurs every 21 days. Luteolysis (in- duced naturally by PGF2a from the uterus) causes the animal to enter a new follicular phase and subsequent estrus.

If a single injection of PGF2a is given be- tween day zero and about day six, luteoly- sis will not occur and the cycle will be of normal length. This is because the corpus luteum must reach a certain stage of devel- opment before it is sensitive to PGF2a.

If PGF2a is injected on day 7-17, luteolysis will occur. Progesterone will drop and the animal will come into estrus in about three days after the injection. Such a strategy is used to synchronize estrus in large groups of animals .

V et B oo ks .ir

198 The Luteal Phase

Exogenous Prostaglandin F za is a Potent Luteolysin and Can Synchronize Estrus

The basic strategy for the Ovsynch program is presented in the steps that follow. Step 1- GnRH is injected into cows that are eligible to be inseminated

The Luteal Phase 199

Figure 9-15. Influence of Prostaglandin F2a Upon Cycle Length in the Cow

........ 10 'E

8 .._, ] 6 0 0 ma

t: 4 Qj llO e 2 c.

Estrus

Normal Cycle - Estrus Every 21 Days

5 10 15

Day of cycle (cow)

Estrus

PGFza Injections - Day 0 to Day 6 - No Effect

........ 10 'E

8 ....... ] 6 0 0

t: 4 Qj llO e 2 c.

Estrus

........ 10 'E

8 ....... -g 6 0 0 iiit

t: 4 Qj llO e 2 c.

Estrus

5 10 IS

Day of cycle (cow)

5 7 10 IS

Day of cycle (cow)

' ' I •

Estrus

V et B oo ks .ir

200 The Luteal Phase

I STEP •I PGF2a

t STEP

Figure 9-16. Presynch and Ovsynch as Methods to Synchronize Ovulation in Cows

14d

ACTION

PGF2a

I STEP 21 I STEP •I Gn RH PGF2a

t 12d t Presynch

cow WHEN REASON RESPONSE

Regress existing CL Cow ovula tes Anytime and induce new and produces

ovulation "new" CL

14 days Regress "ne w" CL. New follicular after I st In cows not phase ( + P., ) PGF2a responding to

Jst PGF2a injection - regress "old" CL.

STEP ACTION WHEN REASON

Situation A To cause ovulat ion in existi ng d ominant follicle (greater t h an IOmm a nd t hat have LH receptors)

12 days Situation B I Gn RH after last

PGF2a To cause continued growth o f exis ting immature follicles (few LH rece ptors). A CL from the current cycle is present at the same time ("old" C L).

Situation A To regress "new· CL (resul t! ng fro m previo us Gn RH

7 days injection 2 PGF2a after last Situation B

GnRH To regress either "old ' CL or both the "ol d" CL and t h e "new" CL (from Gn RH Injec t ion)

Situation A

2 days To cause ovul ati on of

3 GnRH after last dominant follicle

PGF2a Situation B To cause ovulati on o f dom inant follicle

lnsemi- 16 hours Sperm in tract

4 nation after before ov ulatio n GnRH fertilization

I STEP 21 I STEP 31

PGF20. l Gn RH

t All STEP41

cow RESPONSE

Situation A Th e LH surge cau ses ovulation of the existing dominant fo llicle, a ne w C L Is formed and a new foll icular wave starts.

Situation B Co ntinued fo ll icula r growth towards fo lli cular d omina nce (CL o f the cycle still present)

Situation A Co w enters fo ll icular phase an d new dominan t fo llicle devel ops

Situation B Follicl e from first GnRH injection (Step 3) co ntinues to grow a nd beco mes d o minant . The"old" CL reg resses.

Situat ion A Ovulation In 24-32 hou rs Situation B Ovulation in 24-32 hou rs

30-40% co nce ption

Further PHENOMENA for Fertility

Female elephants have a uniquely long es- trous cycle (16 weeks) am/ a gestation of 22 months. What does this say about elephant CL?

The regression of the cmpus luteum in 1m- mans and other primates is not controlled by the utems. However, PGF2a will induce lu- teolysis in primates. It is believed that PGF2a of ovarian origin is responsible for causing luteal regression.

The co1pus lutermz of mos t rode1zts (rats, mice, hamsters and g erbils) does not develop unless copulation occurs. P enile stimulation of the cervix causes prolactin release from the female. Prolactin is luteotropic and causes the formation of cotpora lutea.

Some spiders have no p enis. They ej ect sperm from their abdomen onto their web. The male spider picks up the ejaculate with a special set of antennae and searches for a 1·eceptive female who produces a pheromone. Th e male has to be vety careful and deposit the sem en by surprise because th e female will eat hi m if she catches him.

The luteal phase of the estrous cy cle of the kangaroo is longer than preg nancy.

Researchers at N .C. State University obser ved a sow that had 128 corpora Iuten on both of h er ovaries. This is ten times the normal number of corpora lutea. The caus e of such a high number of ovulations is unknown.

The Luteal Phase 201

Key References

Leymarie, P. and Marta! , J. 1993 . " T he corpus luteum from cycle to gestation" in Re production in Mammals and Man . p 413 -434. C. T hibaul t, M.C. Levasseur and R .H.F. Hunter, eds. , E llipses, Paris. ISBN 2- 7298- 9354-7.

McCracken, J .A. 1998. "Luteolysis" in Encvclopedia o(Reproduction . Vol. 2 . p1083 - 1094. Knobil, E. and J. D. Nei ll, eds. Academic Press, San Diego ISBN 0- 12-227022-3.

Niswender, G.D. and T. M. Nett. 1994. "Corpus luteum and its contro l in infr aprimate species" in Th e Phvsiolof?Y of Reproduction, 2nd Editi on. Vol. I p78 1-816. E. Knob il and J .D. Neill, eds. , R aven Press, Ltd., New York. ISBN 0- 78 17-0086-8.

Pate, J. L. and D .H. Townson. 1994. "Novel local regulators in luteal regression." XXI Biennia l Sym- pos ium on Anima l Reproduction. J. Anim. Sci. 72 (Suppl. 3):3 1-42.

Pursley, J.R., M.R. Kosorok and M.C. Wi ltbank, 1997. " Reproductive management of lactating dairy cows using synchronization of ovulation" in J. Daily Sci. 80:30 1-306.

Salamonsen, L.A. 2003. " Tissue inj ury and repa ir in the fe male human reproductive tract." Reprod. 125(3):30 1-3 11 .

V et B oo ks .ir

200 The Luteal Phase

I STEP •I PGF2a

t STEP

Figure 9-16. Presynch and Ovsynch as Methods to Synchronize Ovulation in Cows

14d

ACTION

PGF2a

I STEP 21 I STEP •I Gn RH PGF2a

t 12d t Presynch

cow WHEN REASON RESPONSE

Regress existing CL Cow ovula tes Anytime and induce new and produces

ovulation "new" CL

14 days Regress "ne w" CL. New follicular after I st In cows not phase ( + P., ) PGF2a responding to

Jst PGF2a injection - regress "old" CL.

STEP ACTION WHEN REASON

Situation A To cause ovulat ion in existi ng d ominant follicle (greater t h an IOmm a nd t hat have LH receptors)

12 days Situation B I Gn RH after last

PGF2a To cause continued growth o f exis ting immature follicles (few LH rece ptors). A CL from the current cycle is present at the same time ("old" C L).

Situation A To regress "new· CL (resul t! ng fro m previo us Gn RH

7 days injection 2 PGF2a after last Situation B

GnRH To regress either "old ' CL or both the "ol d" CL and t h e "new" CL (from Gn RH Injec t ion)

Situation A

2 days To cause ovul ati on of

3 GnRH after last dominant follicle

PGF2a Situation B To cause ovulati on o f dom inant follicle

lnsemi- 16 hours Sperm in tract

4 nation after before ov ulatio n GnRH fertilization

I STEP 21 I STEP 31

PGF20. l Gn RH

t All STEP41

cow RESPONSE

Situation A Th e LH surge cau ses ovulation of the existing dominant fo llicle, a ne w C L Is formed and a new foll icular wave starts.

Situation B Co ntinued fo ll icula r growth towards fo lli cular d omina nce (CL o f the cycle still present)

Situation A Co w enters fo ll icular phase an d new dominan t fo llicle devel ops

Situation B Follicl e from first GnRH injection (Step 3) co ntinues to grow a nd beco mes d o minant . The"old" CL reg resses.

Situat ion A Ovulation In 24-32 hou rs Situation B Ovulation in 24-32 hou rs

30-40% co nce ption

Further PHENOMENA for Fertility

Female elephants have a uniquely long es- trous cycle (16 weeks) am/ a gestation of 22 months. What does this say about elephant CL?

The luteal phase of the estrous cy cle of the kangaroo is longer than preg nancy.

The Luteal Phase 201

Key References

McCracken, J .A. 1998. "Luteolysis" in Encvclopedia o(Reproduction . Vol. 2 . p1083 - 1094. Knobil, E. and J. D. Nei ll, eds. Academic Press, San Diego ISBN 0- 12-227022-3.

Pate, J. L. and D .H. Townson. 1994. "Novel local regulators in luteal regression." XXI Biennia l Sym- pos ium on Anima l Reproduction. J. Anim. Sci. 72 (Suppl. 3):3 1-42.

Pursley, J.R., M.R. Kosorok and M.C. Wi ltbank, 1997. " Reproductive management of lactating dairy cows using synchronization of ovulation" in J. Daily Sci. 80:30 1-306.

Salamonsen, L.A. 2003. " Tissue inj ury and repa ir in the fe male human reproductive tract." Reprod. 125(3):30 1-3 11 .

V et B oo ks .ir

The Puerperium & Lactation

Parturition

Fetal Attachment & Gestation

Early Embryogenesis & Maternal Recognition of Pregnancy

Ovulation & Fertilization

Cyclicity

Regulation of Reproduction

Tract Function

Puberty

Prenatal Development

... , ... , \ I

Spermatogenesis

Regulation of Reproduction

Tract Function

Puberty

Prenatal Development

Take Home Message Reproductive behavior is an obligatmy component oftlze reproductive process. It consists

ofprecopulatory, copulatory and postcopulatory stages. In the female, sexual receptivity occurs only during estrus and is characterized by distinct behavior and mating posture (lm·- dosis). In the male, reproductive behavior can occur potentially any time. Sexual arousal in the male involves a cascade of endocrine and neural events that result in erection oftlte penis, mounting oftlze sexually receptive female, intromission and ejaculation. Erection of the penis involves specific neural and biochemical events that culminate in penile vasodila- tion. Ejaculation is a reflex that is initiated by stimulation of the glans penis and concludes with expulsion ofsemen

Reproductive behavior has evolved as one of the s trongest drives in the animal kingdom and usually takes precedence over all other forms of activity such as eating, resting and s leeping. The purpose of reproduc- tive behavior is to promote the opporhmi ty fo r copula- tion and thus increase the probability that the spem1 and the egg will meet. The ultimate goals of copulation are pregnancy, successful embryogenesis and parturition.

Reproductive behavior in the male consists of three distinct stages:

• the precopulatory stage • the copulatory stage • the postcopulatory stage

Reproductive behavior in the male can be di- vided into thr ee distinct stages. These stages are : the precopulatory stage; the copulatory stage; and the postcopulatory stage. T he specific events that occur during each of these stages are presented in F igure 11-1 .

Reproductive behavior in the female can be considered to serve the following functions:

• attractivity • proceptivity • receptivity

Figure 11-1. Stages of Male Reproductive Behavior and

Specific Events in Each Stage

Precopulatory Behavior

Search for sexual partner

• Courtship -

Sexual arousal - Erection -

Penile protr usion

• .. Copulatory Behavior

Mounting

Intromission

Ejaculation

Postcopulatory Behavior

I Dismount I - I T I Refractory period - I Memory I

V et B oo ks .ir

The Puerperium & Lactation

Parturition

Fetal Attachment & Gestation

Early Embryogenesis & Maternal Recognition of Pregnancy

Ovulation & Fertilization

Cyclicity

Regulation of Reproduction

Tract Function

Puberty

Prenatal Development

... , ... , \ I

Spermatogenesis

Regulation of Reproduction

Tract Function

Puberty

Prenatal Development

Take Home Message Reproductive behavior is an obligatmy component oftlze reproductive process. It consists

Reproductive behavior in the male consists of three distinct stages:

• the precopulatory stage • the copulatory stage • the postcopulatory stage

Reproductive behavior in the female can be considered to serve the following functions:

• attractivity • proceptivity • receptivity

Figure 11-1. Stages of Male Reproductive Behavior and

Specific Events in Each Stage

Precopulatory Behavior

Search for sexual partner

• Courtship -

Sexual arousal - Erection -

Penile protr usion

• .. Copulatory Behavior

Mounting

Intromission

Ejaculation

Postcopulatory Behavior

I Dismount I - I T I Refractory period - I Memory I

V et B oo ks .ir

230 Reproductive Behavior

Precopulatory, copulatory and postcopulatory behaviors in the female can be considered as serving the functions of: attractivity, proceptivity and receptiv- ity. Attractivity refers to behaviors and other signals that serve to attract males. This can include postures, vocalizations, behaviors and chemical cues such as pheromones that attract the male to approach and en- gage in precopulatory behavior. Proceptivity refers to the behaviors exhibited by females toward males that stimulate the male to copulate or that reinitiate sexual behavior after copulation. For example, head butting of the male and mounting the male are two of the most common preceptive behaviors exhibited by females. Proceptivity may also include behaviors among fe- males, such as female-female mounting that sexually stimulate males. Finally, r·eceptivity is the copulatory behavior of females that ensures insemination. This may include the immobility or standing response (lor- dosis) as well as tail deviation or backing-up toward the male.

As you have already learned, sexual activity of the postpubertal female is confined to estrus (heat). This short period of sexual receptivity limits the time during which precopulatory behavior occurs in most females. In contrast, the male is potentially capable of initiating reproductive behavior at any time after puberty. The initiation of courtship-specific behavior is generally under the influence of the female. She will send subtle, or sometimes overt signals to the male (attractivity) to initiate courtship behavior. Fac- tors such as sexual signaling pheromones, vocaliza- tion, increased physical activity and subtle postural changes are signals provided by the female that will initiate more intense courtship behavior on the part of the male. In addition, it has been hypothesized that female-female (proceptivity) interactions such as ho- mosexual mounting activity among cattle may serve as signals to initiate male-female courtship behavior. In general, the postpubertal male is almost constantly searching for signals sent by the female to indicate that she is sexually receptive.

Identification of a sexual partner probably requires mostofthe senses (olfactory, visual, auditory and tactile). The relative importance of these sensory stimuli has not been described critically in most spe- CleS.

Females of almost all species appear to show a marked increase in general physical activity as they come into estrus (See Figure 11-2). Elevated physical activity is generally manifested by increased locomotion. In addition, milling around, exploration, increased vocalization and agonistic behavior towards other females can be observed. In almost all species studied, including humans, there is a marked increase

1/) a.. w I- I/)

Figure 11-2. Relationship Between Physical Activity and

Reproductive Cycles in Various Female Mammals

!cows I

Estrus Estrus

sows

Estrus Estrus

RATS

Estrus Estrus

!woMEN!

Menses Menses

Physical activity increases significantly around the time of estrus and /or ovulation.

in physical activity that accompanies the time of ovula- tion. Presumably, this physical activity is associated with searching for a mate . This increased physical activity can be measured by equipping females with pedometers. Pedometers are devices that monitor and quantitate steps taken by the animal and are currently used in commercial dairy enterprises for detection of estrus.

Courtship-specific behavior is initiated after a sexual partner has been identified.

Once a sexual partner has been identified, a series of highly specific courtship behav iors begin. Courtship-specific behaviors include sniffing of the vulva by the male, urination by the female in the pres- ence of the male, exhibiting flehmen behavior (See Figure 11 -5), chin resting, circling and increased pho- nation. In many species the sense of vision appear s to be the most important w ith regard to sexual arousal in the male. This should not be interpreted to mean that other stimuli, such as auditory or o lfac tmy are not important.

Copulatmy behavior varies significantly among species with regard to duration.

Lordosis (mating posture) by the fema le (re- ceptivity) triggers significant sexual arousal behavior on the part of the male. Once the male discovers that the fema le will display lordosis, he becomes sexually stimulated. It should be emphasized that lordos is is a highly specific female motor response associated with the "willingness" to mate.

Sexual arousal is followed by erection and penile protrusion.

Following expo sure to the appropriate stimuli , erection and protmsion of the penis occur. T hese highly specific m otor events are controlled by the central nervous system. The mechanisms of peni le protru sion and erection will be presented later. Typical behavior dming search, courtship and sexual arousal for domestic animals is presented in Tabl e 11 -1 .

Reproductive Behavior 231

After significant sexual s timulation, mount- ing, intromission and ejaculation follow. In genera l, mammals can be c la ssified as sustained copulators or short copulators. The bull, ram, buck and tom are short copulators w hile the boar, dog and camel ids are sustained copulators. The stallion is intem1edi ate with regard to duration of copulation.

Mounting behavior generally requires immobi- lization of the female and elevation ofthe front legs o f the male to straddle the caudal region of the female ( See Figure 11-1 0). Intromission is ent rance of the penis into the vagi na. Ejaculation is expulsion of semen from the penis into the female reproductive tract.

Copulatory behavior on the par t of the male is learned. Past sexual experiences are important in order for the male to develop appropriate reproductive behavior. For example, negative experiences during the precopulatory and copulatory stages will generally result in less enthusiasm on the part of the male. From a practical standpoint, m anagement of the breeding male should always be directed towards providing the ma le w ith totally p ositive sti m ul i. U ti lizing non-estrus femal es to collect semen fi·om stallions, boars, rams and bulls should be avo ided because these fe males do not w illingly stand to be mounted. Inj my to both the female and the male can occur under these circumstances.

Postcopulatmy behavior is a period of refractivity.

Postcopulatory behavior involves dism ounting and a per iod during which either the male, the fem ale or both will not engage in copulatory behavior. T his refractory period is a peri od of time during w hich a sec ond copulation w ill not take pl ace. Memory is important in both a positive and negative way. Positive mating experiences promote reproductive behavior and negative inhibit reproductive behavior. When semen is collected for artificial insemination, it is important to re- duce the duration of the refi·actmy period when multiple ejaculations need to be collected in the shortest possible time . Techniques to reduce the refractory period will be presented later in the chapter. Both ma les and females often display specific postcopulatory behavior such as vocal emissions, genital groomi ng, changing postural relationships and various tacti le behaviors , such as licking and nuzzl ing.

V et B oo ks .ir

230 Reproductive Behavior

1/) a.. w I- I/)

Figure 11-2. Relationship Between Physical Activity and

Reproductive Cycles in Various Female Mammals

!cows I

Estrus Estrus

sows

Estrus Estrus

RATS

Estrus Estrus

!woMEN!

Menses Menses

Physical activity increases significantly around the time of estrus and /or ovulation.

Courtship-specific behavior is initiated after a sexual partner has been identified.

Copulatmy behavior varies significantly among species with regard to duration.

Sexual arousal is followed by erection and penile protrusion.

Reproductive Behavior 231

Postcopulatmy behavior is a period of refractivity.

V et B oo ks .ir

Ill

232 Reproductive Behavior

Table 11-1 . Typical Behavior During Search, Courtship and Consummation by Female and Male Domestic Animals

Species Search

Cow Increased locomotion, increased vocalization, twitching & elevation of the tail

Mare Increased locomotion, tail erected ("flagging")

Ewe Short period of restlessness ram "seeking"

Sow Mild restlessness

Bitch Roaming

Queen Vocalization (calling)

Species Search

Bull Approach sexually active group of females testing for lordosis, flelm1en

Stallion Visual search, flehmen

Sniffing and licking of ana-genital region, nudging ewe, flehmen

Moving among females

Roaming around territory

Prowling

FEMALE

Courtship

Increased grooming, mounting attempts with other females

Urination stance, urination in presence of stallion

Urination in presence of ram

Immobile stance

limnobile stance

Crouching, affectionate head rubbing, rolling

MALE

Courtship

Nuzzling and licking of perineal region: chin resting, testing for lordosis

High degree of excitement

Neck outstretched and head held horizontally

Nuzzling, grinding of teeth, foams at mouth

Sniffing, licking of the vulva

Biting queen on dorsal neck

Consummation

Homosexual mounting & immobile stance (standing to be mounted)

Presents hindquarters to male, clitoral exposure by labial eversion, pulsati le contractions of labia

Immobile stance

Tail defl ected to one side Urination in presence of ma le affectionate head rubbing

Elevation of rear quarters and hyper- extension ofback (lordosis), presentation of vulva, tai l deviation

Consummation

Penile protrusion w ith dribbling of seminal fluid with few sperm- atozoa, erection and attempted mounts

Penile protrusion with no preejaculatmy expulsion of seminal fluid

Repeated dorsal retraction of scrotum, penile protrusion with no dribbling of seminal flu id

Penile protrusion, shallow pelvic thrusts, attempted mounting

Erection, protrusion of penis, mounting

Mounting

Reproductive Behavior is Programmed During Prenatal Development

During embryogenesis, sexual differentiation occurs, during which the brain is programmed to be either male or female. Recent findings suggest that the very early embryo is neutral with regard to sex (gender). Under the influence of extremely small quantities of estradiol the brain becomes fem inized. Feminiza- tion is the development of female-like behavior. As you learned in Chapter 6, during feta l development, a.-fetoprotein is produced that prevents most fetal and maternal estradiol from crossing the blood-brain barrier and entering the brain. When a.-fetoprote in prevents estradiol from entering the brain, the embryo becomes "fully feminized," because it has not been exposed to estrogen (See Chapter 6). Alpha-fetoprotein does not bind to testosterone, which can then enter the brain and be converted to estradiol. In developing males this high concentration of estradiol in the brain causes defemini- zation and masculinization of the brain. Defeminiza- tion reduces the likelihood that the animal will express female-like behavior postpubertally. Masculinization results in the potential of the animal to develop male- like behavior after puberty.

Sex differences in specific brain structures for the control of reproductive behavior have been observed. For example, in the male, the preoptic area

Reproductive Behavior 233

of the hypothalamus is larger than in female s. I n the male, the size of neurons, the neuron nuclei and the dendritic arborizations are greater. In the fema le, the ventromedial hypothalamus is more important with regard to reproductive behavior.

In most mammals, reproductive behaviors are sexually differentiated. For example, mounting, erection and ejaculation are typically male behavi ors, while standing to be mounted (lordosis), crouching and increased locomotion are typically female behaviors. These behaviors are endocrine controlled. For example, sequential treatment with progesterone and estradiol induces sexual receptivity in ovariectomized fem ales and testosterone will restore reproductive behavior in castrated males. In some species, inj ections of testos- terone into castrated females will even induce male-like reproductive behavior. Female fetuses exposed to androgens prenatally will display significantly reduced female behavior (defeminized) and acquire male-like behavior pos tna tally (mascu lini zed). In contrast, males exposed to estrogen or progesterone prena- tally are unaffected. A class ic example illustrating the behavioral manifestations of prenatal exposure to andro- gens is the freemartin heifer. As previous ly discussed (See Chapter 4), thi s animal has abnormal development of the reproductive tract for two reasons . First, from a genetic perspective, freemartins are chimeras that are XX/XY and therefore they have an ovotestis. Second,

Figure 11-3. Influence of Various Steroid Treatments Upon Reproductive Behavior

PRE NATAL Fetus + E2 ------• f Estrous behavior + male-like behavior Fetus + Testosterone f Estrous behavior + male-like behavior

'b Fetus + E2 or P4 No effect (normal 'b behavior) 'b Fetus+ Testosterone - No effect (normal 'b behavior)

POSTNATAL

No estrous behavior Estrous behavior

------+- Maximum estrous behavior + E2 ------- + P4 and E2 + Testosterone Male-like behavior

----------.- f Sexual behavior + Testosterone ----•

Ovaries remo ved (ovariect omy)

Sexual behavior restored Sexual behavior restored

removed (orchidectomy)

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Ill

232 Reproductive Behavior

Table 11-1 . Typical Behavior During Search, Courtship and Consummation by Female and Male Domestic Animals

Species Search

Cow Increased locomotion, increased vocalization, twitching & elevation of the tail

Mare Increased locomotion, tail erected ("flagging")

Ewe Short period of restlessness ram "seeking"

Sow Mild restlessness

Bitch Roaming

Queen Vocalization (calling)

Species Search

Bull Approach sexually active group of females testing for lordosis, flelm1en

Stallion Visual search, flehmen

Sniffing and licking of ana-genital region, nudging ewe, flehmen

Moving among females

Roaming around territory

Prowling

FEMALE

Courtship

Increased grooming, mounting attempts with other females

Urination stance, urination in presence of stallion

Urination in presence of ram

Immobile stance

limnobile stance

Crouching, affectionate head rubbing, rolling

MALE

Courtship

Nuzzling and licking of perineal region: chin resting, testing for lordosis

High degree of excitement

Neck outstretched and head held horizontally

Nuzzling, grinding of teeth, foams at mouth

Sniffing, licking of the vulva

Biting queen on dorsal neck

Consummation

Homosexual mounting & immobile stance (standing to be mounted)

Presents hindquarters to male, clitoral exposure by labial eversion, pulsati le contractions of labia

Immobile stance

Tail defl ected to one side Urination in presence of ma le affectionate head rubbing

Elevation of rear quarters and hyper- extension ofback (lordosis), presentation of vulva, tai l deviation

Consummation

Penile protrusion w ith dribbling of seminal fluid with few sperm- atozoa, erection and attempted mounts

Penile protrusion with no preejaculatmy expulsion of seminal fluid

Repeated dorsal retraction of scrotum, penile protrusion with no dribbling of seminal flu id

Penile protrusion, shallow pelvic thrusts, attempted mounting

Erection, protrusion of penis, mounting

Mounting

Reproductive Behavior is Programmed During Prenatal Development

Sex differences in specific brain structures for the control of reproductive behavior have been observed. For example, in the male, the preoptic area

Reproductive Behavior 233

Figure 11-3. Influence of Various Steroid Treatments Upon Reproductive Behavior

PRE NATAL Fetus + E2 ------• f Estrous behavior + male-like behavior Fetus + Testosterone f Estrous behavior + male-like behavior

'b Fetus + E2 or P4 No effect (normal 'b behavior) 'b Fetus+ Testosterone - No effect (normal 'b behavior)

POSTNATAL

No estrous behavior Estrous behavior

------+- Maximum estrous behavior + E2 ------- + P4 and E2 + Testosterone Male-like behavior

----------.- f Sexual behavior + Testosterone ----•

Ovaries remo ved (ovariect omy)

Sexual behavior restored Sexual behavior restored

removed (orchidectomy)

V et B oo ks .ir

234 Reproductive Behavior

androgen exposure per se causes abnonnal develop- ment of the female tract. In addition, the freemartin displays more male-like behavior than do her normal heifer counterparts. Figure 11-3 summarizes the influ- ence of reproductive steroids on behavior in the male and the female.

The presence of gonadal steroids (estradiol and testosterone) is obligatory for normal reproduc- tive behavior in both the male and the female. For example, ovariectomized females display no estrous behavior (See Figure 11-3 ). Likewise, castrated males have significantly reduced reproductive behav- ior. However, the abolition of reproductive behavior depends on the duration of time between castration and the opportunity to copulate. For example, males that have reached puberty and established a sustained pattern of reproductive behavior require a longer period of time between abolition of sexual behavior after castration than do males that have not estab- lished a sustained pattern of reproductive behavior.

Females will display male reproductive be- havior following injections of testosterone.

When ovariectomized fema les receive injections of estradiol, estrous behavior is reestablished, but at a less than maximum leve l. Among farm animals, ovariectomized fe males that are tTeated fi rst with pro- gesterone (to mimic the luteal phase of the cycle) and then treate d with estradio l displ ay maximum estrous behavior. In other species, estradiol must precede progesterone to produce maximal behavior. It is not clear why progesterone "priming" of the central ner- vous system fo r maximal stimulation is necessary. It would be logical to propose that progesterone promotes upregulation of estradiol receptors in the brain. Ova- riectomized females that are treated with testosterone develop male-like behavior. They w ill even develop secondary sex characteristics (reduced pitch of voice, hump on the back of the neck and atrophy of the fe male reproductive tract).

Figure 11-4. Hypothetical NeNous Pathway Eliciting Reproductive-Specific Motor Behavior

• Visual • Olfactory • Auditory • Tactile

• Estrogen receptors • t E2 -+ t increased

nerve excitability • Neurons produce

behavior specific peptides

OC = Optic Chiasm AL = Anterior Lobe

of Pituitary PL = Posterior Lobe

of Pituitary

• "Receiving zone" for hypothalam ic peptides

• Speeds up impulses

and mounting

Spinal cord

• Generates signals to specific muscles fo r lordosis and moun ting

Specific muscles responsible

for lordos is and

Reproductive Behavior is Controlled by the Central Nervo us System

The neural pathways and key anatomical com- ponents for the control of reproductive behavior are pre- sented in Figure 11 -4 . Reproductive behavior can take place only if the nemons in the hypothalamus have been sensitized to respond to sensory signals. Testosterone in the male is aromatized to esh·adio l in the brain and estradiol promotes reproduc tive behav ior. Recall that tes tosterone is produced in small episodes every 4 to 6 hours. Therefore, there is a relatively constant supply of testosterone and thus estradiol, to the hypothalamus in the male. This allows the male to initiate reproductive behavior at any time. In contrast, the female experi- ences high esh·adiol during the follicu lar phase only and will display sexual receptivity during estrus only.

Figure 11-4 outlines a generic neural pathway for sexual behavior. Under the influence of estrogen, sensory inputs such as olfaction, audition, v ision and tactility send neural messages to the hypothalamus. These neurons synapse directly on neurons in the ven- h·omedial hypo thal amus as well as the preoptic and anterior hypothalami c regions. These sensory inputs cause neurons in the hypothalamus to release behav ior spec ific peptides that serve as ne urotransmitters. These neurotransmitters act on neurons in the midbrain. The neurons in the midbrain serve as receiving zones for the peptides produced by the hypothalamic neurons. The midbrain h·anslates neuropeptide signals released by hypothalamic neurons into a fast response . Neu- rons in the midbrain synapse with neurons in the brain stem (medulla). These nervous s ignals are integrated in the medulla. From the medulla, nerve tracts extend to the spinal cord where the nerves synapse with mo- tor neurons that innervate muscles that cause lordosis and mounting. It should be emphasized that the model presented in Figure 11-4 does not account for all of the nerve pathways involved in reproductive behavior.

Reproductive behavior is initiated by: • olfaction • vision • audition • tactility

The primary sensory inputs for reproductive behavior are olfaction, audition, vision and tactility. The degree to which these sensory inputs influence repro- ductive behavior, particularly precopulatmy behavior, varies significantly among species.

Reproductive Behavior 235

Figure 11-5. Flehmen Response in the Stallion and

Bull and the Vomeronasal Pathway

0;2/ Flu ids

Nasopalatine Flu ids duct

The flehmen res ponse involves curling of the upper lip so that airflow through the nasal pas- sages is restricted. A subatmospheric pres- sure is thus created in the nasopalatine duct. Therefore , flu ids can be aspirated through the duct and into the sensory surfaces of the vomeronasal organ. Arrows in the bull indicate the approximate openings of the nasopalatine ducts. (Photo of stallion courtesy of Dr. A T1bary, Washington State University, College of Veterinary Medicine; Photo of bull courtesy of Select S ires, Inc. www.selectsires.com)

V et B oo ks .ir

234 Reproductive Behavior

Females will display male reproductive be- havior following injections of testosterone.

Figure 11-4. Hypothetical NeNous Pathway Eliciting Reproductive-Specific Motor Behavior

• Visual • Olfactory • Auditory • Tactile

• Estrogen receptors • t E2 -+ t increased

nerve excitability • Neurons produce

behavior specific peptides

OC = Optic Chiasm AL = Anterior Lobe

of Pituitary PL = Posterior Lobe

of Pituitary

• "Receiving zone" for hypothalam ic peptides

• Speeds up impulses

and mounting

Spinal cord

• Generates signals to specific muscles fo r lordosis and moun ting

Specific muscles responsible

for lordos is and

Reproductive Behavior is Controlled by the Central Nervo us System

Reproductive behavior is initiated by: • olfaction • vision • audition • tactility

Reproductive Behavior 235

Figure 11-5. Flehmen Response in the Stallion and

Bull and the Vomeronasal Pathway

0;2/ Flu ids

Nasopalatine Flu ids duct

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236 Reproductive Behavior

The Olfactory and Vomeronasal Systems Respond to Pheromones that Trigger

Reproductive Behavior

Secretions from the female reproductive tract serve to sexually stimulate and attract the male to the female. Vaginal and urinary secretions from females in estrus smell different to the male than secretions from females not in estrus. There is good scientific evidence that females produce pheromonal substances that are identifiable both within species and among species. However, their action is species specific. Recall that a phe1·omone is a volatile substance secreted or released to the outside of the body and perceived by the olfac- tory system and/or activated by the vomeronasal organ. Releasing pheromones can cause specific behavior in the recipient. Pheromones can also be priming phero- mones that have physiologic rather than behavioral effects on the recipient.

Males also produce sex pheromones that attract and stimulate females. Among food producing animals, the best documentation for a male sex pheromone is in swine. Boars produce specific substances that cause sows and gilts to become sexually aroused when they are in estms. Two sexual attractants are produced by boars. One of these attractants is a preputial pouch secretion. The second pheromonal-like substance is present in saliva secreted by the submaxillary salivary glands. During sexual excitement and precopulatory interac- tions, the boar produces copious quantities of foamy saliva. The active components in saliva are the androgen metabolites 3a.-androstenol and 5a.-androstenone. Both compounds have a musk-like odor.

It has been demonstrated that dogs have the ability to identifY cows in estrus by olfactory discrimi- nation. In addition, rats can be trained to press a lever in response to air bubbled through urine from cows in estms. Rats did not press the lever when air was bubbled through urine fi·om nonestrous cows. Clearly, urine from cows in estrus contains a material that can be identified by olfaction by other species (dogs and rats).

Figure 11-6. "Warm-Up" Stalls Used for Stimulating Sexual Behavior in Bulls Providing Semen for Artificial Insemination

Bulls waiting to be ejaculated (arrows) watch mounting and ejaculatory behavior of another bull. Such a practice "prestimulates" bulls and reduces stimulation time when they enter the collection arena. It also in- creases sperm harvest. A false-mount is being performed by the bull mounting the stimulus animal (SA). (Photo courtesy of Select Sires, Inc., www.selectsires.com)

Flehmen Behavior is a Close-Range Investigative Behavior

Some pheromones appear to be less volati le and need to be detected by the vomeronasal organ in the bull, ram, stallion and to some extent, the boar. The male needs to closely approach the source of pheromones and he wi ll nuzzle the genital region of the female. The vomeronasal organ (See Figure 11-5) is an accessory ol factory organ. It is connected to two small openings in the anterior roof of the mouth j ust behind the upper lip. Fluid-borne, less vo latile chemicals can enter the vomeronasal organ through the oral cavity by means of the nasopalatine ( incis ive) ducts. Many species, such as bulls, rams and stallions, perfom1 a special investigative maneuver when in close proxim- ity to a female. Vaginal secretions and urine evoke an investigative behavior known as the flehmen response. Flehmen behavior allows less vo latile materials to be "examined" by sensory neurons in the vomeronasal organ. Flehmen behavior is characterized by head el- evation and curling of the upper lip (See Figure 11 -5). Curling of the upper lip closes the nostrils and allows a negative pressure to forn1 in the nasopalatine duct. Thus, less vo latile materials (like mucous and urine) can be aspirated through the duct into the vomeronasal organ where they can be "evaluated" by sensory neurons in the organ. Olfactory bulbec tomy in goats inhibits the flehmen response. Flehmen behavior in males is likely to be performed whether the material is from an estrus or nonestrus female. It is believed that the fleh- men behavior is used to help a male identifY mating opportunities. Flehmen is occasionally performed by females during sexual encounters with males. Cows will frequently perform the maneuver when sniffing other cows that are in estrus or proestrus. As in the male, females will display flelunen to novel compmmds, including fluids associated with the placenta, newborn animals and other volatile materia ls. Flehmen is fre - quently displayed by post-parturient fe males as they make identity discriminations between their own versus other's neonates.

Auditory stimulation can serve as a long-range signal.

In many species, sexual readiness is accompa- nied by some fom1 of unique vocalization or "mating calls". For example, cows are known to increase their bellowing during the time of estrus. Sows display a characteristic grunting sound associated with estrus. Queens often "yeow" repeatedly to call the tom. By

Reproductive Behavior 237

comparison, mares and ewes are relatively silent. El- evated vocalization serves to alert or send a signa l to males that sexual readiness is imminent. The auditory stimulus is more useful in long-range discrimination, rather than close discrimination. The classic example of reproductive driven vocalization is bugling of the bull e lk during rut (the breeding season).

Visual signals are valuable for close encounters.

All females display a fonn of sexual postur- ing that can be perceived by males. While posturing can be quite subtle, especially to human observers, the identifica tion of postures probably takes place easily among members of the same species.

Tactile stimulation is generally the final stimulus before copulation.

Almost all males experience a degree of sexual stimulation when they observe mating behavior among other individuals of the same species. It is well documented that in bulls, visual observation of mating behavior enhances sexual stimulation. This observa- tion has led to the common practice of placing bulls used for artificial insemination in "warm-up" stalls (See Figure 11-6). Bulls are brought to the "warm-up" stalls and are allowed to observe the mounting behav- ior and collection of semen from other bulls prior to entering the collection area themselves. This causes an elevated level of sexual excitement and reduces the time required for final sexual stimulation and collection of semen. This is important because labor requirements for semen collection are significant. This procedure is also important because it tends to increase spem1 concentration in the ejaculate.

Tactile stimuli fro m male s appears to be im- portant in evoking sexual postures or standing postures by females. For example, biti ng on the neck and the withers of mares by stallions appears to be important for sexual stimulation. Biting of the neck of the queen by the tom is also a characteristic reproductive behav- ior among cats. Rubbing of the flanks and genitalia of mares, whether done by the stallion or by a human handler, evokes behavior signals of estrus from the mare that othe1wise would not be displayed. Chin resting by a bull on the back of a cow just prior to mounting may have some stimulatory effect on the cow.

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236 Reproductive Behavior

The Olfactory and Vomeronasal Systems Respond to Pheromones that Trigger

Reproductive Behavior

Males also produce sex pheromones that attract and stimulate females. Among food producing animals, the best documentation for a male sex pheromone is in swine. Boars produce specific substances that cause sows and gilts to become sexually aroused when they are in estms. Two sexual attractants are produced by boars. One of these attractants is a preputial pouch secretion. The second pheromonal-like substance is present in saliva secreted by the s ubmaxillary salivary glands. During sexual excitement and precopulatory interac- tions, the boar produces copious quantities of foamy saliva. The active components in saliva are the androgen metabolites 3a.-androstenol and 5a.-androstenone. Both compounds have a musk-like odor.

Figure 11-6. "Warm-Up" Stalls Used for Stimulating Sexual Behavior in Bulls Providing Semen for Artificial Insemination

Flehmen Behavior is a Close-Range Investigative Behavior

Some pheromones appear to be less volati le and need to be detected by the vomeronasal organ in the bull, ram, stallion and to some extent, the boar. The male needs to closely approach the source of pheromones and he wi ll nuzzle the genital region of the female. The vomeronasal organ (See Figure 11-5) is an accessory ol factory organ. It is connected to two small openings in the anterior roof of the mouth j ust behind the upper lip. Fluid-borne, less vo latile chemicals can enter the vomeronasal organ through the oral cavity by means of the nasopalatine ( incis ive) ducts. Many species, s uch as bulls, rams and stallions, perfom1 a special investigative maneuver when in close proxim- ity to a female. Vaginal secretions and urine evoke an investigative behavior known as the flehmen response. Flehmen behavior allows less vo latile materials to be "examined" by sensory neurons in the vomeronasal organ. Flehmen behavior is characterized by head el- evation and curling of the upper lip (See Figure 11 -5). Curling of the upper lip closes the nostrils and allows a negative pressure to forn1 in the nasopalatine duct. Thus, less vo latile materials (like mucous and urine) can be aspirated through the duct into the vomeronasal organ where they can be "evaluated" by sensory neurons in the organ. Olfactory bulbec tomy in goats inhibits the flehmen response. Flehmen behavior in males is likely to be performed whether the material is from an estrus or nonestrus female. It is believed that the fleh- men behavior is used to help a male identifY mating opportunities. Flehmen is occasionally performed by females during sexual encounters with males. Cows will frequently perform the maneuver when sniffing other cows that are in estrus or proestrus. As in the male, females will display flelunen to novel compmmds, including fluids associated with the placenta, newborn animals and other volatile materia ls. Flehmen is fre - quently displayed by post-parturient fe males as they make identity discriminations between their own versus other's neonates.

Auditory stimulation can serve as a long-range signal.

Reproductive Behavior 237

Visual signals are valuable for close encounters.

Tactile stimulation is generally the final stimulus before copulation.

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238 Reproductive Behavior

Penile Erection and Protrusion Completes the Pr·ecopulatory P hase

of Reproductive Behavior

When sexua l receptiv ity of a f emale is es - tablished and sufficie nt a rousal is acc omplished in the ma le, erection and protrusion of the penis ensue . Successful penil e erection requires a complex series of neura l and vas omotor (blood v essel) reactions . Erection of the penis is necessary for copulation and

depos ition of semen in the female repro ductive h·act. Erection is character ized by a marked increase in the rigidity of the peni s. T he increased rigidity is the result of a marked incr ease in ar teri al inflow of blood w hen compared to the ven ous outflow of blood. Erecti on requires that blood b e trapped within the cavernous sinuses of the penis . Increased blood flow to the pe- nis is brought ab out by vasodilation of the arterioles supplying it. In the bull, ram and boar erection not only involves increased bl oo d flow and a subsequent

Figure 11-7. Steps in Penile Erection as They Relate to Cavernous Blood Pressure and Contraction of the Bulbospongiosus and

Ischiocavernosus Muscles

";) :I E E -f :s Ul Ul

f D. "C 0 0

a5 Ill :s 0 c Q)

u Sexual arousal (visual, tactile,

olfactory)

(Modified from Beckett, et al. 1972. Bioi. of Rep rod. 7:359)

. .•. ,ljJJ..:::: of bulbospongiosus

--- - - ------ --41'1-___,- , . A.\1 Contractions of ischiocavernosus

t Blood flow to cavernous tissue +

+ venous outflow

Vasodilation of helicine arteries

(tblood flow)

Time (seconds)

Cavernous pressure

increase in pressur e, but a simultaneous relaxation ofthe reh·actor peni s muscles. Thus, erection and protrusion a lso involve s traighten ing of the penis to eliminate the s igmo id flexure. The penis of the bull, boar and ram is fi broe lastic in nature and therefore does not increase significantly in diameter during erection and protrusion. In contrast, the penis of the stallion increases signifi- cantly in diameter during erection. The stallion has a retractor penis muscle that, as in other spec ies, relaxes during erection. H owever, the sta llion does not have a sigmoid flex ure. Engorgement w ith blood plays a much mor e sig nifi cant role in the highly vascular penis of the stallion, dog and man than in the bu11, ram, boar and camelids.

Erection of the p enis requires: • elevated arterial blood inflow • dilation of corporal sinusoids • restricted venous outflow • elevated intrapenile pressure • relax ation of the retractor . penis m uscle

Contractions of the ischiocavernosus muscles cause compression of the penile veins. T his compres- sion causes blockage of venous retum thus enabling the cavem ous tissue to retain blood for maintenance of an erection. As you w ill recall, the isch iocavernosus muscles surround the two crura. Intem1ittent contrac- tions of the muscles creates a pump-like action at the base of the penis. T hese contractions result in a buildup of blood within the corpus cavemosum of the penis and exceptionally high pressures resu lt. For example, during the fina l stages of erection, the pressures w ithin the cavernous tissue of the goat penis can reach 7,000 nun Hg (S ee Figme 11-7). When the penis is flacc id, pressures w ithin the corpus cavernosum are only 19 mm Hg. Pressures in the bull penis are around 1,700 mm Hg during peak erection and a bou t 30 mm Hg when the cavernous spaces are collapsed. Figure 11-7 summarizes the steps of penile erection and intrapenile pressures as they relate to contraction of the ischiocav- emosus and bulbospongiosus muscles.

One of the most publ icized phannaceuticals ever introduced is a material called Si ldenafil C itrate (Viagra®). This pharmaceutical provides a therapy for erectile dysfunction in men. Erectile dysfunction is defined as the inability to achieve and maintain a penile erection (tumescence) . Reports indicate that 10% of men between the ages 40 and 70 years old are affl icted

Reproductive Behavior 239

Figure 11-8. Basic Steps in the Erectile Process

STEP I Erotogenic stimuli cause sensory nerves to fire

r STEP 2

Sensory nerves activate "Reproductive Behavior Center"

in hypothalamus- (See Figure 11- 4)

I ... STEP 3

St imu lat ion of parasympat heti c nerves that innervate peni le arterio les

STEP4 Parasympathetic ne rve te rminals rele ase

nitric oxide (NO) - (See Figure 11-9)

STEP 5 Nitric oxide init iates biochemical cascade that causes e rection - (See Figure 11 -9)

by complete erectile failure. Other reports have esti- mated that up to 30 million men in the United States may have some fonn of erectile dysfunction. E rectile dysfunction is rare among dom estic anima ls because such males ar e rapidly eliminated from the gene poo l by artificia l selection (culling) or by natural selection (no erection-no copulation-no offspring).

Erection of the Penis Requires Sensory Inp ut and a Local Vascular Response

As mentioned earli er in the chapter, penile erec- tion is a complex series of neural and vasomotor events. These events can be broadly su bd ivided into a nervous component (cerebral and sp inal) and a local vascular component within the penis. The nervous component is arousal-driv en. For example, there must be ap- propriate sensory stim uli (tactile, visual, auditory and olfactory) in order for the central nervous system to be appropriately stimulated so that efferent neural events

V et B oo ks .ir

238 Reproductive Behavior

Penile Erection and Protrusion Completes the Pr·ecopulatory P hase

of Reproductive Behavior

Figure 11-7. Steps in Penile Erection as They Relate to Cavernous Blood Pressure and Contraction of the Bulbospongiosus and

Ischiocavernosus Muscles

";) :I E E -f :s Ul Ul

f D. "C 0 0

a5 Ill :s 0 c Q)

u Sexual arousal (visual, tactile,

olfactory)

(Modified from Beckett, et al. 1972. Bioi. of Rep rod. 7:359)

. .•. ,ljJJ..:::: of bulbospongiosus

--- - - ------ --41'1-___,- , . A.\1 Contractions of ischiocavernosus

t Blood flow to cavernous tissue +

+ venous outflow

Vasodilation of helicine arteries

(tblood flow)

Time (seconds)

Cavernous pressure

Reproductive Behavior 239

Figure 11-8. Basic Steps in the Erectile Process

STEP I Erotogenic stimuli cause sensory nerves to fire

r STEP 2

Sensory nerves activate "Reproductive Behavior Center"

in hypothalamus- (See Figure 11- 4)

I ... STEP 3

St imu lat ion of parasympat heti c nerves that innervate peni le arterio les

STEP4 Parasympathetic ne rve te rminals rele ase

nitric oxide (NO) - (See Figure 11-9)

STEP 5 Nitric oxide init iates biochemical cascade that causes e rection - (See Figure 11 -9)

Erection of the Penis Requires Sensory Inp ut and a Local Vascular Response

V et B oo ks .ir

240 Reproductive Behavior

Figure 11-9. Vascular and Biochemical Contra! of an Erection (Modified from Korenman. 1998. Am. J. Med. 105.135.)

Su perficial dorsal vei n

vein

· Erect Penis

Arte rl ol inflo w

Internal pudendal

-- .

Circ umfl ex vein

Emissory --+

Cavernosal artery

Flaccid Penis

ve in

PDEs

+ Inhibi tion

Erect Penis

Sinusoid smooth muscle relaxes

I ERElJoNI

Anatomy The shaft of the penis co nsists of tw o dorso-lateral co rpora cave r- nosa and the corp us spongiosum. Arteria l blood is supplied by the in- ternal pudendal artery that supplies the dorsal and deep cave rnosal ar- teries. Corpo ral sinusoids are sup- plied by helici ne arteries. The deep dorsal vein and superficial dorsal vein drain the erectil e tissues.

Flaccid penis The sinuso ids are flattened be- cause adrenergic nerves secrete norepinephe rine that causes vaso- constriction. Blood flow to the cav- ernous tissue therefore is quite low for the majority of the time. Since no erotogenic sti muli a re p res- ent, nonadrenergic noncholine rgic (NANC) parasympathetic neurons do not fire and thus do not release nitric oxide (NO ). Therefore , vaso- constriction takes precedence over vasodilation.

Erect penis Wh en erotoge nic stimul i are pres- ent the NANC neu rons fire and release nitric oxide (NO) from their termina ls. When NO is released, it activ ates a n enzyme called guanylate cyclase. This enzyme co nverts guanylate tri phosphate (GTP) to cyclic guanyosine mono- phosphate (cGMP) and cau ses the smooth muscle of the corporal sinusoids to relax (vasodilatation). The cave rnous sinusoids engorge with blood and intracorpo ral pres- sure in creases dramatically. This compresses the ve nules through whi ch bl ood exits the penis. Blood is then trapped within t he penis causing an erection.

Reproductive Behavior 241 can cause an erection. These extrinsic stim uli are called erotogenic stimuli. As shown in Figure 11-4, these stimuli cause afferent sensory nerves to fire. Their tern1inals synapse with neurons in the so-called "behav- ior center" in the hypotha lamus. These hypothalamic neuro ns synapse with parasympathetic and sympathetic efferent neurons that control peni le vascular smooth muscle (vascular tone). The basic step s in the erecti le process are outlined in Figure 11 -8.

Mounting postures and cha racteri sti cs of copulatory behavior for various species are presented in Figures 11 - I 0 and I I -l I. T he purpose of mounting is fo r the male to position himself so that intromission can occur. Introm iss ion is the successful entrance of the penis into t he vagina. F ollowing intromission, ejaculation takes place in r esponse to sensory stimula- tion of the glans penis. T he time of ejaculation relative to intromission varies significantly among species (See F igures 11- 10, I 1- 11 and 11-12). F or examp le, in the bull and the ram ejaculation occurs within one or two seconds after intromission . In these species ejaculati on is stimulated by the warm temperature of th e vagina. Vag inal pressure is relatively unimportant in inducing ejaculation in the ram and bull. In contrast, the boar may have a sustained ejaculation for periods of up to 30 minutes. The stallio n has a mating duration of between 30 seconds and one minute. The llama and the dog are perhaps the most sustained copulators with reports of copulation occuring continually for up to 50 minutes.

Erection is caused by the firing of nonadrener- gic, noncholonergic (NAN C) parasymp athetic neurons that release nitric oxide (NO), a gas, fro m their ter- minals. N itric oxide is the principal neurotransmitter that "dr ives" the erecti le process. Nitric oxide causes its effect by stimulating an enzyme, guanylate cyclase, to convert gua ny late triphosphate (GTP ) to cyclic guanosine monophosphate ( cGMP). Cyclic guanosine monophosphate causes corporal smooth m uscle relax- ation (vasodilation) and an erection results.

Under n onerotogen ic c o nditions, cGMP is acted upon by PDE 5 (Phosphodie sterase 5) and this enzyme promotes the conversion of cGMP to GM P. This breakdown causes increased vascular tone result- ing in outflow of blood fro m the corpora cavernosa and loss of an erection. Si ldenafil blocks the action of PDE5 thus prolonging the vasodi lation effect of cGMP and an erection develops that can be maintained for a sustained period of time. It should be emphasized tha t w ithout nitric oxide production by the parasympathetic nerve terminals Si ldenafil can have no effect because nitric o xi de must be pr esent for cG MP to be produced. The usual fla ccid state of the penis (contracted corporal arteries) results from a to nic contraction of the arteria l and corporal smooth m uscles mediated by sympa thetic adrenergic neurons. Such vasoconstriction keep s pe- nile blood flow to a minimum under no n-erotogenic conditions.

When t he corp o ral smooth muscles relax because of cGMP, the resistance to blood flow by the penile arteri oles and corporal sinusoids decreases and blood flow to the p enis triples or q uadruples w hen the appropriate erotogenic stimul i are present. When an erection occurs, the sinusoid pressure is so great that the emissary veins are collapsed. Therefore, blood cannot return through them because venous outflow is blocked. Penile erectio n can be maintained fo r as long as vasodi- lation of the corporal smooth muscle takes place. TI1ese reactions are summarized in Figure l l-9 .

Ejaculation is a simple neural reflex caused by:

• intromission • stimulation of the glans penis • forceful muscle contraction

E jaculation is defined as the reflex expulsion of spermatozoa and semina l plasma from the male repro- d uctive tract. The basic mechanism for ejaculation of semen is quite s imilar among all mamm als. Expulsio n of semen is the result of sensory stimulatio n, primarily to the glans penis, that causes a series of coordinated muscular contractions . Once intromissio n has been achieved, reflex imp ulses ar e initiated . These neural impu lses are derived mainly fro m sensory nerves in the glans penis. Up on thresho ld stimulation, impu lses are transmitted from the glans penis by way of the internal pudendal nerve to the lumbosacral region of the spinal cord (See F igure 11 -13 ). The sensory impulses resu lt in fi ring of nerves in the spina l cord and the forcing of semen into t he urethra is accompl ished by nerves in the hypogastric p lexus that innervate the target m uscles . Of primary importance for ejaculation are the urethralis m uscle (that sun ounds the pelvic uretlu-a), the ischiocavernosus and the bulbospongiosus muscles.

Copulatory behavior includes: • mounting • intromission • ejaculation

F igure l l -13 summarizes the nerve pathways r esulting in emission and ejaculation. It should be emphasized that emission is defined as the movement of seminal fl uids from the accessory sex glands into the pelvic uretlu-a so they can mix with spennatozoa. Emission occurs befor e and during ejaculation. In some

V et B oo ks .ir

240 Reproductive Behavior

Figure 11-9. Vascular and Biochemical Contra! of an Erection (Modified from Korenman. 1998. Am. J. Med. 105.135.)

Su perficial dorsal vei n

vein

· Erect Penis

Arte rl ol inflo w

Internal pudendal

-- .

Circ umfl ex vein

Emissory --+

Cavernosal artery

Flaccid Penis

ve in

PDEs

+ Inhibi tion

Erect Penis

Sinusoid smooth muscle relaxes

I ERElJoNI

Ejaculation is a simple neural reflex caused by:

• intromission • stimulation of the glans penis • forceful muscle contraction

Copulatory behavior includes: • mounting • intromission • ejaculation

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242 Reproductive Behavior

Figure 11-10. Characteristics of Copulation, Site of Seminal Deposition and Number of Ejaculations to Satiation and Exhaustion in the Ram,

Bull, Stallion and Boar

Mating pair

Photos of:

Deposition Ejaculations Ejaculations to Satiation to Exhaustion

1 to 2 sec- .8 to 1 ml e xt e r n a I onds (1 pel- (.1 to 2ml) cervical os

1 to 3 sec- 3-5ml fornix vagina onds (1 pel- (.5 to 12ml)

commences that is ac- companied by somnolence)

75-120ml

200-250ml

external cer- v ical as b ut semen enters uterus at high pressu re

ce rvix and uterus

10 30 to 40

20 60 to 80

3 2 0

3 8

Ram/Ewe-courtesy of Drs. G.S. Lewis and J.B. Taylor. U.S. Sheep Experimental Station http://p wa.ars.usda.go v/dubois!index Bull/Cow-courtesy of Dr. L.S. Katz, Rutgers University Stallion/Mare-courtesy of Dr. A. Tibary, Washington State University, College of Veterinary Medicine

Reproductive Behavior 243

Figure 11-11. Characteristics of Copulation , Site of Seminal Deposition and Number of Ejaculations to Satiation and Exhaustion in the Camel

and Llama

Duration of Volume of Site of Average Maximum

Mating pair Copulation Ejaculate Semen Number of Number of (Range) Deposition Ejaculations Ejacu lations

to Satiation to Exhaustion

6-20 minutes, 3-8m I Partly intrauter- 23 matings Data not ex t ension of ine, partly intrac- in 24 hr available neck, straining erv ic al , some of the body, intravaginal multiple ejacu- l a t i ons per copulation

20 - 30 min - 1-5ml intrauterine Data not Data not u tes, bo d y ava ilable available tremo rs an d pelvic thrusts

(Photos courtesy o f Dr. A. Tibary, Washington State Un iversity, College of Veterinary Medicine)

species, such as the boar, stallion and dog, emission oc- curs in a sequence resulting in an ej aculate that consists of various fluid fractions (See Chapter I 2).

Postcopulatory behavior involves refractivity and recovery.

Following ej aculation , all males experience a refractory per iod before a second ejaculation can occur. The length oftime of this refractory period depends on several factors. These fac tors are ; degree of sexual rest prior to copulation, age of the male, species of the male, degree of fe ma le novelty and number of previous ej aculations. The postcopulatory refractory period is sometimes erroneously refeiTed to as sexual exhaustion. The refractory period should be considered as part of satiation rather than exhausti on. With natural service, it is quite nonnal for a male to copulate repeatedly with the same female. For example, a stall ion will breed a mare in heat 5 to I 0 times during one estrus period. Rams are noted to remate with the same ewe 4 to 5 tim es. Bulls also remate w ith estrous cows repeatedly. In fact, it has been noted in most species that if more than one fema le is in heat at the same time, some males will generally copulate preferentially with one and sometimes wi ll not copulate with a second female. Boars nonnally serve sows severa l times over a period of 1 to 2 days.

Sexual satiation refers to a condition in whi ch fi1rther stimul i will not cause immediate responsive- ness or motivation under a given set of stimulus con- ditions. Restimu lation may occur after the refractory period. F igures I I - 1 0 and I 1- I I compare the normal number of ejaculations to satiety and the number of ejacu lations to exhaustion among species. Exhaustion is the condition whereby no further sexu a l behavior can be induced even if sufficient stimuli are present. As you can see fro m Figures 11 - 1 0 and 11-11 , there is a large variation in the behav ioral reserves (the behav ioral cap acity, or libido) among species. There is also a large variation in libido within species. For example, beef bulls have significantly lower behav- ioral reserves than dairy bulls. While the factors that contro l the degree of reproductive behavior among males are poorly understood, they are almost certa inly governed by genetic factors as well as environmental fac tors.

Reproductive behavior can be enhanced by:

• introducing novel stimulus animals • changing stimulus settings

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242 Reproductive Behavior

Figure 11-10. Characteristics of Copulation, Site of Seminal Deposition and Number of Ejaculations to Satiation and Exhaustion in the Ram,

Bull, Stallion and Boar

Mating pair

Photos of:

Deposition Ejaculations Ejaculations to Satiation to Exhaustion

1 to 2 sec- .8 to 1 ml e xt e r n a I onds (1 pel- (.1 to 2ml) cervical os

1 to 3 sec- 3-5ml fornix vagina onds (1 pel- (.5 to 12ml)

commences that is ac- companied by somnolence)

75-120ml

200-250ml

external cer- v ical as b ut semen enters uterus at high pressu re

ce rvix and uterus

10 30 to 40

20 60 to 80

3 2 0

3 8

Reproductive Behavior 243

Figure 11-11. Characteristics of Copulation , Site of Seminal Deposition and Number of Ejaculations to Satiation and Exhaustion in the Camel

and Llama

Duration of Volume of Site of Average Maximum

Mating pair Copulation Ejaculate Semen Number of Number of (Range) Deposition Ejaculations Ejacu lations

to Satiation to Exhaustion

20 - 30 min - 1-5ml intrauterine Data not Data not u tes, bo d y ava ilable available tremo rs an d pelvic thrusts

(Photos courtesy o f Dr. A. Tibary, Washington State Un iversity, College of Veterinary Medicine)

species, such as the boar, stallion and dog, emission oc- curs in a sequence resulting in an ej aculate that consists of various fluid fractions (See Chapter I 2).

Postcopulatory behavior involves refractivity and recovery.

Reproductive behavior can be enhanced by:

• introducing novel stimulus animals • changing stimulus settings

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244 Reproductive Behavior

Figure 11-12. Copulation in the Dog

First Stage Coitus (1-2 min)

The Turn (2-5 sec)

The male and female remain "tied" together be- cause the bulbus glandis of the penis remains engorged with blood after the turn. Contractions of the muscles near the base of the penis prevent venous outflow of blood from the bulbus glandis. Also, the sphincter muscles of the vulva constrict thus compressing the dorsal veins of the penis preventing blood from leaving. (Figures modified from Grandage. 1972. Vet. Rec. 91:141)

The vascu lature of the dog penis has been in- jected with latex and the tissue dissolved away leaving cast of the vascu lature. Red vessels are arteries and the blue vessels are veins. IL=IIeum, MCA=Medial Caudal Artery, LCA=Lateral Caudal Artery, IS=Ischium , A=Ace tabulum , CS=Corpus Spong iousum, CC=Corpus Caver- nosum, DPA=Dorsal Penile Artery, DPV=Dorsal Penile Vein, OP=Os Penis, BG=Bulbus Glandis, PLG=Pars Longa Glandis, PA=Prostatic A rtery, I P=lnternal Pudendal Artery, IIA=InternallliacArtery (Specimen courtesy of the W orthman Veterinary Anatom y Teach- ing Museum, College of Veterinary Medicine, Washington State University. Specimen prepared by Dr. R.P. Worthman)

First Stage Coitus The male mounts the female in a manner typical of a quadraped . The female holds the tail to one side and the penis is introd uced into the vagina by a few thrusting movements. This stage of copula- tion lasts for only 1-2 minutes. The first and second fractions of semen are ejaculated during the first stage coitus.

The Turn This is the transition between first stage and se c- ond stage co itus. Shortly after ejaculation, the dog dismounts and turns around while lifting one hind leg over the bitch.

Second Stage Coitus After the turn, the animals stand with their hind quarters in contact and their heads facing opposite directions. The third fraction of semen is ejaculated during this stage. Second stage coitus may last from 5-45 minutes. It is believed that the purpose of second stage coitus is to encourage uterine rather than vag inal insemination. Turning around discourages detumescence of the penis and there- fore maintains high intravaginal pressu re. The dog steadily ejaculates up to 30-ml of seminal fluid that is delivered through the cervix into the uterus. This phenomenon tends to force the sperm-rich fraction into the uterus. The copulatory behavior described here is perfectly natural. Unfortunately this behavio r is often interpreted as being unnatural and attempts to break the "tie" are often made by the uninformed. Such intervention com promises fertility because delivery of semen to the uterus over a sustained period of time is reduced.

Reproductive Behavior 245

Figure 11-13. Major Steps in Ejaculation

Afferent

Sensory stimulat io n of glans penis (temperature and pr essure)

Int r omissio n

Reproductive Behavior and Spermatozoal Output can be Manipulated

The degree of novelty of both the copulatmy partner and the copulatmy environment can be of great importance when managing reproductive behavior in breeding males. U nder condi tions of artificial insemina- tion, where repeated seminal collectio n is necessary to maximize the harvest of spermatozoa, understanding the influence of novelty and mating situations is impor tant. The " Coolidge Effect" is defined as the restoration of mating behavior in mal es (that have reached sexual satiation) when the origina l fema le is replaced by a novel female . In other words, a sexually sati ated ma le can be restimulated if exposed to a novel female. (For derivation of the term "Cool idge E ffect" see Further Phenomena for Fertility)

Semen collection in bull studs can occur as frequently as 4 to 6 ejaculations per week. In o rder for this collection frequency to be successful, the male

0 Sudden and pow erful contractio n of

urethralis, bulbospo ngiosus and ischiocavernosus m uscles

0 Expulsion of semen

m ust first be sexu ally stimulated. Sexual stimula tion is defin ed as the presentation of a stimulus situation that w ill achieve mounting and ej acu lation. The purpose of sexual stimulation is to o btain ejaculation or mating in the shmtest time possible so that manpower involved in managi ng the mating of animals can be minimized. Ther e are three approaches used to re-induce sexual stimul atio n in bulls u sed for artificia l insemination. These approaches ar e: to introduce a novel stimulus animal; to change the stim ulus setting; or both. Pre- sentatio n of nove l stim ulus animals reinitiates sexual behavior after sexual satiation in bulls (See F igure 11 - 14, "Novel Fema les"). A second approach to achieve sexual stimulation after satiation is to present familiar stimulus animals in new stimulus situations . In other words, changing the location or setting has a stimulatmy effect on the satiated male (See F igure 11- 14 "New Lo- cation"). In cases where sexual stimulation is difficult to achieve, presenting a novel stimulus animal, coupled with changing locatio ns, often has positive effects.

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244 Reproductive Behavior

Figure 11-12. Copulation in the Dog

First Stage Coitus (1-2 min)

The Turn (2-5 sec)

The Turn This is the transition between first stage and se c- ond stage co itus. Shortly after ejaculation, the dog dismounts and turns around while lifting one hind leg over the bitch.

Reproductive Behavior 245

Figure 11-13. Major Steps in Ejaculation

Afferent

Sensory stimulat io n of glans penis (temperature and pr essure)

Int r omissio n

Reproductive Behavior and Spermatozoal Output can be Manipulated

Semen collection in bull studs can occur as frequently as 4 to 6 ejaculations per week. In o rder for this collection frequency to be successful, the male

0 Sudden and pow erful contractio n of

urethralis, bulbospo ngiosus and ischiocavernosus m uscles

0 Expulsion of semen

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246 Reproductive Behavior

Figure 11-14. Introduction of Novel Females and a Change of Locations has a Positive Effect on Mounting Behavior

(Hypothetical examples, not experimental data)

Familiar Female

A familia r female may stimulate a

lllllllll ------ "------lll -- --------" ----- ----1 bull to mount about 12 times in an 8

112 Mounts I hour period. SS= sexual satiation

I I I I I I I I I 0 I 2 3 4 5 6 7 8

Time (h)

Familiar Female and New Location Bulls can be restimulated to mount + New location + New location (after satiation) by changing the

! lllllllll --- ·;.II IIlli --- Jiilll stimulus setting (new location) . This induces more total mounts (18 liB Mounts I mounts) than the familia r female (12 I I mounts). 0 I 2 3 4 5 6 7 8

Time(h)

Novel Females When the novel females (1-5) are introduced after a period of sexual

llillllll Ji IIlli _,[ 1111 --- - I satiation, mounting behavior is stimulated beyond that realized with change of location and exposure to a sing le familiar female (24 mounts I I I I I I I I I 0 I 2 3 4 5 6 7

Time (h)

There has been little research conducted on the effect of introducing novel animals upon stimulation of mounting behavior in the female. However, it has been shown that dairy cows will mount novel cows with a greater frequency than they do familiar cows. As you might expect, the effect of novelty is confounded with the stage of the cycle.

Sexual preparation prolongs sexual stimulation and increases

spermatozoa per ejaculation.

In order to maximize the output of spermatozoa per ejaculate, sexual preparation is necessary. Sexual preparation is extending the period of sexual stimula- tion beyond that needed for mounting and ejaculation.

8 vs. 18 and 12 respectively).

Sexual preparation pro longs the precopulatory stage of reproductive behavior. The purpose of sexual prepara- tion is to collect semen containing the greatest possible number of spemmtozoa per ejaculation. Figure ll-15 illustrates the phys iologic me chanisms believed to be responsible for enhancing spermatozoal numbers in the ejaculate. Three approaches are used to sexually prepare a male. These are: false-mounting, restraint and false-mounting plus restraint.

Sexual preparation may include: • false-mounting • restraint • false-mounting plus restraint

Reproductive Behavior 24 7

Figure 11-15. Major Steps in Sexual Preparation Resulting in Transport of Spermoatozoa from the Tail of the Epididymis into the Pelvic Urethra

Sensory stimulation (optic, olfactory, tactile and aud itory)

Affe re nt

Transport of spermatozoa into an ejaculatory position

• Stim ulation o f nerves in the supraoptic and paraventricular nuclei

0 Contractions of smooth muscle in distal tail of

epididymis and ductus deferens

[ill I

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246 Reproductive Behavior

Figure 11-14. Introduction of Novel Females and a Change of Locations has a Positive Effect on Mounting Behavior

(Hypothetical examples, not experimental data)

Familiar Female

A familia r female may stimulate a

lllllllll ------ "------lll -- --------" ----- ----1 bull to mount about 12 times in an 8

112 Mounts I hour period. SS= sexual satiation

I I I I I I I I I 0 I 2 3 4 5 6 7 8

Time (h)

Familiar Female and New Location Bulls can be restimulated to mount + New location + New location (after satiation) by changing the

! lllllllll --- ·;.II IIlli --- Jiilll stimulus setting (new location) . This induces more total mounts (18 liB Mounts I mounts) than the familia r female (12 I I mounts). 0 I 2 3 4 5 6 7 8

Time(h)

Novel Females When the novel females (1-5) are introduced after a period of sexual

Time (h)

Sexual preparation prolongs sexual stimulation and increases

spermatozoa per ejaculation.

8 vs. 18 and 12 respectively).

Sexual preparation may include: • false-mounting • restraint • false-mounting plus restraint

Reproductive Behavior 24 7

Figure 11-15. Major Steps in Sexual Preparation Resulting in Transport of Spermoatozoa from the Tail of the Epididymis into the Pelvic Urethra

Sensory stimulation (optic, olfactory, tactile and aud itory)

Affe re nt

Transport of spermatozoa into an ejaculatory position

• Stim ulation o f nerves in the supraoptic and paraventricular nuclei

0 Contractions of smooth muscle in distal tail of

epididymis and ductus deferens

[ill I

V et B oo ks .ir

248 Reproductive Behavior

False mounting consists of manually deviat- ing the penis during a mount so that intromission can- not occur. If intromission does not occur, ejaculation usually does not occur. Restraint prevents the male from mounting even though he wishes to do so. Gen- erally, restraint is for two to tlu·ee minutes within two or three feet of the stimulus animal. A combination of false mounting and restraint will result in the greatest improvement of spemmtozoal output.

In dairy bulls, the recommended procedures for sexual preparation are: one false mount followed by two minutes of restraint, followed by two additional false mounts before each ejaculation. In beef bulls, sexual preparation involves three false mounts with no restraint. In general, beef bulls have lower behavioral reserves (libido) than dairy bulls and thus have a less rigorous sexual preparation regimen.

While sexual preparation is taking place, re- lease of oxytocin from the posterior pituitary occurs. Oxytocin causes contraction of the smooth musculature surrounding the tail of the epididymis and the ductus deferens. These contractions transport spem1atozoa from the tail of the epididymis into the duchts deferens and eventually into the pelvic urethra. Once spem1 gain entrance into the pelvic urethra, they begin to mix with secretions from the accessory sex glands.

Homosexual-like Behavior

Homosexual-like behavior is common among domestic animals and is particularly common in cattle. The tenn homosexuality implies a sexual preference for same-sex partners. In animals, there is not a preference, but rather indiscriminate orientation or same-sex di- rected behavior. Thus, an alternative term that is appli- cable to sub-primate animals would be homosexual-like behavior. Cows and bulls exhibit strong homosexual- like behavior. Similar behavior is seen in sheep and dogs and to a Jesser extent in swine and horses. Such behavior has profound usefulness for detecting cattle in estrus. When a female stands to be mounted by another cow, this alerts the management team that the cow is in estrus and artificial insemination can be performed. A favorite question of managers and stu- dents of reproductive physiology alike is, "What is the evolutionary advantage of animals displaying this kind of behavior?" While a definitive answer is not known, two theories exist to explain female-female mounting behavior in cattle.

The first explanation theorizes that cows mounting each other provide a visual signal that attracts a bull to the cow in estrus. In other words, when a bull sees cows mounting each other he will investigate and if the cow is in standing estrus, he will breed her.

The second theory explaining the evolution of homosexual-like behavior among cows involves inad- vertent genetic selection by man fo r this behavior. It has been proposed that cattle of European descent were se- lected by humans for their estrous behavior. In Medieval Europe, cattle husbandry involved the use of a few cows by each peasant farmer for three purposes: dra ft, milk and meat. Peasant f.·mners could not afford to maintain a bull for breeding purposes since the bulls gave no milk, gave birth to no calves and had obnoxious behavior that made them unsuitable for everyday management. In addition, most bulls apparently were owned by wealthy land holders who probably controlled the breeding, as well as the financial aspects of cattle management. Since most cows were k ept in groups without intact males, the herdsmen needed some sign to tell him when his cows should be bred. Obviously, the cow that showed the most intense mounting behavior was the one most likely to be observed by the peasant and most likely to be bred by the nobleman's bull. Those that showed little mount- ing behavior did not become pregnant in a reasonable amount of time. This theory suggests that cows with a high degree of mounting behavior were inadvertently selected because they were noticed by man and offered a greater opporhmity to become pregnant. T hus, this behavioral trait was transmitted to their offspring.

Artificial Insemination Requires an Understanding of Reproductive Behavior

and Physiology

Th ere are two fundamental ways to collect semen from the male. The prefe rred method utilizes an artificial vagina or a device that simulates vaginal conditions of a female in estrus. The second method relies on electrical stimulation of the accessory sex glands and the pelvic urethra and this method is called electroejaculation. Electroejaculation is generally used in males of high genetic value that cannot physically perform mounting and ejaculation. In the beef industry, electroejaculation is used in range bulls.

Typical artificial vaginas for domestic animals are shown are Figure 11-I 6. In general, artificial vaginas consist of an outer casing fashioned of reinforced rubber and a liner that is generally made of rubber that can be lubricated. Tempera hire and pressure are controlled by the water that is p laced between the casing and the liner. One end of the artificial vagina is attached to a funnel- like cone that in tum is attached to a collection vessel, usually a nonbreakable graduated test tube.

From a behavioral perspective, males that are to be collected with an artificial vagina need some form of training. Males with previous sexual experience will readily mount a surrogate animal (artificial animal or "dummy"). The degree to which animals will mount

Reproductive Behavior 249

Figure 11-16. Artificial Vaginas for Various Animals Outer casing

l -- Warm water

Rubbe r liner - Warm water

Rubber co llection

funne l

-· ; - .--......... The typical artificia l vagina consists of a sturdy outer casing, a rubber liner, a chamber fi ll ed with warm water, a rubber collection funnel and a collection tube.

tube

The artificial vag ina for the stallion consists of a leather outer casing (C) equiped with a port to drain water (arrow). The collection vessel (CV) and the protective covering (PC) are shown. Ide- ally, ej aculation takes place in the collection cone (CC) so that most of the semen will drain directly into the collection vessel. (Artific ial vagina courtesy of Northwest Equine Reproduction Laboratory, University of Idaho, www.avs.uidaho.edu/nerl)

The artificial vagina for the bull consists of a black casing (C), a rubber liner (RL) a collection cone (CC) and a collection vessel (CV). Wate r is placed between the casing and the liner. The proper tem- peratu re is critical for successful ejaculation in the bull . Wh ile not shown in the photograph a protec- tive covering is placed over the cone and collection vessel to prevent cold shock of the semen.

The artificial vagina for the boar consists of a bulb that can apply pressure to the artificial vag ina. High pressure is obligatory fo r stimulation of the glans pen is and ej aculation in the boar. The artificial vagi na for the boar also consists of an outer casing (C), a liner (L) and a protective covering (PC) that houses the collection vessel. (Photograph courtes y of MinitO b Germany, www.minitilb.de)

The artificial vagina for coll ection of semen from rams and bucks consists of a rubber casing (C) with a valve (arrow) through which water can be added or emptied, a rubber liner and a collection vessel (CV). The protective covering (PC) is shown above the artificial vagina. (Photograph courtesy of MinitOb Germany, www.minitilb.de)

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248 Reproductive Behavior

Homosexual-like Behavior

Artificial Insemination Requires an Understanding of Reproductive Behavior

and Physiology

Reproductive Behavior 249

Figure 11-16. Artificial Vaginas for Various Animals Outer casing

l -- Warm water

Rubbe r liner - Warm water

Rubber co llection

funne l

-· ; - .--......... The typical artificia l vagina consists of a sturdy outer casing, a rubber liner, a chamber fi ll ed with warm water, a rubber collection funnel and a collection tube.

tube

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250 Reproductive Behavior

Figure 11-17. Surrogate Stimulus Animals for Semen Collection

"Phantom" for Stallion Semen Collection

In general, males of most species can be trained to mount and ejaculate using surrogate stimulus animals. A surrogate stimulus animal provides ease of cleaning and minimizes the risk of injury and disease transmission . Further, surrogate stimulus animals do not require feed, hous- ing and labor for maintenance as does a live stimulus animal. The use of artificial stimulus animals req uires previous training of the male. Once the male has been trained he will gener- ally mount the "dummy" readily. The size can be adjusted easily to accomodate various males. Mobile surrogate stimulus animals are used fo r collection of semen in bulls because the location can be changed with ease.

The surrogate stimulus animal used to collect semen from the stallion is generally referred to as a "phantom". The "phan- tom" contains a biting belt (arrow) to provide the stallion with a surface to bite during mounting thus provid ing a means for natural behavior. All of the devices shown have a built-in artificial vagina in which the temperature and pressure can be controlled. (Photographs courtesy of Mini tOb Germany, www.minitDb.de)

dummies depends on the amount of training provided. A surrogate stimulus animal provides the advantage of safety, reduced expense and they can be designed to accomodate males of various stature. The disadvantage of using surrogate stimulus animal is that changing lo- cations and teasers is difficult. Figure 11-17 illustrates examples of surrogate animals for semen collection.

Sometimes it is diffi cult to train animals to mount either a stimulus animal or a surrogate stimulus animal. In this event, semen can be collected by plac- ing a condom-like structure inside the vagina of the female in estrus. When the male mounts the female and ejaculates , the semen is deposited inside the vessel. Such techniques are valuable when animals have not been adequately trained.

The design of an artificial vagina should accomplish the following:

• provide a suitable environment for stimulation of the glans penis

• provide an environment that prevents damage to the penis

• provide an environment that maxi- mizes sperm recove1y and minimizes sperm insult

Further PHENOMENA for Fertility One day President and Mrs. Coolidge were visiting a government farm. Soon after their arrival they were taken off on sepa- rate tours. Wizen M rs. Coolidge passed the chicken pens, she paused to ask the man in charge if the rooster copulated more than once each day. "Dozens of times," was the reply. "Please tell that to the President, " Mrs. Coolidge requested. Wizen the President passed the pens am/ was told about the rooste1; he asked, "Same h en eve1y day?" "Oft no, Mr. President, a different one each time. " The President nodded slowly and then said, "Please tell that to Mrs. Coolidge."

The praying mantis has mwsual reproduc- tive As soon as the male mounts the female and accomplishes intromission, the f emale bites his head off. She imme- diately eats the top half of his body while intromission is still taking place. The rea- son for this behavior is because ejaculation is permanently inhibited in the male and can take place only after the head has been removed. It is not known whether the slang phrase "bite-your-head-off' was derived from this behavior.

Roman snails shoot love darts at one an- other before copulation to determine if they are both members of the same species.

Some male insects (certain flies and mos- quitoes) have evolved mmsual adaptations to ins me that their genetics will be passed on. Males have a sharp, specialized penis that can enter a pupa. The male insemi- nates the rmbom female.

When a grey squirrel comes into estrus, up to a dozm males noisily chase her through the trees. This chase is necessary, because the female will not ovulate without it.

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To mate, the queen bee leaves the hive and p elforms a mating flight in an area where drones are congregated. The fastest dron e is the first to copulate with the queen. Copu- lation is a11 hz-jfight event that lasts from 1 to 3 seconds. Wizen the copulating bees separate, the entire male genitalia is ripped from the male and stays with the queen. The male soon dies am/ another male will then mate with the queen. Up to 17 matings in one mati11g flight have been observed.

Females of some species are quite choosy about who gets to fertilize their eggs. In these cases, mate choice is determined by nuptial gifts presented by the male. Th e female black-tipped hangffy accepts nuptial gifts in the form of food in exchange for copulation. Wizen edible food is presented by the male, the duration of copu/atio11 is depende11t on the size of the gift. If the gift is small and can be consumed in 5 minutes or less, the female will not allow mating. If the gift is large (cannot be consumed in 20 minutes), the female will allow mating to take place. If the gift provides a meal of only 12 minutes she will/eave the gift-giver prematurely and seek another gift-giver as a mate.

Satin bowerbirds build their nests only with blue objects. Males gather blue flo wers, pen caps, berries and ribbons and arrange them under bushes or in other cozy spots. If a female "likes" what she sees, she will choose the nest's decorator as Iter mate.

A male newt begins his courtship by jump- ing on the back of the female and rubbing his jaw against h er snout. This releases a scent that drives the female newt "crazy with desire. "

When female rhinoceri are in heat they will run away from a male, then suddenly tum and fight him horn-to-horn, sometimes for longer than a day. Only if he is fit enough to pursue will she submit. There are no "wimp genes" in the rhinocerous gene pool.

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250 Reproductive Behavior

Figure 11-17. Surrogate Stimulus Animals for Semen Collection

"Phantom" for Stallion Semen Collection

The design of an artificial vagina should accomplish the following:

• provide a suitable environment for stimulation of the glans penis

• provide an environment that prevents damage to the penis

• provide an environment that maxi- mizes sperm recove1y and minimizes sperm insult

Roman snails shoot love darts at one an- other before copulation to determine if they are both members of the same species.

When a grey squirrel comes into estrus, up to a dozm males noisily chase her through the trees. This chase is necessary, because the female will not ovulate without it.

Reproductive Behavior 251

A male newt begins his courtship by jump- ing on the back of the female and rubbing his jaw against h er snout. This releases a scent that drives the female newt "crazy with desire. "

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252 Reproductive Behavior

During courtship the female balloon fly will eat the male if given the chance. To achieve copulation and keep from getting eaten, the male will present the female with a balloon-shaped cocoon as a "present". Unwrapping this "present" keeps the female occupied long enough for the male to mate her and fly off.

When box turtles copulate, the male mounts the female and remains in an upright posi- tion in order to facilitate insemination. The pair may remain in this position for hours to ensure adequate insemination. A t the conclusion of the event the female will sud- denly move away, sometimes causing the male to fall p recariously on his hack where he may remain until his death if h e can't right himself.

Most frogs and toads copulate in the dark. They are often so eager to mate that the male will try to momzt any thing that passes by. They have been observed keeping a .firm grip on strange objects and even other small animals in the hope that they might turn out to he females.

Th e long neck of the giraffe plays an im- portant role in their reproductive First the male samples the urine to ascertain whether she is in estrus. If so, the two giraffes then indulge in a form of sexual preparation by entwining and rubbing their necks together. Physiologically, this behav- ior is like a false-mount and no doubt causes the release of oxytocin that moves sperm in the distal tail of the epididymis into an ejaculatory position.

The p ressure within the penis of the bull at the time of ejaculation is equivalent to 10 times the pressure within a normal vehicle tire.

Key References

A lbright, J.L., and C.W. Arave. 1997. The Behaviour o(Cattle. CAB International, Wellingford, UK. ISBN 0- 851 99-1 96-3.

Cra ig, J. V. 198 1. Domestic Animal Behavior: causes and implications (or animal care and management. Prentice-Ha ll, Inc . New Jersey. ISBN 0-13-2 18339- 0.

Evans, H.E. 1993. Anatomv o( the Do[, 3rd Edition. W.B .Saunders Co. Philadelphia. ISBN 0-721 6- 3200-9.

Grandage, J. 1972. "The erect dog pe nis: a paradox of flexibl e rig idity." Vet Rec: 9 1: 14 1- 147.

Hart, Benjamin L. 1985. The Behavior o[Domestic Animals. W.H. Freeman and Co., New York. ISBN 0-7 167-1595-3.

Houpt, K.A . 1998. Domestic Animal Behavior for Veterinarians and Animal Scientists. 3rd Edition. Iowa State Uni versity Press, ISBN 0-8138-1061 -2.

Katz, L. S. and T.J. McDonald. 1992. "Sexual Behavior of farm animals" in Repoduction in Far m A ni mals : Science, App lic ation and Mode ls . Theriogenology 38:240-254.

Korenman, S.G. 1998 . "New ins ights into erectile dys- function : a practical approach." Am. J. Med. 105:135- 144.

Signoret, J.P. and J. Balthazart. 1993 "Sexual behavior" in Reproduction in Mammals and Man . C. Thibault, M.C. Levasseur and R.I-I.F. I-I under, eds. Ell ipses, Paris. ISBN 2-72 98-9354-7.

Tibary, A. and A. Anouassi. 1997. Theriogenolorsy in Camelidae. United Arab Emirates. Ministry of Cul- ture and Infor mation. Publ ication authorization N o. 3849/ 1116. ISBN 9981 - 801 -32- 1.

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