Write Answers for the questions from the assigned Chapters

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The Puerperium & Lactation

Early Embryogenesis & Maternal Recognition of Pregnancy

Ovulation & Fertilization

Cyclicity

Regulation of Reproduction

Tract Function

Puberty

Prenatal Development

Spermatogenesis

Regulat ion of Reproduction

Tract Function

Puberty

Prenatal Development

Take Home Message Gestation is the period of time that a female is pregnant. During gestation, the

placenta forms a major organ of pregnancy that provides an inteJface for metabolic exchange between the dam and the f etus. Placentas are described mmplwlogically according to the distribution of villi on the chorionic smface ami the degree of separa..:. tion between matemal and fetal blood. The placenta is also an endocrine organ that secretes hormones responsible for: 1) maintenance of pregnancy; 2) stimulation of the matemal mammmy gland and 3) ensures fetal growth. Parturition is brought about by secretion of fetal corticoitls and requires removal of the progesterone block. Par- turition consists of three stages. They are: 1) initiation of myometrial contractions; 2) expulsion ofthe f etus and 3) e.:\:pulsion oftlzefetalmembranes.

The word gestation literally means "the act of carry ing or being carried". Thus, gestation means the action or process of carrying or being carried' in the uterus between conception and birth . · Gest ation and pregnancy are synonymous and thus, gestation length means the length of pregnancy. Attachment of the conceph1s to form an intimate, but temporary, relationship with the uterus is an evolutionary step that provides significant advantage to the conceph1s . The phenomenon of intrauterine development ensures that the developing conceptus will receive adequate nutri- tion and protection during its development. In contrast, lower fon11S of animals lay eggs (oviparous). The survival of potential offspring of oviparous animals is jeopardized because the female cannot completely protect the eggs from environmental and predatmy dan- ger. Thus, from an evolutionary perspective, eutherian mammals (mammals with a placenta), are "equipped" with an in-utero protection mechanism that is highly successful after the placenta is formed.

The final prepartum steps of reproduction are:

• formation of a placenta

• acquisition of endocrine function of the placenta

• initiation of parturition

The term implantation is often used to mean attachment of the placental membranes to the endo- metrium in most animals. Achmlly, true implantation is a phenomenon in humans in which the conceptus "buries" itself into the uterine endometrium. The con- ceptus temporarily disappears beneath the surface. In

most other species, the conceph1s does not truly implant, but rather attaches to the endometrial surface and never disappears from the luminal compartment.

The placenta is an organ of metabolic inter- change between the conceph1s and the dam. It is also an endocrine organ. The placenta is composed of a fetal component derived fi·om the chorion and a maternal component derived from modifications of the uterine endometrium. The discrete regions of contact between the chorion and the endometr ium form specific zones of metabolic exchange. The placenta also produces a variety of hormones . This endocrine function is important for the maintenance of pregnancy and the induction of parh1rition.

Parturition (giving birth to young) is the step in the reproductive process that immediately precedes lactation, uterine repair and return to cyclicity. It is ini tiated by the feh1s and involves a complex cascade of endocrine events that promote myometrial contrac- tions, dilation of the cervix, expulsion of the feh1s and expulsion of the extraembryonic membranes.

Placentas Have Different Distributions of Chorionic Villi

As you have learned in the previous chapter, the conceptus consists of the embryo and the extraembry- onic membranes (amnion, allantois and chorion). The chorion is the fetal contribution to the placenta. The functional uni t of the fetal placenta is the chorionic villus. The chorionic villus is an "exchange apparatus" and provides increased surface area so that exchange is maximized. Chorionic villi are small, finger- like projections that are on the surface of the chorion. These tiny villi protrude away from the chorion toward the uterine endometrium. Placentas are classified according to the distribution of chorionic vill i on their surfaces,

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The Puerperium & Lactation

Early Embryogenesis & Maternal Recognition of Pregnancy

Ovulation & Fertilization

Cyclicity

Regulation of Reproduction

Tract Function

Puberty

Prenatal Development

Spermatogenesis

Regulat ion of Reproduction

Tract Function

Puberty

Prenatal Development

Take Home Message Gestation is the period of time that a female is pregnant. During gestation, the

The final prepartum steps of reproduction are:

• formation of a placenta

• acquisition of endocrine function of the placenta

• initiation of parturition

most other species, the conceph1s does not truly implant, but rather attaches to the endometrial surface and never disappears from the luminal compartment.

Placentas Have Different Distributions of Chorionic Villi

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294 Placentation, Gestation and Parturition

giving each placental type a distinct anatomical appear- ance. Placentas may also be classified by number of tissue layers separating maternal and fetal blood.

Placentas are classified acc01·ding to the distribution of chorionic viili. These classifications are:

• diffuse • zonary • discoid • cotyledonary

The diffi.tse placenta of the pig has a velvet-like surface with many closely spaced chorionic villi that are distributed over the entire surface of the chorion (See Figure 14-1 ). Initial attachment occurs around day 12 and is well established by day 18 to 20 after ovulation (See Chapter 13).

Diffuse placentas have uniform distribution of chorionic villi that cover the swface of the chorion.

Example= pig

The mare placenta is also classified as diffuse, however it is characterized by having many specialized "microzones" of chorionic villi known as microcoty- ledons (See Figure 14-1 ). These microcotyledons are microscopically discrete regions at the fetal-maternal interface. As in the pig, they are also distributed over the entire chorionic surface.

The mare placenta also contains unique tran- sitory structures known as endometrial cups. These are discrete areas that range from a few millimeters to several centimeters in diameter. The endometrial cups are of both trophoblastic and endometrial origin. There are 5 to I 0 endometrial cups distributed over the surface of the placenta (See Figure 14-6). Endometrial cups produce equine chorionic gonadotropin (eCG) and develop between days 35 and 60 of pregnancy. Following day 60, the endometrial cups are sloughed into the uterine lumen and are no longer functional. Attachment of the conceptus to the endometrium is initiated at about day 24 and becomes well established by 36 to 38 days (See Chapter 13).

Zonary placentas have a band-like zone of chorionic villi.

Example = dogs and cats

The zonary placenta (found in dogs and cats) includes a prominent region of exchange that fonns a broad zone around the chorion near the middle of the conceptus (See F igure 14-2). A second region consists of a highly pigmented ring at either end of the central zone. This pigmented zone consists of small hematomas (blood clots). The pigmented zone is also refetTed to as the paraplacenta and is thought to be important in iron transport from the dam to the fehts. The function of this zone is not well understood. A third region is the transparent zone on the distal ends of the chorion that has poor vascularity. This zone may be involved in absorption of materials directly from the uterine lumen.

Discoid placentas form a regionalized disc.

Example = rodents and primates

The discoid placenta (See Figure 14-2) is found in rodents and primates. It is characterized by having one or two distinct adj acent discs. These discs contain chorionic vi lli that interface with the endometrium and provide the region for gas, nutrient and metabolic waste exchange.

Cotyledonary placentas have nu- merous, discrete button-like structures called cotyledons.

Example = ruminants

Ruminants have a cotyledonary placenta (See Figure 14-3). A cotyledon is defined as a placental unit of trophoblastic origin cons isting of abundant blood vessels and connective tissue. In sheep, there are between 90 and 100 cotyledons distributed across the surface of the chorion and, in cattle, 70 to 120 cotyledons have been observed. The placentome (point of interface) in the cotyledonary placenta consists of a fetal cotyledon contributed by the chorion and a maternal cotyledon , originating from the caruncular regions of the u terus. At about day 16 in sheep and day 25 in cattle the chorion initiates attachment to the cm·uncles of the uterus. Prior to this time the placenta is essentially diffi.tse. During the formation of the placentomes, chorionic v illi protrude into crypts in the caruncular tissue. This relationship .lli not implantation but an anatomically specialized forn1 of attachment. Attachment is well established by day 30 in ewes and day 40 in cows (See Chapter 13).

In the cow, the placentomes form a convex structure, whi le in the ewe they are concave (See Figure 14-3). During gestation, the cotyledons will

increase many-fold in diameter. In fact, cotyledons in the cow near the end of gestation may measure 5 to 6 centimeters in diameter. Such growth provides enormous surface area to support placental transfer of nutrients from the dam and metabolic wastes from the fetus.

Placental Classification by Microscopic Appearance is Based on the Number of Placental Layers that Separate the Fetal

Blood from the Maternal Blood

The nomenclature for describing placental in- timacy is derived by first describing the tissues of the maternal placenta in the prefix of the word. The tissues of the fetal placenta constitute the suffix. Exchange can occur through as many as six tissue layers and as few as three. The name of the prefix and suffix of each type of placenta changes depending on the number of tissue layers that exist.

Prefix =maternal side Suffix =fetal side "epithelia" "chorial"

epitheliochorial

Placentation, Gestation and Parturition 295

cells originate from trophoblast cells and are thotwht to be fanned continuously throughout gestation. Bi- nucleate giant cells constitute around 20% of the fetal placenta. During development, the binucleate giant cells migrate from the chorionic epithelium and invade the endometrial epithelium (See Figure 14-4). The binucleate giant cells are believed to transfer complex molecules from the fetal to the maternal p lacenta. There is evidence that they secrete placental lactogen. Also, these cells secrete pregnancy specific protein B (PSPB) that are also called pregnancy associated glycoproteins (PAG). These proteins are unique to pregnancy in ruminants. The binucleate giant cells are also important sites of steroidogenesis, secreting progesterone and estradiol. These cells will no doubt emerge as increasingly important "players" in the func- tion of the ruminant placenta with further research.

I Endotheliochorial = 5 layers I The endotheliochorial placenta is character-

ized as having complete erosion of the endometrial epithelium and underlying interstitium. T hus, maternal capillaries are directly exposed to epithelial cells of the chorion (See Figure 14-5). The chorionic epithelium packs around the vessels on the maternal side. Note in Figure 14-5 that this type of placenta is more intimate

I Epitheliochorial 6 layers I than the epitheliochoriat placenta because the en dome-- _ trial epithelium no longer exists. Dogs and cats possess ..__ _ _ ____ _ _ _ _____ ____ _. endotheliochorial placentation. The epitheliochorial placenta (See Figure

14-5) is the least intimate among the placental types. In the epitheliochorial placenta, both the endometrial epithelium (maternal side) and epithelium of the chori- onic villi are intact. In other words, there is a complete intact layer of epithelium in both the maternal and fetal components. The epitheliochorial placenta is found in the sow and the mare. Recall that the placentas of the sow and the mare are diffitse and villi occupy a large proportion of the surface area of the chorion.

Ruminants also have an epitheliochorial pla- centa. However, the endometrial epithelium transiently erodes and then regrows, causing intennittent exposure of the maternal capillaries to the chorionic epithelium. This type of placenta has been tenned syndesmocho- rial.

In addition to the feature of partial erosion of the endometrial epithelium, a unique cell type is found in the ruminant placenta. These cells are called binucleate giant cells. As their name implies, they are characterized as being quite large and have two nuclei. Binucleate giant cells appear at about day 14 in the sheep and between days 18 and 20 in the cow. These

I Hemochorial = 3 layers I The hemochorial placenta (See Figure 14-5)

is characterized as having the chorionic epithelium in direct apposition to maternal pools ofblood. Thus, nu- trients and gases are exchanged directly from maternal blood and must move tlu-ough only tlu-ee tissue layers. This highly intimate relationship is found in primates and rodents (See Figure 14-5).

The Placenta Regulates the Exchange Between the Fetus and Dam

Placental exchange involves a number of mechanisms found in other tissues. These are simple diffusion, facilitated diffusion and active tr a nsport. Gases and water pass from high to low concentrations by simple diffusion. The p lacenta contains act ive transport pumps for sodium and potassium, as well as calcium. Glucose and other metabolically important materials such as amino acids are transported by facili- tated di ffusion uti lizing specific carrier molecules.

141

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294 Placentation, Gestation and Parturition

giving each placental type a distinct anatomical appear- ance. Placentas may also be classified by number of tissue layers separating maternal and fetal blood.

Placentas are classified acc01·ding to the distribution of chorionic viili. These classifications are:

• diffuse • zonary • discoid • cotyledonary

Diffuse placentas have uniform distribution of chorionic villi that cover the swface of the chorion.

Example= pig

Zonary placentas have a band-like zone of chorionic villi.

Example = dogs and cats

Discoid placentas form a regionalized disc.

Example = rodents and primates

Cotyledonary placentas have nu- merous, discrete button-like structures called cotyledons.

Example = ruminants

In the cow, the placentomes form a convex structure, whi le in the ewe they are concave (See Figure 14-3). During gestation, the cotyledons will

Placental Classification by Microscopic Appearance is Based on the Number of Placental Layers that Separate the Fetal

Blood from the Maternal Blood

Prefix =maternal side Suffix =fetal side "epithelia" "chorial"

epitheliochorial

Placentation, Gestation and Parturition 295

I Endotheliochorial = 5 layers I The endotheliochorial placenta is character-

I Hemochorial = 3 layers I The hemochorial placenta (See Figure 14-5)

The Placenta Regulates the Exchange Between the Fetus and Dam

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296 Placentation, Gestation and Parturition

Figure 14-1. The Diffuse Placenta

Sow

Endometrium

=---

The diffuse placenta of the sow consists of many chorionic villi distributed over the entire surface of the chorion. They penetrate into the endometrium forming the fetal-maternal interface. Vessels from each chorionic vil- lus merge and eventually form large vessels that enter the umbilical cord. A= Allantois, AC= Allantochorion, AM= Amnionic Cavity, E= Endometrium, M= Myometrium

Mare

Endometrium

Myometrium

The diffuse placenta of the mare consists of many microcotyledons distributed over the entire surface of the chorion. These mi- crocotyledons are the site of fetal-maternal exchange. A= Allantois, AC= Allantochorion, AM= Amnionic Cavity, E= Endometrium, M= Myometrium, YS= Yolk Sac

Placentation, Gestation and Parturition 297

Figure 14-2. The Zonary and Discoid Placentas

YS .....-"'

Bitch

The zonary placenta consists of three distinct zones; a transfer zone (TZ), a pigmented zone (PZ) and a relatively nonvascular zone, the allantochorion (AC). In the zonary placenta, a band of tissue forms around the conceptus where nutrient transfer occurs. The pigmented zone (PZ) or paraplacenta represents local regions of maternal hemorrhage and necrosis. A= Allantois, AC= Allantochorion, AM= Amnionic Cavity, E= Endometrium, M= Myometrium, YS= Yolk Sac

Primates

The discoid placenta consists of a round patch of chori- onic tissue that forms the fetal-maternal interface. Ves- sels from the exchange zone merge to form the umbilical vessels that supply the fetus with blood. The vasculature of the chorion (within the disc) is immersed in pools of blood where metabolic exchange takes place.

A= Allantois, AC = Allantochorion, AM= Amnionic Cavity, E = Endometrium, EZ = Exchange Zone, M = Myometrium

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296 Placentation, Gestation and Parturition

Figure 14-1. The Diffuse Placenta

Sow

Endometrium

=---

Mare

Endometrium

Myometrium

Placentation, Gestation and Parturition 297

Figure 14-2. The Zonary and Discoid Placentas

YS .....-"'

Bitch

Primates

A= Allantois, AC = Allantochorion, AM= Amnionic Cavity, E = Endometrium, EZ = Exchange Zone, M = Myometrium

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298 Placentation, Gestation and Parturition

Figure 14-3. The Cotyledonary Placenta

Convex (cow, giraffe)

In the photograph above, the fetal membranes and the fetal cotyledons (FC) can be visualized. The membrane labeled AC is the allantochorion. The umbilical cord, (UC-arrow) of the fetus receives blood vessels (BV) from the fetal cotyledons (FC). Glycogen plaques (GP) can be visualized on the surface of the chorion and the amnion. These plaques are localized squamous proliferations called verrucae.

Concave (sheep, goat)

The cotyledonary placenta is characterized by numerous "button-like" structures distributed across the surface of the chorion . These are called fetal cotyledons. When they jo in with the maternal caruncle they form a placentome. Aconvex cotyledon becomes covered with the chorion. Many finger-like villi (red) originating from the chorionic tissue protrude toward the lumen of the uterus. In the concave cotyledon, the chorionic tissue pushes inward, forming a concave interface between the chorion and the maternal caruncle.

Placentation, Gestation and Parturition 299

Figure 14-3. The Cotyledonary Placenta

The diagram in the upper left illustrates the distribution of the extraembryonic membranes prior to complete at- tachment. The extraembryonic membranes consist of the amnion (blue sac), yolk sac (YS) and the allantois (A). Even though the fetus is located in one uterine horn, the chorion invades the contralateral uterine horn and forms placentomes.

Cow Some fetal cotyledons (FC) have been partially separated from maternal cotyledons (MC). The chorion (C) is the outer fetal membrane. Arrows indicate the border of the amnion (A). The myometrium (M) is indicated by the ar- rows. Notice that the fetal cotyledon (FC) is attached to the surface of the caruncle creating a convex cotyledon. E= Endometrium

Ewe-A The chorion can be seen entering the placentome (P). The chorionic stalk (CS) contains the fetal vasculature.

Ewe-8

)

A portion of the chorion has been incised so that the fetal vasculature can be visualized clearly. The fetal vessels (arrow) and chorionic tissue "push" into the caruncular tissue forming a concave cotyledon. A set of arteries (A) and veins (V) emerge from each cotyledon and eventually merge in the umbilical cord (UC). P= Placentoma

Ewe-C A concave placentoma is clearly visible. The chorionic stalk is draped over the needle holder. Notice the vessels (arrows) within the chorionic tissue. The reddish-beige tissue is the maternal cotyledon (MC) that is covered by the allantochorion. The dark tissue in the center (arrows) is the fetal component of the placentome.

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298 Placentation, Gestation and Parturition

Figure 14-3. The Cotyledonary Placenta

Convex (cow, giraffe)

Concave (sheep, goat)

Placentation, Gestation and Parturition 299

Figure 14-3. The Cotyledonary Placenta

Ewe-A The chorion can be seen entering the placentome (P). The chorionic stalk (CS) contains the fetal vasculature.

Ewe-8

)

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300 Placentation, Gestation and Parturition

Glucose is the major source of energy for the fetus. The majority of glucose is derived from the ma- temal circulation. Near the end of gestation, glucose consumption by the fetus is exceptionally high and can lead to a metabolic drain of glucose away from the dam. Such a glucose drain favors the development of ketosis in the dam. Ketosis results from the metabo- lism of body fat that generate ketones for energy when glucose is limited. Periparturient ketosis is common in dairy cows where postpartum metabolic demands are exceptionally high because of high milk produc- tion. Some materials cannot be transported across the placenta. With the exception of some immunoglobu- lins, matemal proteins do not cross the placental banier. Immunoglobulins can be transported from the matemal to the fetal side in a hemochorial or an endotheliochorial placenta. However, the fetus synthesizes the majority of its own proteins from amino acids contributed by the dam. Nutritionally-based lipids do not cross the placenta. Instead, the placenta hydrolyzes triglycer- ides and matemal phospholipids and synthesizes new lipid materials to be used by the fetus. Large peptide honnones such as thyroid stimulating hom1one, adrenal cortical stimulating hormone, growth honnone, insu- lin and glucagon do not cross the placenta. Smaller molecular weight hormones such as steroids, thyroid hormone and the catecholamines (epinephrine and norepinephrine) cross the placenta with relative ease. Vitamins and minerals are transfened to the fetus at

variable rates. Fat soluble vitamins do not cross the placenta with ease, while water soluble vitamins (Band K) pass across the placenta with relative ease. Nutrients are also transferred by pinocytosis and phagocytosis. Areolae from the chorion form over the openings of the uterine glands and are thought to absorb secretions from these glands.

Of significant importance is the ability of the placenta to transfer toxic and potentially pathogenic ma- terials. Many toxic substances easily cross the placental banier. These include ethyl alcohol, lead, phosphorus and mercmy. Also, opiate drugs and numerous common phmmaceuticals such as barbiturates and antibiotics can cross the placental banier. Some substances may be highly teratogenic. Teratogenic means inducing ab- normal development (birth defects). These substances include LSD, amphetamines, lithium, diethylstilbestrol and thalidomide. It is well documented that these ma- terials induce abnormal embtyonic development and cause serious birth defects.

It is known that a wide range of microorgan- isms can contaminate the fetus. Viruses can cross the placental banier with ease and thus many viral diseases can be transmitted from the dam to the fetus. Such human diseases as German measles, Herpes virus and HIV can be transmitted from the pregnant mother to the fetus. Bacteria such as syphilis can also be transmitted to the fetus.

Figure 14-4. The Migration of Binucleate Giant Cells in the Ruminant Placenta

r::

Fetal (chorion)

Maternal (endometrial epithelium)

Basement membrane

Maternal circulation

Binucleate giant cells (BNGC) migrate from the chorion to the en- dometrial epithelium in ruminants. These cells are thought to se- crete placental lactogen and pregnancy specific protein B.

(www. biotracking. com)

Placentation, Gestation and Parturition 301

Figure 14-5. Placental Classification Based on Separation Between Fetal and Maternal Blood Supplies

Fetal (chorion)

Maternal (endometrial epithelium)

Fetal (chori on)

Fetal (chorion)

Maternal (endometrium)

Epitheliochorial

Endotheliochorial

Hemochorial

Epithel iochorial (pigs, horses and ruminants)

6. Chorionic capillaries 5. Chorionic interstitium 4. Chorionic epithelium 3. Endometrial epithelium 2. Endometrial interstitium 1. Endometrial capillaries

Endotheliochorial (dogs and cats)

5. Chorionic capillaries 4. Chorionic interstitium 3. Chorionic epithelium 2. Endometrial interstitium 1. Endometrial capillaries

Hemochorial (primates and rodents)

3. Chorionic capillaries 2. Chorionic interstitium 1. Chorionic epithelium RBC= Red blood cell

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300 Placentation, Gestation and Parturition

Figure 14-4. The Migration of Binucleate Giant Cells in the Ruminant Placenta

r::

Fetal (chorion)

Maternal (endometrial epithelium)

Basement membrane

Maternal circulation

Binucleate giant cells (BNGC) migrate from the chorion to the en- dometrial epithelium in ruminants. These cells are thought to se- crete placental lactogen and pregnancy specific protein B.

(www. biotracking. com)

Placentation, Gestation and Parturition 301

Figure 14-5. Placental Classification Based on Separation Between Fetal and Maternal Blood Supplies

Fetal (chorion)

Maternal (endometrial epithelium)

Fetal (chori on)

Fetal (chorion)

Maternal (endometrium)

Epitheliochorial

Endotheliochorial

Hemochorial

Epithel iochorial (pigs, horses and ruminants)

6. Chorionic capillaries 5. Chorionic interstitium 4. Chorionic epithelium 3. Endometrial epithelium 2. Endometrial interstitium 1. Endometrial capillaries

Endotheliochorial (dogs and cats)

5. Chorionic capillaries 4. Chorionic interstitium 3. Chorionic epithelium 2. Endometrial interstitium 1. Endometrial capillaries

Hemochorial (primates and rodents)

3. Chorionic capillaries 2. Chorionic interstitium 1. Chorionic epithelium RBC= Red blood cell

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' I I 302 Placentation, Gestation and Parturition

The Placenta is a Major Endocrine Organ During Pregnancy

In addition to serving as a metabolic exchange organ, the placenta serves as a transitory endocrine or- gan. Hormones from the placenta gain access to both the fetal and the matemal circulation.

The placenta secretes hormones that can: • stimulate ovarian function • maintain pregnancy • influence fetal growth • stimulate mammary function • assist in parturition

The placenta of the mare produces a gonado- tropin called equine chorionic gonadotropin (eCG). Equine chorionic gonadotropin is also called pregnant mare's serum gonadotropin (PMSG). Equine cho- rionic gonadotropin is produced by the endometrial cups of the placenta. Endomeh·ial cups are a transient placental endocrine gland. They begin producing eCG at the time of attachment of the conceptus to the endo- metrium. The relationship between the fom1ation of the endometrial cups in the mare and the synthesis of eCG is presented in Figure 14-6. As you can see, the production of eCG is closely related to the weight of the endometrial cups.

Equine chorionic gonadotropin acts as a lu- teotropin and provides a stimulus for maintenance of the primary cm·pus luteum. The primary corpus luteum in the mare is defined as the corpus luteum fom1ed from the ovulated follicle. In addition, eCG is respon- sible for controlling the formation and maintenance of supplementary (accessory) corpora lutca. As eCG increases, the pregnant mare will often ovulate, thus generating accessory corpora lutea. The eCG-induced ovulations occur between days 40 and 70 of preg- nancy. Luteinization (promoted by eCG) also occurs in antral follicles that do not ovulate. Thus, eCG has a significant positive impact on the ability of the ovary to produce progesterone. Indeed, if one examines the progesterone profile, it can be seen that there is a close relationship between the concentrations of proges- terone and the production of accessory corpora lutea (See Figure 14-7).

In addition to its luteotropic action, eCG has powerful FSH-like actions when administered to fe- males of other species. In fact, eCG will cause marked follicular development in most species. It is used com- monly to induce superovulation where embryo transfer is performed (cow, sheep, rabbit). In mares, however, eCG does not exert significant FSH-like action.

--'E Db c .._, l!l u Cll

Figure 14-6. Production of Equine Chorionic Gonadotropin (eGG) is Closely Related to the

Weight of the Endometrial Cups (Modified from Ginther,

Reproductive Biologv of the Mare)

175 10

ISO 9

125 I 0 I I I 100 I 7 I I

75 I 6 I I I

50 I 5

4 25 I ---- 40 60 80 100 120 140 160 18 0 200

Days of Gestation

Endometrial cups (EC) are seen here in a U-shaped configuration. The fetus (F) is surrounded by the amnion (not visible). The membrane indicated by arrows is the allantochorion . This specimen was re- moved from a mare at 50 days of gestation. (Photograph courtesy of Dr. O.J. Ginther from Reproductive Biology of the Mare. 2nd Ed.)

,..., Ill a. :I u iii 'i: ... Ill E 0 'C c w .... 0 ..... J:

Placentation, Gestation and Parturition 303

Figure 14-7. Luteal Progesterone Output During the First Half of Gestation in the Mare

(Modified from Ginther, Reproductive Biologv of the Mare)

Progesterone { P4) from the primary corpus luteum increases rapidly after ovulation and then decreases (hatched region) . Without eCG, P4 would continue to decrease {dashed line) and the pregnancy would terminate.

Ill c 0 -:p ns :1.. ns -4J c c

:1.. Cl) Cl) u

-4J c ns 0 :ru Cl) Cl) > c ·.p 0 ns :1..

Q) Cl) -4J cc: Ill Cl) b.O 0 :1..

' ,,

Upon stimulation by eCG, the primary CL is stimulated and P4 in the maternal blood again increases. If eCG were not produced, P4 would continue to decrease (dashed line).

As eCG continues to increase, accessory CL develop and P4 increases until about day 100. After day 100, the placenta assumes the major P4 producing ro le.

0 30 60 90 120 ISO 180 2 10 240 270

Days of Gestation

Figure 14-8. The Production of hCG and Progesterone During Gestation in the Pregnant Woman

Human chorionic gonadotropin peaks at about 2.5 months of gestation and then declines. This period of time is critical for maintenance of pregnancy because the corpus luteum assumes primary responsibility for progesterone secretion.

Ovarian P4

hCG

2 3 4

At about 2 .5 to 3 months of the placenta begins to assume the primary responsibility for proges- terone secretion and continues this role until the time of parturition . hCG increases slightly between months 6 and 9 because of the increased placental mass.

Parturition Placental P4

5 6 7 8 9

Months of Gestation

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' I I 302 Placentation, Gestation and Parturition

The Placenta is a Major Endocrine Organ During Pregnancy

In addition to serving as a metabolic exchange organ, the placenta serves as a transitory endocrine or- gan. Hormones from the placenta gain access to both the fetal and the matemal circulation.

The placenta secretes hormones that can: • stimulate ovarian function • maintain pregnancy • influence fetal growth • stimulate mammary function • assist in parturition

--'E Db c .._, l!l u Cll

Figure 14-6. Production of Equine Chorionic Gonadotropin (eGG) is Closely Related to the

Weight of the Endometrial Cups (Modified from Ginther,

Reproductive Biologv of the Mare)

175 10

ISO 9

125 I 0 I I I 100 I 7 I I

75 I 6 I I I

50 I 5

4 25 I ---- 40 60 80 100 120 140 160 18 0 200

Days of Gestation

,..., Ill a. :I u iii 'i: ... Ill E 0 'C c w .... 0 ..... J:

Placentation, Gestation and Parturition 303

Figure 14-7. Luteal Progesterone Output During the First Half of Gestation in the Mare

(Modified from Ginther, Reproductive Biologv of the Mare)

Ill c 0 -:p ns :1.. ns -4J c c

:1.. Cl) Cl) u

-4J c ns 0 :ru Cl) Cl) > c ·.p 0 ns :1..

Q) Cl) -4J cc: Ill Cl) b.O 0 :1..

' ,,

Upon stimulation by eCG, the primary CL is stimulated and P4 in the maternal blood again increases. If eCG were not produced, P4 would continue to decrease (dashed line).

As eCG continues to increase, accessory CL develop and P4 increases until about day 100. After day 100, the placenta assumes the major P4 producing ro le.

0 30 60 90 120 ISO 180 2 10 240 270

Days of Gestation

Figure 14-8. The Production of hCG and Progesterone During Gestation in the Pregnant Woman

Ovarian P4

hCG

2 3 4

Parturition Placental P4

5 6 7 8 9

Months of Gestation

Ve tB oo ks .ir

304 Placentation, Gestation and Parturition

The second major gonadotropin of placental origin is human chorionic gonadotropin (hCG). This hormone is not only found in the human but in many other primates. Often hCG (and eCG) may simply be referred to as "CG". It originates from the trophoblas- tic cells of the chorion and is secreted as soon as the blastocyst hatches from the zona pellucida. Human chorionic gonadotropin can be detected in the blood and urine of the pregnant woman as early as days 8 to 1 0 of gestation. It increases rapidly in the urine of the pregnant woman, reaching a maximum value at about 2.5 months (See Figure 14-8). Its presence in the urine constitutes the basis for over-the-counter pregnancy diagnosis kits.

The primary role of hCG during early preg- nancy is to provide a luteotropic stimulus for the ovulatory corpus luteum as it transitions into the CL of pregnancy. Luteal LH receptors also bind hCG resulting in sustained progesterone production. Administration of hCG to non-primate females can cause ovulation. In fact, hCG is used commonly to induce ovulation in superovulation protocols.

The Placenta Secretes Progesterone and Estrogens

Progesterone is obligatory for early embry- onic development because it provides the stimulus for elevated secretion by the endometrial glands. High progesterone is also responsible for the so-called "pro- gesterone block" that inhibits myometrial contractions. Progesterone increases in the blood of the pregnant female and peaks at different stages of gestation for different species. The absolute levels of progesterone also vary significantly among species (See Figure 14-9). While progesterone is always produced by the corpus luteum in early pregnancy, the role of the corpus luteum in maintenance of pregnancy varies among species. In some species (ewe, mare and woman), the corpus luteum is not needed for the entire gestational period because the placenta takes over production of progesterone. For example, in the ewe the corpus 1uteum is responsible for initial production of progesterone, but the placenta assumes responsibility for its production after only 50 days of gestation (See Table 14-1 ). In other species (sow or rabbit), lutectomy (surgical removal of corpora lutea) will terminate pregnancy regardless of when this occurs during gestation. Lutectomy in the cow up to 8 months of gestation will result in abortion. It should be pointed out that even though the placenta takes over for the corpus luteum of pregnancy, the corpus luteum secretes progesterone throughout gestation.

In addition to progesterone, estradiol also is an important product of the placenta, particularly during the last part of gestation. In fact, the peak of estradiol in most species signals the early preparttu·ient period. The profiles of estradiol during gestation are presented in the subsequent section on parhrrition.

Cea·tain Placental Hormones Stimulate Mammaa·y Function of the Dam

and Fetal Growth

The placenta is known to produce a polypep- tide hom1one known as placental lactogen that is also called somatomammotropin. Placental lactogens have been found in rats, mice, sheep, cows and humans. They are believed to be similar to growth hormone, thus promoting the growth of the fehts. Placental lactogen also stimulates the mammary gland (lactogenic) of the dam. The degree to which fetal somatotropic (growth) versus lactogenic effects occur depends on the species (See Figure 14-10). For example, in the ewe ovine placental lactogen (oPL) has a more potent lactogenic activity than somatotropic activity. A similar condition exists in humans, but not in the cow. Placentallactogens have been shtdied most intensely in the ewe. They are produced and secreted by the binucleate giant cells of the placenta. The secretory products of the binucleate cells are transferred into the maternal circulation.

It is hypothesized that the sire may have an effect on the degree to which the fehts can produce placental lactogen. Such an effect could cause elevated concentrations of placental lactogen by the ferns. In- creased placental lactogen secretion would cause enhanced stimulation of the maternal manunary gland and thus promote elevated milk production. This theory suggests that it might be possible for the sire to influence fetal placental lactogen and enhance milk production in the dam. This sire-on-fetus-hypothesis has not been tested critically, but could hold promise for the genetic improvement in dairy, beef cattle and goats.

Placental relaxin is secreted in humans, mares, cats, dogs, pigs, rabbits and monkeys. Its function is to cause softening and "relaxation" ofthe pelvic ligaments to facilitate expulsion of the ferns. The stimulus for relaxin secretion is not known. Relaxin is not present in the bovine placenta during any stage of gestation. It is likely (with the exception of the rabbit) that relaxin, during the time of parrnrition, originates from both the ovary and the placenta. The role of relaxin is therefore questionable in the cow. Maternal blood relaxin levels are the basis for a commercial pregnancy diagnostic test at about 30 days of gestation in the bitch.

Placentation, Gestation and Parturition 305

Figure 14-9. Progesterone Profiles in Various Pregnant Females so (P = Parturition) -E 40 -..

00 1: 30 -

"C 20 0 0 iil 10

t e 2 3 4 Months of Gestation

"1 - 100 E -.. 00 1: - 20

"C 0

..5! al 10

® t e 2 4 6 Months of Gestation 8 10 II

14-1. Length and Time of Placental Takeover for Progesterone Production in Vanous Spec1es

SPECIES

Alpaca Bitch Camel Cow Ewe Goat Llama Mare Queen Rabbit Sow Woman

GESTATION LENGTH

11.4 mo 2 mo (65 days)

12.3 mo 9 mo 5 rna 5 mo

11.3 mo 11 mo 2 mo (65 days) 1 mo

3.8 mo 9mo

TIME OF PLACENTAL TAKEOVER

11.4 mo (none) 2mo (none)

12.3 mo (none) 6-8 mo

50 days 5 mo (none)

11.3 mo (none) 70 days

2 mo (none) 1 mo (none)

3.8 mo (none) 60-70 days

Ve tB oo ks .ir

304 Placentation, Gestation and Parturition

The Placenta Secretes Progesterone and Estrogens

Cea·tain Placental Hormones Stimulate Mammaa·y Function of the Dam

and Fetal Growth

Placentation, Gestation and Parturition 305

Figure 14-9. Progesterone Profiles in Various Pregnant Females so (P = Parturition) -E 40 -..

00 1: 30 -

"C 20 0 0 iil 10

t e 2 3 4 Months of Gestation

"1 - 100 E -.. 00 1: - 20

"C 0

..5! al 10

® t e 2 4 6 Months of Gestation 8 10 II

14-1. Length and Time of Placental Takeover for Progesterone Production in Vanous Spec1es

SPECIES

Alpaca Bitch Camel Cow Ewe Goat Llama Mare Queen Rabbit Sow Woman

GESTATION LENGTH

11.4 mo 2 mo (65 days)

12.3 mo 9 mo 5 rna 5 mo

11.3 mo 11 mo 2 mo (65 days) 1 mo

3.8 mo 9mo

TIME OF PLACENTAL TAKEOVER

11.4 mo (none) 2mo (none)

12.3 mo (none) 6-8 mo

50 days 5 mo (none)

11.3 mo (none) 70 days

2 mo (none) 1 mo (none)

3.8 mo (none) 60-70 days

Ve tB oo ks .ir

306 Placentation, Gestation and Parturition

Figure 14-10. Placental Lactogen in Blood Near Termination of Gestation

(From Martal in Reproduction in Man and Mammals) Woman

4000_1 '5:b c - 600 c

CIJ

Q Ewe

0 Somatotropic activity 0 Lactogenic activity

!)0 0 .... u Ill

...J iii .... c

CIJ u Ill 0::

500 400 300 200 100 Cow

0 270 120 ISO 270

Day of Gestation

Parturition is a Complex Cascade of Physiologic Events

Rat

The fetus triggers the onset of parh1rition by initiating a cascade of complex endocrine/biochemical events. The fetal hypothalamo-pihlitary-adrenal axis is obligatory for the initiation of parturition. During the conclusion of gestation, fetal mass approaches the in- herent space limitations of the uterus. This space limita- tion has been considered by some to be the stimulus that causes adrenal corticotropin (ACTH) to be secreted by the fetal pih1itary. The fetal pituitary then stimulates secretion of adrenal corticoids from the fetal adrenal cortex. The elevation of fetal corticoids initiates a cascade of events that cause dramatic changes in the endocrine condition of the dam. These endocrine changes cause two major events to occur: 1) removal of the myometrial "progesterone block," enabling myome- trial contractions to begin and 2) increased reproductive tract secretions, particularly by the cervix.

The three stages of parturition are: • I: initiation of myometrial

contractions (removal ofprogesterone block)

• II: expulsion of the fetus

• III: expulsion of the fetal mebranes

Placental lactogen has both lactogenic actions and soma- totrophic actions. The lac- togenic activity of placental lactogen promotes mammary function in the dam, while the somatotropic activity promotes fetal growth.

Removal of the "progesterone block" occurs because fetal cortisol promotes the synthesis of three enzymes that convert progesterone to estradiol. The conversion pathway is illustrated in Figure 14-11. Progesterone, that is high at the placental interface, is converted to 17a-hydroxyprogesterone by the en- zyme !?a-hydroxylase. Fetal cortisol also triggers the enzyme 17-20 desmolase to convert 17a-hydroxy- progesterone to androstenedione. Androstenedione is converted to estrogen by activation of an aromatase enzyme. This involves aromatization of the A ring of the steroid and removal of the 19 carbon. The conver- sion of progesterone to estradiol accounts, at least in part, for the dramatic drop in progesterone and dramatic elevation of estradiol. The relationship between pro- gesterone and estradiol during gestation is presented in Figure 14-12.

In addition to converting progesterone to es- h·adiol, fetal corticoids also cause the placenta to syn- thesize PGF2a.. The synthesis of PGF 2a helps abolish the "progesterone block." As both estradiol and prosta- glandin become elevated, the myometrium becomes in- creasingly more active and begins to display noticeable contractions. Also, PGF 2a causes the CL of pregnancy to regress, facilitating the decline in progesterone. The drop in progesterone in some species is brought about both by the conversion of progesterone into estradiol and by the luteolytic process brought about by PGF2a· Endocrine events associated with parhrrition are sum- marized in Figures 14-13 and 14-14.

The fetus initiates Stage I of parturition.

Figure 14-11. Conversion of Progesterone to Estradiol as

Parturition Nears Corticoids from the fetus activate 17 a-hydroxylase, 17-20 desmolase and aromatase that convert progesterone to estradio l. This conversion removes the "progesterone block" to myometrial activity.

17 a Hydroxyprogesterone

Androstenedione

CHJ I

)

JJ-SD CH1 I

· ' 1' 0

117: 20 I

l I Aromotase I o)D'"

. As the pressure inside the uterus continues to mcrease, the feh1s in the cow, mare and ewe rotates so

the fi·ont feet and head are positioned to the poste- of the dam (See Figure 14-15). Such a rotation is

tmportant to insure a proper delivery. If the fetus fails to position itself correctly, dystocia (difficult birth) may occur.

. As the levels of estradiol increase, coupled With the e l_evation in levels of PGF2a , the contracting

begms to push the fetus toward the cervix, ap- plymg pressure to the cervix. The endocrine events that pro?1ote the firs t stage of parturition (dilation of the cervtx and entry of the feh1s into the cervical canal) are summarized in Figure 14-14.

Pressure ?n the cervix brought about by in- myometnal contractions activates pressure-

sensttl_ve neurons located in the cervix that synapse in the spmal cord and evenhmlly synapse with oxytocin

Ill c 0

'.P 1.': ..., c Q) v c 0 u N w

"'C c Rl

Placentation, Gestation and Parturition 307

Figure 14-12. Estradiol and Progesterone Profiles During Gestation in the Mare, Cow,

Woman, Ewe and Sow (P = Parturition)

Mare

I Woman I

I Sow I

t 10 20 30 40 so e Weel<s of Gestation

Ve tB oo ks .ir

306 Placentation, Gestation and Parturition

Figure 14-10. Placental Lactogen in Blood Near Termination of Gestation

(From Martal in Reproduction in Man and Mammals) Woman

4000_1 '5:b c - 600 c

CIJ

Q Ewe

0 Somatotropic activity 0 Lactogenic activity

!)0 0 .... u Ill

...J iii .... c

CIJ u Ill 0::

500 400 300 200 100 Cow

0 270 120 ISO 270

Day of Gestation

Parturition is a Complex Cascade of Physiologic Events

Rat

The three stages of parturition are: • I: initiation of myometrial

contractions (removal ofprogesterone block)

• II: expulsion of the fetus

• III: expulsion of the fetal mebranes

The fetus initiates Stage I of parturition.

Figure 14-11. Conversion of Progesterone to Estradiol as

17 a Hydroxyprogesterone

Androstenedione

CHJ I

)

JJ-SD CH1 I

· ' 1' 0

117: 20 I

l I Aromotase I o)D'"

. As the pressure inside the uterus continues to mcrease, the feh1s in the cow, mare and ewe rotates so

the fi·ont feet and head are positioned to the poste- of the dam (See Figure 14-15). Such a rotation is

tmportant to insure a proper delivery. If the fetus fails to position itself correctly, dystocia (difficult birth) may occur.

. As the levels of estradiol increase, coupled With the e l_evation in levels of PGF2a , the contracting

Pressure ?n the cervix brought about by in- myometnal contractions activates pressure-

sensttl_ve neurons located in the cervix that synapse in the spmal cord and evenhmlly synapse with oxytocin

Ill c 0

'.P 1.': ..., c Q) v c 0 u N w

"'C c Rl

Placentation, Gestation and Parturition 307

Figure 14-12. Estradiol and Progesterone Profiles During Gestation in the Mare, Cow,

Woman, Ewe and Sow (P = Parturition)

Mare

I Woman I

I Sow I

t 10 20 30 40 so e Weel<s of Gestation

Ve tB oo ks .ir

308 Placentation, Gestation and Parturition

producing neurons in the hypothalamus (See Figure 14- I 5). Oxytocin, released into the systemic circula- tion, acts to facilitate the myomeh·ial contractility initiated by estradiol and by PGF2u· As the pressure against the cervix continues to increase, so does the oxytocin secretion, and thus the force of conh·action of the myometrial smooth muscle begins to peak. When this occurs, the fetus enters the cervical canal and the first stage of parturition is complete.

Expulsion of fetus (Stage II) requires strong myometrial and abdominal

muscle contractions.

Another important hormone involved in suc- cessful parhrrition is relaxin. Relaxin is a glycopro- tein that is produced by either the corpus luteum or the placenta, depending upon the species. The synthesis of relaxin is stimulated by PGF2a · Relaxin causes a softening of the connective tissue in the cervix and promotes elasticity of the pelvic ligaments. Thus, this hormone prepares the birth canal by loosening the supportive tissues so that passage of the fehts can occur with relative ease.

One of the dramatic effects of estradiol elevation prior to parturition is that it initiates secre- tory activity of the reproductive tract in general and particularly the cervix. As estradiol increases, the cervix and vagina begin to produce mucus. This mucus washes out the cervical seal of pregnancy and thoroughly lubricates the cervical canal and the vagina. Mucus reduces friction and enables the fetus to exit the reproductive tract with relative ease. As myometrial contractions continue to increase, the feet and head of the fehts begin to put pressure on the fetal membranes. When the pressure reaches a certain level, the membranes rupture, with subsequent loss of amniotic and allantoic fluid. This fluid also serves to lubricate the birth canal. As the fetus enters the birth canal, it becomes hypoxic (deprived of adequate levels of oxygen). This hypoxia promotes fetal movement that, in tum, promotes further myometrial contrac- tion. This positive feedback system creates a set of conditions where the time of parhtrition is reduced because an increased strength of contraction follows fetal movement. In a sense, the fehts is controlling its exit from the uterus. The uterine contractions are accompanied by abdominal muscle contractions of the dam that further aid in expulsion of the fetus.

VI 1: 0

"" "'"' 1: Ql u 1: 0 u Ql 1: 0 E "" 0 :r: Ql >

1i r:x:

Figure 14-13. Relative Hormone Profiles in the Cow

During the Periparturient Period

Estrogens

I Prostaglandin

-I 0 -B -6 -4 -3 -2 -I 0 I 2 3 4 5 t Parturition

Days

Note that as fetal cortisol levels rise, P4 levels fall.

In most species, expulsion of the fetal mem- branes quickly follows expulsion of the fetus. Expulsion of the fetal membranes requires that the chorionic villi become dislodged from the crypts of the matemal side of the placenta. This release of the chorionic villi is believed to be brought about by powerful vasoconstric- tion of arteries in the villi. Vasoconstriction reduces pressure and thus allows the villi to be released from the crypts. Obviously in some fonns of placentation, there must be some maternal vasoconsh·iction. For ex- ample, in animals that have hemochorial placentation, matemal blood is adjacent to the fetal placenta. Thus, if vasoconstriction does not occur on the matemal side, hemorrhage is likely.

The duration of parhlrition is variable among species and this variation is summarized in Table 14-2. Extension beyond what is considered to be the normal upper-end duration of parturition constitutes a difficult birth (dystocia). Such prolonged parturition can result in serious complications to both the fetus and the dam.

Placentation, Gestation and Parturition 309

Figure 14-14. Cascade of Events Prompted by Fetal Cortisol

f t FETAL ACTH f /I Fetal cortisol j \

Placental P4 Relaxin enzymes [!iJ I PGF2a I ...,.I .-----------. t / t ....._____+ -l

I Luteolysis t Secretion by <;;?tract

Lubrication

t Myometrial contractions

I+ Pressure f

t Cervical stimulation

t Oxytocin

t Maximum pressure

Pelvic ligament stretching

Ve tB oo ks .ir

308 Placentation, Gestation and Parturition

Expulsion of fetus (Stage II) requires strong myometrial and abdominal

muscle contractions.

VI 1: 0

"" "'"' 1: Ql u 1: 0 u Ql 1: 0 E "" 0 :r: Ql >

1i r:x:

Figure 14-13. Relative Hormone Profiles in the Cow

During the Periparturient Period

Estrogens

I Prostaglandin

-I 0 -B -6 -4 -3 -2 -I 0 I 2 3 4 5 t Parturition

Days

Note that as fetal cortisol levels rise, P4 levels fall.

Placentation, Gestation and Parturition 309

Figure 14-14. Cascade of Events Prompted by Fetal Cortisol

f t FETAL ACTH f /I Fetal cortisol j \

Placental P4 Relaxin enzymes [!iJ I PGF2a I ...,.I .-----------. t / t ....._____+ -l

I Luteolysis t Secretion by <;;?tract

Lubrication

t Myometrial contractions

I+ Pressure f

t Cervical stimulation

t Oxytocin

t Maximum pressure

Pelvic ligament stretching

Ve tB oo ks .ir

31 0 Placentation, Gestation and Parturition

Figure Pressure on the Cervix Causes Oxytocin Release and Subsequent Myometrial Contractions

As the fetus moves through the birth canal , elevated pressure on the cervix stimulates sensory neurons. A neural pathway ter- minates in the paraventricular nu- cleus (PVN) and causes to be secreted from the postenor pituitary lobe. Oxytocin contraction of the myometnum.

Afferent neurons

Hypothalamus

Difficulties in parturi tion usually occur in the second stage (expulsion of the fetus). One cause of dystocia is excessive size of the fetus. Fetal size is controlled by both the dam and the sire. In primiparous dams, it is always advisable to breed females to a male of small body size so that fetal size does not exceed the ability of the female to give birth successfully.

A second cause of dystocia is failure of proper fetal rotation. About 5% of all births in cattle are char- acterized by abnormal positioning of the fetus during parturition. Such abnormal positioning results in dif- ficult births and sometimes impossible presentations/ positions that require caesarean section.

A third cause of dystocia is multiple births in monotocous species. Twins generally cause dystocia. This is because: 1) both twins may be presented simul- taneously, 2) the first fetus is positioned abnommlly and therefore blocks the second or 3) the uterus becomes fatigued by difficult and sustained contractions. A dis- cussion of obstetrical procedures used to correct these problems is beyond the scope of this book, but c .... atfbe researched by consulting the appropriate references at the conclusion of this chapter.

Placentation, Gestation and Parturition 311

Expulsion of fetal membranes (Stage III) requires myometrial

contractions.

Myometrial contractions continue after expul- sion of the fetus although they are not as strong. These contractions are responsible for expelling the placenta. The time required for expulsion of the placenta varies significantly among species. This variation is presented in Table 14-2. Retention of the fetal membranes (also referred to as "retained placenta"), is not uncommon in ruminants, especially dairy cows. This condition will occur in 5-15% of parturitions in healthy dairy cows. The underlying cause of retained placenta appears to be that placental connective tissue is not enzymatically degraded by cotyledonary proteolytic enzymes. Thus, fetal cotyledons remain attached to matemal cotyledons. Retained placenta is rare is mares, sows, bitches and queens.

Table 14-2. Stages and Duration of Parturition Among Various Species

Stage I Stage II Stage III (Mllometrial Contractions/ (Fetal (Fetal Membrane

Cervical Dilation)

Alpaca 2 to 6h 5 to 90 min 45 to 180 min

Bitch 6 to 12h 6h (24h in large litters) most placentas pass with neonate or within 15 min of birth

Camel 3 to 48h 5 to 45 min 40 min

Cow 2 to 6h 30 to 60 min 6 to 12h

Ewe 2 to 6h 30 to 120 min 5 to 8h

Llama 2 to 6h 5 to 90 min 45 to 180 min

Mare 1 to 4h 12 to 30 min 1h

Sow 2 to 12h 150 to 180 min 1 to 4h

Queen 4 to 42h 4 kittens/litter, most placentas pass with 30-60 min/kitten neonate

Woman 8+h 2h 1h or less

Ve tB oo ks .ir

31 0 Placentation, Gestation and Parturition

Figure Pressure on the Cervix Causes Oxytocin Release and Subsequent Myometrial Contractions

Afferent neurons

Hypothalamus

Placentation, Gestation and Parturition 311

Expulsion of fetal membranes (Stage III) requires myometrial

contractions.

Table 14-2. Stages and Duration of Parturition Among Various Species

Stage I Stage II Stage III (Mllometrial Contractions/ (Fetal (Fetal Membrane

Cervical Dilation)

Alpaca 2 to 6h 5 to 90 min 45 to 180 min

Bitch 6 to 12h 6h (24h in large litters) most placentas pass with neonate or within 15 min of birth

Camel 3 to 48h 5 to 45 min 40 min

Cow 2 to 6h 30 to 60 min 6 to 12h

Ewe 2 to 6h 30 to 120 min 5 to 8h

Llama 2 to 6h 5 to 90 min 45 to 180 min

Mare 1 to 4h 12 to 30 min 1h

Sow 2 to 12h 150 to 180 min 1 to 4h

Queen 4 to 42h 4 kittens/litter, most placentas pass with 30-60 min/kitten neonate

Woman 8+h 2h 1h or less

Ve tB oo ks .ir

312 Placentation, Gestation and Parturition

Further PHENOMENA for Fertility The term "caesarean" was derived from the false notion that Julius Caesar was born by removing him from his mother through an incision in the abdominal and uterine wall. His family name, Caesar was derived from the belief that Julius' ancestors (centuries before him) were hom in such a way. The name Caesar is derived from the Latin word "caesus" that means "to cut". The name also fits the way Julius died.

In a number of teleost fishes (fishes with a more or less ossified skeleton) the female incubates the eggs in her mouth and in some species the male does the same. The term "keep your mouth shut" has a special meaning in this species.

In pipe fishes and sea horses the female lays her eggs in a brood pouch of the male and he is responsible for gestation. In fact, several females may lay eggs in one male's brood pouch. The brood pouch offers a special environment for developing offspring and is under the control of prolactin.

Lampreys (a predatory eel) build nests in sandy bottomed sh·eams. They assemble rock walls to slow the water running over the nest. At spawning, they stir up the sand that sticks to the eggs. The sand weights the eggs and prevents them from floating downstream. It also reduces predation. This is mwtherform of attachment that enables successful embryo- genesis.

Infant kangaroos in their mother's pouches nurse from two nipples, and two babies of different ages commonly nurse at the same time. So, the mother kangaroo produces two kinds of milk- on one side, fully rich for the younger and 011 the other side, a sort of skim for the elder.

The most prolific mammal in existence is the tiny J'Odent known as the multimammate rat. One female is capable of producing up to 120 offspring a year if conditions are favorable. This is because she has 24 teats, the most of any female mammal. It is rare that all of them are used but when they are a multimammate population explosion catt occur.

The female Egyptian spiny mouse acts as a midwife to other females. She bites through the umbilical cord and licks the neonates while the mother continues to deliver the litter.

The female African elephant has a gestation period of 1.8 years. The calf weighs about 300 pounds at birth and nurses for about three years.

Durittg the 19th Century, adultery was so feared that the chastity belt was invented. Such belts were devices that were locked around the

genitalia to prevent copulation. It has been recorded that afaitliful wife locked into a chastity belt discovered that she was pregnant some months after her husband had left 011 a crusade. Her husband had the o11ly key. Her pregnancy progressed and eventually the vil- lage blacksmith had to be called in to remove the chastity belt.

During the Middle Ages, prostitution was considered to he an honest and essential pro- fession. This was because prostitution was considered as a means to prevellt adulte1y, homosexual behavior and masturbation. The Church actually condoned prostitution for tlzis reason.

The Mayans believed in a maize god. Since corn was a nutritional staple for these people, they revered it and believed that corn was symbolic of both the male am/female. From a nutritional perspective they believed that corn was nurturing like a breast and that each individual kernel had powerful fertilizing capabilities like spermatozoa. Once the seeds were planted in the earth and the mature com was produced, the cob represented the penis and the husk represented the vagina. Thus, the ear of com was also symbolic of copulation.

Kev References

Arthur, G.H., D.E. Noakes, H. Pearson and T.J. Parkin- son. 1996. Veterinarv Reproduction and Obstetrics. 7th Edition. W.B. Saunders Co. Philadelphia. ISBN 0-7020-1 758-X.

Catchpole, H.R. 1991. "Hormonal mechanisms in pregnancy and parturition" in Reproduction in Domestic Animals. 4th Edition. P.T. Cupps, ed., Academic Press, San Diego. ISBN 0-1 2-196575-9.

Flood, P.F. 199 I. "The development of the conceptus and its relationship to the uterus" in Reproduction in Domestic Animals. 4th Edition. P.T. Cupps, ed., Aca- demic Press, San Diego. ISBN 0-1 2-196575-9.

Fuchs, A.R. and M.J. Fields. 1999. "Parturition, no.!Jhtl- man mammals" in Encvclopedia o(Reproducilon: Vol. 3 p703-7 I 6. Knobil , E. and J.D . Neill, eds. Academic Press, San Diego. ISBN 0- 12-227023- 1.

Ginther, OJ . 1992. Reproductive Biology o{the Mare. 2nd Edition. Equiservices, Cross Plains, WI. Library of Congress Catalog No. 9 1-075595.

Johnston, S.D. M.V. Root, Kustritz and P.N.S. Olson. 200 I. Canine and Feline Theriogenologv. W.B. Saun- ders, Philadelphia. ISBN 0-7216-5607-2.

Morrow, D.A. 1986. Current Therapy in Theriogenol- 2nd Edition. W.B. Saunders Co. Philadelphia.

ISBN 0-7216-6580-2.

Mossman, H.W. 1987. Vertebrate Fetal Membranes. Rutgers University Press, New Brunsw ick. ISBN 0-8135-1132-1.

Thibault, C., M.C. Levasseur and R.H.F. Hunter.eds. I 993. Reproduction in Man and Mammals. Ellipses, Paris. ISBN 2-7298-9354-7.

Placentation, Gestation and Parturition 313

Ve tB oo ks .ir

312 Placentation, Gestation and Parturition

The female Egyptian spiny mouse acts as a midwife to other females. She bites through the umbilical cord and licks the neonates while the mother continues to deliver the litter.

The female African elephant has a gestation period of 1.8 years. The calf weighs about 300 pounds at birth and nurses for about three years.

Durittg the 19th Century, adultery was so feared that the chastity belt was invented. Such belts were devices that were locked around the

Kev References

Arthur, G.H., D.E. Noakes, H. Pearson and T.J. Parkin- son. 1996. Veterinarv Reproduction and Obstetrics. 7th Edition. W.B. Saunders Co. Philadelphia. ISBN 0-7020-1 758-X.

Catchpole, H.R. 1991. "Hormonal mechanisms in pregnancy and parturition" in Reproduction in Domestic Animals. 4th Edition. P.T. Cupps, ed., Academic Press, San Diego. ISBN 0-1 2-196575-9.

Ginther, OJ . 1992. Reproductive Biology o{the Mare. 2nd Edition. Equiservices, Cross Plains, WI. Library of Congress Catalog No. 9 1-075595.

Johnston, S.D. M.V. Root, Kustritz and P.N.S. Olson. 200 I. Canine and Feline Theriogenologv. W.B. Saun- ders, Philadelphia. ISBN 0-7216-5607-2.

Morrow, D.A. 1986. Current Therapy in Theriogenol- 2nd Edition. W.B. Saunders Co. Philadelphia.

ISBN 0-7216-6580-2.

Mossman, H.W. 1987. Vertebrate Fetal Membranes. Rutgers University Press, New Brunsw ick. ISBN 0-8135-1132-1.

Thibault, C., M.C. Levasseur and R.H.F. Hunter.eds. I 993. Reproduction in Man and Mammals. Ellipses, Paris. ISBN 2-7298-9354-7.

Placentation, Gestation and Parturition 313

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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 Immediately following parturition, the female hegins to lactate and enters a period

of reproductive repair called the pue1perium. For a period of time these two processes overlap. During the puerperium uterine involution and return of ovarian function oc- curs. Involution is the reduction in size and "remodeling" of the endometrium so that the uterus can initiate and sustain another pregnancy.

Mammary gland development is initiated prenatally in the f emale fetus and con- tinues through puberty and pregnancy. The anatomy and distribution of mammary glands is diverse among mammals. Accumulation of secretions in the mammmy gland hegins about two weeks before parturition. Lactation provides the neonate with the opportunity to nurse and he nourished with minimal expenditure of energy. It also provides immunoprotection for the neonate because initialmammmy secretions called colostrum contain antibodies that provide passive immunity. Lactation continues until the neonate is weaned. After weaning, the mammary glands undergo involution and retum to a non-secretory state.

The puerperium and lactation are. init iated immediately after parturition and for a period of time these processes occur simultaneously. Lactation is the synthesis, secretion and removal of milk from the mammary gland. The puerperium is the period after parturition when the reproductive tract retums to its nonpregnant condition so that the female may become pregnant again. This chapter will describe the basics of these two important processes. Parturition results in loss of placental function and deterioration of the mater- nal tissue contributing to the placenta. Tissue damage results . During the puerperium damaged reproductive tissues are repaired and ovarian function returns.

The Puerperium

The puerperium begins immediately after par- turition and lasts until reproductive function is restored so that another pregnancy can occur. The time required for complete uterine involution (repair) and ovarian activity to resume in the postpartum female varies sig- nificantly among species (See Table 15-1 ).

The four major events of the puerperium are: • myometrial contractions and expulsion of lochia

• endometria/repair • resumption of ovarian function • elimination of bacterial contamination of the reproductive tract

It must be emphasized that in many polyestrous animals, the shortest possible puerperium is desirable because eligibility for a subsequent pregnancy is of high economic importance. For example, in dairy cows frequent pregnancies are required for maximum lifetime milk yield. In swine and beef cows, the shorter the interval between pregnancies the more offspring are produced and the more efficient the production of meat becomes. Conversely, the longer the puerperium, the longer the delay of a subsequent pregnancy and the less efficient the production process becomes. Figure 15-1 summarizes the events that occur from parturition to the subsequent pregnancy. These events will be described in more detail below.

Reduction in Uterine Size and Volume is Brought About by Myometrial Contractions

Immediately after parturition, the myome- trium undergoes strong repeated contractions. The purpose of these contractions is threefold. First, they facil itate discharge of fluids and tissue debris from the uterus. Secondly, the contractions compress the uterine vasculature and help minimize the possibility ofhemorrhage. Third, myometrial contractions reduce the overall size of the uterus. Of the species presented in this text, timely uterine involution is most important in the postpartum dairy cow. In most species, frequent postpartum suckling occms and oxytocin is secreted (See Figure 15-13). In suckled animals, uterine contrac- tions occur on a frequent basis. In the dairy cow how- ever, the calf is usually removed within 24 hours after parturition and milking takes place only two or three times per day. Consequently, oxytocin episodes are

Ve tB oo ks .ir

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 Immediately following parturition, the female hegins to lactate and enters a period

The Puerperium

The four major events of the puerperium are: • myometrial contractions and expulsion of lochia

• endometria/repair • resumption of ovarian function • elimination of bacterial contamination of the reproductive tract

Reduction in Uterine Size and Volume is Brought About by Myometrial Contractions

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316 The Puerperium and Lactation

Figure 15-1. Major Events From Parturition to Subsequent Conception (Ruminant Model)

Conception

Uterine Involution • .J. Uterine size (length and diameter) • .J. Uterine volume • Expulsion of lochia • Endometrial repair

Table 15-1. Time Required for Uterine Involution and Resumption of Ovarian Activity in Various Species

Species

Alpaca Beef Cow Bitch Camel Dairy Cow Ewe Llama Mare Queen Sow Woman

Time Required for Complete Uterine Involution

20d 30d 90d 30-50d 45-50d 30d 20d 21-28d 30d 28-30d 40-45d

L = Lactation inhibits ovarian activity (See Chapter 7)

T ime Required for Resumption of Ovarian Activity

5-10d 50-60d (L) 150d (A) 25-40d or up to 1 yr (L) 18-25d 180d (SOB) 5-1 0d 5-12d 30d 7d (L) 6-24mo (L) (See Chapter 7)

SOB = Short Day Breeder- ewes giving birth in the spring will not cycle until fall A = Long natural postpartum anestrus (See Chapter 7)

reduced, myometrial contractions are not as frequent and uterine involution can be delayed. In this light, much of the material presented on uterine involution will focus on the dairy cow since delayed uterine in- volution is an important factor limiting fertility in this animal.

Immediately after parturition the uterus un- dergoes rapid but highly coordinated atTophy so that in a relatively short period of time the uterine mass is reduced to its nonpregnant size. In all species, marked size reduction occurs during the first several days af- ter parhrrition. In fact, in the dairy cow, myometrial cell size decreases from 700pm on the firs t day after parturition to a few days later. In most spe- cies, myometrial contractions occur in three to four minute intervals for the firs t several postpartum days. These strong, high frequency myometrial contractions subside within several days. The exact time that these contractions stop depends on the species. The dramatic postpartum size reduction of the uterus in the dairy cow is illustrated in Figure 15-2. '

..-. E

... tl() c

J! QJ c ·;::: QJ ...

::J @

Figure 15-2. Changes in Uterine Length and Weight at

Various Postpartum Days

80 70 60 so 40 30 20 10

0 I s 10 IS 20

Days of postpar tum

The uterine length values here are used in Figures 15-4 through 15-8 to illuslrate approximate size changes. (From Gier, H.T. and G.B. Marion, 1968. Amer. J. Vet. Res. 29: 83-96)

During and After Myometrial Contractions a Bloody Fluid is Discharged fr·om the Tract

Shortly after parturition, a discharge called lochia is expelled from the vulva. Lochia is typically a blood-tinged fluid containing remnants of the fetal placenta and endometrial tissue. L ochial discharge occurs between 2 and 9 days in postpartum dairy cows. An increase in blood and tissue debris in the lochia is nonnal and occurs between 5 and 1 0 days. This is due to the sloughing of caruncular surfaces that leaves vascular "stubs" that leak blood. Lochial discharge is

The Puerperium and Lactation 317

physiologically nomml in all species. However, it is often interpreted by observers to be the result of uterine pathology (especially in the dairy cow). Therefore, the first " instinct" of the reproductive management team is to treat the animal for nonexistent pathology. Unwar- ranted treatment is financ ially wasteful, not effective and often prolongs uterine involution especially if the uterine lumen is invaded (infusion of antibiotics, various solutions or to remove manually retained fetal membranes).

Obviously, with s ignificant myometrial con- tractions occmTing for the first 7 to I 0 days there will be a reduction in the volume oflochia within the uterus. In the dairy cow, up to 2000ml oflochia can be expelled from the uterus during the first two to three days after parturition. By 14 to 18 days, lochial discharge is al- most nonexistent in most cows (See Figure 15-3).

..-. I

E ::l 0 > iii ' ij 0

...1

Figure 15-3. Changes in Lochial Volume at Various

Postpartum Days 1,500

1,000

soo

s 10 IS

Days of postpartum

(From Gier, H.T. and G.B. Marion, 1968. Amer. J. Vet. Res. 29: 83-96)

Caruncular Repair Requires Vasoconstriction, Necrosis and Sloughing of Tissues Followed

by Growth of Surface Epithelium

After separation of the fetal cotyledons from the maternal caruncle (within 8-12 hours after delivery of the neonate) vasoconstriction takes place in the stalk of the matemal caruncle. Necrosis of the caruncular tissue follows. Necrosis is irreversible cell death that leads to sloughing of the caruncular mass leaving ne- crotic tissue in the lochial fluid inside the uterus. Some blood is released from the canmcular stalk generating a blood-tinged fluid. About 5 days after parturition, the caruncles begin to lose their cellular organization and integrity. This results in chunks of the caruncles detaching from the surface of the caruncle leaving remnants of blood vessels exposed to the surface. After the decidual tissue of the caruncle has sloughed into

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316 The Puerperium and Lactation

Figure 15-1. Major Events From Parturition to Subsequent Conception (Ruminant Model)

Conception

Uterine Involution • .J. Uterine size (length and diameter) • .J. Uterine volume • Expulsion of lochia • Endometrial repair

Table 15-1. Time Required for Uterine Involution and Resumption of Ovarian Activity in Various Species

Species

Alpaca Beef Cow Bitch Camel Dairy Cow Ewe Llama Mare Queen Sow Woman

Time Required for Complete Uterine Involution

20d 30d 90d 30-50d 45-50d 30d 20d 21-28d 30d 28-30d 40-45d

L = Lactation inhibits ovarian activity (See Chapter 7)

T ime Required for Resumption of Ovarian Activity

5-10d 50-60d (L) 150d (A) 25-40d or up to 1 yr (L) 18-25d 180d (SOB) 5-1 0d 5-12d 30d 7d (L) 6-24mo (L) (See Chapter 7)

SOB = Short Day Breeder- ewes giving birth in the spring will not cycle until fall A = Long natural postpartum anestrus (See Chapter 7)

..-. E

... tl() c

J! QJ c ·;::: QJ ...

::J @

Figure 15-2. Changes in Uterine Length and Weight at

Various Postpartum Days

80 70 60 so 40 30 20 10

0 I s 10 IS 20

Days of postpar tum

The uterine length values here are used in Figures 15-4 through 15-8 to illuslrate approximate size changes. (From Gier, H.T. and G.B. Marion, 1968. Amer. J. Vet. Res. 29: 83-96)

During and After Myometrial Contractions a Bloody Fluid is Discharged fr·om the Tract

The Puerperium and Lactation 317

..-. I

E ::l 0 > iii ' ij 0

...1

Figure 15-3. Changes in Lochial Volume at Various

Postpartum Days 1,500

1,000

soo

s 10 IS

Days of postpartum

(From Gier, H.T. and G.B. Marion, 1968. Amer. J. Vet. Res. 29: 83-96)

Caruncular Repair Requires Vasoconstriction, Necrosis and Sloughing of Tissues Followed

by Growth of Surface Epithelium

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318 The Puerperium and Lactation

Fig'!re 15-4. Bovine Reproductive Organs- Day 1 Postpartum

Ovaries- There are no functional structures on the right ovary. The left ovary contains two cor- pora lutea (arrows 1 and 2) indicating a double ovulation. Only one con- ceptus developed. There is no evidence of follicular development on either ovary.

Cervix (caudal view)- The brownish mucus (M) is a remnant of the cervical seal of pregnancy. Mu- cosal hemorrhaging (MH) has resulted from abrasive trauma to the cranial vagina (CV) , fornix vagina (FV) and portions of the cervix (CX) during expulsion of the fetus. A stainless steel rod has been positioned in the cervical canal to provide spatial reference in Figures 15-4 through 15-8.

Uterus. in-situ- This photograph and all subsequent in-situ pho- tographs were taken from post- partum dairy cows in which the viscera was removed so that the cranial surface of the reproduc- tive tract can be viewed. Here, the approximate overall length of the uterus is 85cm. The right uter- ine horn (RUH) is larger than the left uterine horn (LUH) because the right uterine horn housed the fetus. The broad ligament (BL) and rectum (R) are obvious.

Uterine Interior-The uterus contains many large caruncles (C) that consist of intact tissue. Only a few caruncles have started to undergo necrosis (N) as judged by the blackened regions. There is very little lochia (L) pres- ent. The caruncular stalks (CS) are quite long and house the vasculature that supplied the maternal cotyledon with blood during pregnancy. The enlarged photograph illustrates a caruncular crown (CC) that has been sliced open. The incision has extended into the center of the caruncular stalk (CS). The entire layer of decidual tissue (DT) will soon slough into the uterine lumen because of vasoconstriction of the caruncular arterioles.

CLP \

The Puerperium and Lactation 319

Figure 15-5. Bovine Reproductive Organs- Day 4 Postpartum

RIGHT OVARY

..... -= .... - X Cervix (caudal view)- Lo- chia (L) has been expel led through the cervix (CX) and it has pooled in the ventral reg ion of the cranial vagina (CV) here. In the live cow, lochia would be discharged to the exterior.

LEFT OVARY

Ovaries- A regressing CL from the pregnancy (CLP) is present on each of the right and left ova- ries indicating a double ovulation. Only one conceptus developed. A regressing CL (RCL) from a cycle prior to the pregnancy is present on the right ovary. There is no evidence of follicular development in either ovary.

Uterus. in-situ- The most dramatic reduction in uterine size occurred between day 1 and day 5. Uterine length is reduced from about 85cm (day 1) to 58cm (day 4 ). The left uterine horn (LUH) housed the conceptus during pregnancy and is larger than the right uterine horn (RUH). The broad ligament (BL) and rectum (R) can be observed. Uterine Interior- Much of the decidual tissue of the caruncles (C) has sloughed into the uterine lumen along with blood and other fluids forming lochia (L). This material is normally expelled from the uterus. The pres- ence of lochia (L) in the uterus and its discharge from the vulva is normal.

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318 The Puerperium and Lactation

Fig'!re 15-4. Bovine Reproductive Organs- Day 1 Postpartum

CLP \

The Puerperium and Lactation 319

Figure 15-5. Bovine Reproductive Organs- Day 4 Postpartum

RIGHT OVARY

LEFT OVARY

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320 The Puerperium and Lactation

Figure 1·5-6. Bovine Reproductive Organs- Day 1 0 Postpartum

Cervix (caudal view)- Sites of mucosal hem- orrhaging (MH) are still apparent in the floor of the cranial vagina (CV) in this cow. The cervix (CX) has decreased in diameter be- cause its overall tone has increased.

LEFT - -oVARY '

LEFT ---.. OVARY -·

Ovaries- The right ovary contains several corpora albicantia (CA) and a few antral foll icles (AF). The left ovary contains the regressing corpus luteum of pregnancy (CLP). It also contains an antral follicle (AF) indicating that a new follicular phase is beginning.

Uterus. in-situ- The uterus continues to undergo a reduction in size (41 em). The left uterine horn (LUH) remains larger than the right uterine horn (RUH) because the left uterine horn housed the conceptus. The rectum (R) and broad ligament (BL) are visib le. Uterine Interior- The decidual tissue of each caruncle has been sloughed into the uterine lumen. Some lochia (L) is still present but it is more viscous and mucus-like. The endometrial and caruncular epithelium is now beginning to cover the surface. The enlarged photograph illustrates the marked reduction in size of the caruncle (compare to days 1 and 4 ). The caruncular stalk is nonexistent. This size reduction is a function of vasoconstriction of the caruncular blood vessels (BV).

The Puerperium and Lactation 321

Figure 15-7. Bovine Reproductive Organs- Day 15 Postpartum CERVIX

Cervix (caudal view)- Strands of clear mucus (M) secreted by the cervix (CX) and cranial vagina (CV) indicate that th is cow is entering her first foll icular phase after par- tu rition (S ee ovaries). FV = Fornix vagina.

Ovaries- The right ovary con- tains the regressing corpus luteum of pregnancy (CLP). It also contains a developing antral follicle (AF). The left ovary con- tains a large antral follicle (AF) indicative of the first postpartum follicular phase. The follicles present produce estradiol that causes secretion of mucus by the cervix and cranial vagina.

Uterus. in-situ- The right uterine horn (RUH) housed the conceptus and is larger than the left uterine horn (LUH). Continued reduction in size is evident. The broad ligament (BL) and the rectum (R) can be observed. The dark coloration at the tips of the uterine horns represents pooling of blood following exsanguination of the cow. Uterine Interior- The caruncles (C) have decreased further in size and are almost completely covered in mucus. Lochia is almost nonexistent and a puss-like material (P) is present in localized areas. The presence of puss is normal and reflects phagocytosis of deteriorating tissue by leukocytes. Carun- cular blood vessels (arrows) can be seen as small knot-like structures in the incised caruncles. M = Myometrium.

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320 The Puerperium and Lactation

Figure 1·5-6. Bovine Reproductive Organs- Day 1 0 Postpartum

LEFT - -oVARY '

LEFT ---.. OVARY -·

The Puerperium and Lactation 321

Figure 15-7. Bovine Reproductive Organs- Day 15 Postpartum CERVIX

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322 The Puerperium and Lactation

Figure 15-8. Bovine Reproductive Organs- Day 20 Postpartum

RIGHT OVARY

Tlte photographs in Figures 15-4 through 15-8 were part of an Honors Thesis entitled "A Full Color Photographic Description of Postpartum Uterine Involution in the Daily Cow" submitted to Washington State University Honors College by Christina M. Davis, Spring 2002. The Honors project was sponsored by Current Conceptions, Inc.

Cervix (caudal view)- The cranial vagina (CV} and fornix vagina (FV) are free of hemorrhagic foci. The color, diameter and tone of the cervix (CX} are normal. Mucus is present coating the mucosal surfaces.

-..CA

Ovaries- The right ovary contains the regressing corpus luteum of pregnancy (CLP) and an antral follicle (AF). The antral follicle is not observed in the incised ovary because it is out of the plane of sec- tion. The left ovary contains several antral follicles (AF} indicating this cow is entering her first postpartum follicular phase. A corpus albicans (CA} represents a corpus luteum from a cycle prior to the previous pregnancy.

Uterus. in-situ- The uterine horns continue to decrease in size and have almost returned to their normal nonpregnant size. The right uterine horn (RUH) remains larger than the left uterine horn (LUH} because the right uterine horn housed the concep- tus. The broad ligament (BL} and the rectum (R} can be readily observed. Uterine Interior- Caruncles (C) are approaching the size of those normally seen within the nonpregnant uterus. A cross-section of an incised caruncle shows the mass of blood vessels (BV) between the myometrium (M) and the epithelium (E) covering the caruncle. The fluid within the uterine lumen is predominantly mucus.

the uterine lumen the caruncle begins to undergo re- pair and is eventually covered again with endometrial epithelium.

Figure 15-9. Changes in Caruncular Height at Various

Postpartum Days e s u ._, i: 4 00

'ijj J: 3 ,.... ns a 2 c ::l ,.... ns u

Days Postpartum

{From 1978)

At the same time caruncular repair is taking place, the intercaruncular endometrial surfaces also undergo repair. In general, the epithelium of the inter- caruncular area of the endometrium repairs at a faster rate than do the caruncles. The repair of the intercarun- cular endometrium is generally complete by the eighth postpartum day. The delay in caruncular repair, when compared to the intercaruncular epithelium is associ- ated with the large mass of the canmcular tissue that must undergo necrosis and sloughing before surface epithelial repair can take place.

Postpartum Bacterial Contamination of the Uterus is Common in Most Domestic Animals

Generally, parturition in domestic animals oc- curs in a non-sterile environment. As a result, bacte- rial contamination of the reproductive tract, especially the uterus is an inevitable sequela to parturition. The postpartum reproductive tract (containing lochia) is an ideal environment for the growth of bacteria. Even though myometrial contractions tend to remove the large volume of lochia produced in some species, bacte- rial growth can continue. It must be emphasized that bacterial contamination is not always associated with pathology. Nom1al postpartum events tend to eliminate the bacterial flora within a reasonable time. As you recall, elevated estradiol promotes leukocytosis in the uterus and elsewhere in the reproductive tract. Thus, a high degree of phagocytosis can be observed in the postpartum reproductive tract as a result of relatively high postparhnn estradiol concentrations that exist for a few days.

The Puerperium and Lactation 323

In some instances, high numbers of bacteria can ove1whelm the natural defense mechanisms re- sulting in postpartum uterine infection. Conditions that predispose the uterus to infections are: retained fetal membranes, dystocia and delay in lochial expul- sion brought about by weak myometrial contractions. Regardless of the cause, failure to eliminate bacterial contamination will: 1) prolong uterine involution; 2) prolong the puerperium and 3) delay subsequent preg- nancies. Treatment of uterine infections is controversial. There is little evidence that supports the effectiveness of infusing the uterus with various pharmaceuticals in dairy cows. The single most important nahtral factor that aids in elimination of bacterial contamination is a return to cyclicity (estrus) so that estradiol concentra- tions will be elevated.

Photographic descriptions of the changes that occur in the uterus, caruncles, cervix and ovaries of the dairy cow during the first 20 days of the puerpe- rium are presented in Figures 15-4 through 15-8. The specimens were obtained from cows that were defined as clinically normal as judged by palpation per rectum by the veterinarian servicing the herd. All cows gave birth to a single calf. To compare these various days of involution to completely involuted organs, please consult the figures in Chapter 2.

Lactation

Lactation ensures that the neonatal mammal does not have to obtain food on its own. Instead, the dam is responsible for consuming all of the nutritional raw materials and transforming these into a highly nutritious secretion called milk. The neonate benefits from this synthetic and secretory process because its only behavioral requirement in the early postnatal period is suckling the dam. Some animals have been domesticated and selected so they produce quanti- ties of milk that far exceed that needed to nourish the young. The dairy cow is the dominant producer of milk for human consumption. However, goats, sheep, water buffalo, camels and mares are also considered important for their milk producing ability in some parts of the world. The immense milk producing ability of the modern dairy cow has provided a huge variety of dairy products that contribute to a multi-billion dollar industry in the western world. In this light, much of the infom1ation provided in this section will be about the dairy cow. However, the basic principles apply to most mammals. The development of the mammary gland (mammogenesis), anatomical diversity and milk ejection from the gland will be the priority topics in the remainder of this chapter.

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322 The Puerperium and Lactation

Figure 15-8. Bovine Reproductive Organs- Day 20 Postpartum

RIGHT OVARY

-..CA

the uterine lumen the caruncle begins to undergo re- pair and is eventually covered again with endometrial epithelium.

Figure 15-9. Changes in Caruncular Height at Various

Postpartum Days e s u ._, i: 4 00

'ijj J: 3 ,.... ns a 2 c ::l ,.... ns u

Days Postpartum

{From 1978)

Postpartum Bacterial Contamination of the Uterus is Common in Most Domestic Animals

The Puerperium and Lactation 323

Lactation

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324 The Puerperium and Lactation

Mammary Glands are Sophisticated Sweat Glands

Mammary glands arise in the developing embryo along two lateral lines on the ventral surface of the developing conceptus. These lines are slightly thickened ridges of epidermis (skin) and are called mammary ridges (See Figure 15-10). The mammary ridges extend from the axillary region (armpit) of the conceptus to the inguinal region. The number of mam- mary glands that develop from the mammary ridges depends on the species. For example, animals like the pig, dog and cat have a series of individual glands that develop at predictable positions along the entire path of the mammary ridges. In contrast, animals like the human and elephant have paired mammary glands that develop from the thoracic portion of the mammary ridges. Animals like the cow, mare and goat have mam- mary glands that develop from the inguinal region of the mammmy ridge. The diversity among mammals in gland number, anatomic location and teat morphology is presented in Figure 15-11.

The thickened epidermal epithelium creating the mammary ridges gives rise to the primary mam- mary bud (See Figure 15-1 0). The primmy mammary bud pushes into the underlying dermis as it grows. Continued growth results in secondary mammary buds that fonn bud protrusions away from the primmy bud. These seconda1y buds then lengthen and branch throughout the remainder of embryonic development. Finally, these branched buds begin to canalize form- ing tiny ducts in the center of each bud. Each bud then becomes a duct with a lumen. At birth, the mammary glands consist oflactiferous ducts that open into larger ducts and empty to the exterior of the mammary gland through a teat or nipple (See Figure 15-1 0).

Postnatal changes in the mammary gland occur:

• between birth and puberty • between puberty and pregnancy • during pregnancy • during lactation • during involution

Postnatal Growth of the Mammary Gland is Endocrine Mediated

Complete anatomical development of the mammary gland coupled with the ability to synthesize and secrete milk does not occur until the female has reached puberty, becomes pregnant and gives birth to offspring.

Between Birth and Puberty, Mammary Growth is Isometric

Between birth and puberty the mammary gland experiences isometric gr·owth (at the same rate as the other tissues). In other words, there is no noticeable enlargement of the mammary glands when compared to the rest of the body.

Mammary Glands Grow Significantly Between Puberty and Pregnancy

After the onset of puberty, the mammary gland begins to grow at a rate that is disproportionately faster than normal body growth. This type of growth is referred to as allometric growth. During repeated estrous cycles, a duct and alveolar framework is con- structed within the mammary gland. This framework provides the cellular basis for future milk synthesis. During the first several estrous (or menstrual) cycles, the ducts begin to branch and their diameter increases under the influence of estradiol. Under the influence of progesterone (during the luteal phase), the terminal portions of each branch begin to form the initial portions ofthe alveoli. The alveoli fom1 the functional secretmy elements of the mammaty gland (See Figure 15-13). Estradiol alone will cause some duct development but more complete and rapid duct development occurs in the presence of prolactin and growth hormone (somatotropin). Both of these hormones increase dur- ing the onset of puberty. Repeated cyclic exposure of the mammary cells to estrogen and progesterone can stimulate mammogenesis to proceed only so far. The mammary framework formed between puberty and pregnancy needs fuh1re endocrine input during the gestational period for complete development.

Final Mammary Development Occurs During Pregnancy

Complete alveolar development in the dam takes place during the last trimester of pregnancy. During this time the terminal alveoli begin to grow into bunches called lobules. A lobule would be analogous to a group of grapes on a single stem among an entire bunch of grapes (See Figure 15-13 ). A group oflobules that emp-

The Puerperium and Lactation 325

Figure 15-10. Prenatal Mammogenesis

Primary mammary

bud

Secondary mammary

bud

I Secondary mammary bud I

Canalization

Lactiferous ducts

Myoepithelial ---1 cells

Mammary Ridges Mammary ridges are th ickened epider- mal tissue that give rise to the mammary gland.

Primary Mammary Bud The thickened epidermal tissue begins to develop inward and penetrate into the mesenchyme (dermis).

Secondary Mammary Bud The primary mammary bud begins to send out branches that further penetrate into the dermis.

Canalization The fingerlike secondary buds begin to lengthen and branch out. Finally they be- gin to form canals or channels (canaliza- tion) that will form the duct system of the gland. Myoepithelial cells surround the terminal portions of the developing gland.

tsl

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324 The Puerperium and Lactation

Mammary Glands are Sophisticated Sweat Glands

Postnatal changes in the mammary gland occur:

• between birth and puberty • between puberty and pregnancy • during pregnancy • during lactation • during involution

Postnatal Growth of the Mammary Gland is Endocrine Mediated

Between Birth and Puberty, Mammary Growth is Isometric

Mammary Glands Grow Significantly Between Puberty and Pregnancy

Final Mammary Development Occurs During Pregnancy

The Puerperium and Lactation 325

Figure 15-10. Prenatal Mammogenesis

Primary mammary

bud

Secondary mammary

bud

I Secondary mammary bud I

Canalization

Lactiferous ducts

Myoepithelial ---1 cells

Mammary Ridges Mammary ridges are th ickened epider- mal tissue that give rise to the mammary gland.

Primary Mammary Bud The thickened epidermal tissue begins to develop inward and penetrate into the mesenchyme (dermis).

Secondary Mammary Bud The primary mammary bud begins to send out branches that further penetrate into the dermis.

tsl

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326 The Puerperium and Lactation

• Cow • Camel

• Ewe • Goat • Mare

• Sow

Figure 15-11. Diversity in Anatomical Position, Number and Teat Morphology Among Mammals

Inguinal I J

Inguinal

Camel 2 canals per teat

(camel)

Cow

I canal I cistern per teat

(cow, ewe, goat)

2 ducts per teat

(mare, sow)

The Puerperium and Lactation 327

Figure 15-11. Diversity in Anatomical Position, Number and Teat Morphology Among Mammals

• Rat • Mouse • Rabbit • Bitch • Queen

• Primates • Elephant

I Thoracic I

/ ( \ Primate

ties into a common duct is called a lobe. During the final trimester of pregnancy, the lobulo-alveolar structur·es develop to the point where they represent nearly 90% of the cellular mass of the mammary gland at parturi- tion. Prolactin, adrenal cortical hormones and placen- tal lactogen are important in allowing the mammmy epithelium to synthes ize milk. As seen in Chapter 14, all ofthese bonn ones increase dramatically just before the time of parturition . The induction of parturition is carefully timed with the onset of the mammary gland's abil ity to secrete copious quantities of milk so that the neonate has immediate access to mi lk.

Elephant (Elephant photograph courtesy of Dr. Janine L. Brown, Con- servation & Reasearch Center, National Zoological Park)

5 - 6 ducts per teat

(bitch, queen)

8- 10 ducts per nipple/ teat

(primate, elephant)

Lactation Provides Immunoprotection and Nutrition for the Neonate

The first secretions from the mammary gland (called colostrum) are critical to neonatal survival be- cause the milk from the dam contains immunoglobulins (antibodies). These immunoglobul ins are ingested by the neonate and are h·ansported unaltered by the cells of the gut mucosa to provide passive immunity. In ruminants (and other animals) with an epitheliocho- rial placenta, maternal immunoglobulins cannot be transferred in-utero because the placenta is a baiTier. Thus, ingestion of colosh·um soon after birth provides necessary immunologic protection for the newborn. In

Ve tB oo ks .ir

326 The Puerperium and Lactation

• Cow • Camel

• Ewe • Goat • Mare

• Sow

Figure 15-11. Diversity in Anatomical Position, Number and Teat Morphology Among Mammals

Inguinal I J

Inguinal

Camel 2 canals per teat

(camel)

Cow

I canal I cistern per teat

(cow, ewe, goat)

2 ducts per teat

(mare, sow)

The Puerperium and Lactation 327

Figure 15-11. Diversity in Anatomical Position, Number and Teat Morphology Among Mammals

• Rat • Mouse • Rabbit • Bitch • Queen

• Primates • Elephant

I Thoracic I

/ ( \ Primate

Elephant (Elephant photograph courtesy of Dr. Janine L. Brown, Con- servation & Reasearch Center, National Zoological Park)

5 - 6 ducts per teat

(bitch, queen)

8- 10 ducts per nipple/ teat

(primate, elephant)

Lactation Provides Immunoprotection and Nutrition for the Neonate

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328 The Puerperium and Lactation

conh·ast, humans and other animals with hemochorial placentation have placental transfer of immunoglobulins from the dam to the fehts. The feh1s is thus born with at least partial passive immunity. Immunoglobulins in milk are still important to neonatal immunoprotection in primates.

Breastfed infants suffer fewer ear infections, respiratory infections and gastrointestinal disorders compared to infants who are formula fed. Early feed- ing in the neonatal period can have lifelong impact. Women who breastfeed their babies have a lower risk of breast cancer. Adults who were breastfed as infants have a lower incidence of obesity, cardiac disease and Type I diabetes compared to those who received for- mula. Calves fed milk have higher growth rates and lower morbidity and mortality compared to calves fed milk-replacer. In addition, calves that grow faster during the milk-feeding period can produce more milk during their lifetimes.

Colostrum is provided for a brief period (2 to 3 days) and then milk composition remains relatively constant for the remainder of the lactation. During the course of lactation, milk synthesis increases and then peaks shortly after parh1rition. After the secretory peak, the synthetic rate decreases and this generally coincides with the time of weaning. It is important to recognize that growth of the neonate is directly proportional to milk protein production by the dam. In some instances, failure of the neonate to grow can be due to mastitis (inflammation of the mammary gland) or agalactia (failure to synthesize milk). It should be emphasized that in some breeds of sheep and goats the reproductive goal is to produce triplets and quadruplets. This goal conflicts with the anatomy/lactation ability of the dam since these species have only two teats. Nutrition of the neonate may thus be compromised since there may not be enough milk to serve the nutritional demands of the offspring.

Involution is the Return to a Nonsecretory State

As the need for milk as the sole nutritional source begins to decrease, the neonate begins to suckle less frequently. Consequently, there is a buildup of pres- sure within the manunmy gland causing the secretory cells to become less and less functional. This phenom- enon is called pressure atrophy. Pressure atrophy is such a powerful force that milk synthesis can be stopped in just a few days if the intramammaty pressure is al- lowed to buildup suddenly. The milk synthesis occur- ring in the alveolar epithelium decreases to the point that the cells undergo almost complete atrophy. Secre- tory cells will remain nonfunctional until a subsequent pregnancy. With a subsequent pregnancy, prolactin,

adrenal cortical hormones and placental lactogen will restimulate alveolar cells to produce milk for another lactation.

During involution, immune cells such as lym- phocytes and macrophages invade the mammary tissue. Mammary involution is a critical process because it allows the mammaty gland to recover and develop new secretory tissue for a subsequent lactation. Changes in the tissue mass of the mammary gland as a function of reproductive stage are presented in Figure 15-1 2.

Milk Contains Hormones and Growth Factors

Many substances that are found in blood can also be found in milk. Thus, milk naturally contains hormones and growth factors that are derived from the blood of the lactating female. Also, exogenous materi- als such as alcohol, drugs, antibiotics, etc. can be found in milk if they are consumed by the dam. Before the advent of controlled nutrition in dairy cattle, it was not uncommon for milk to have an onion-like flavor in the spring because cows grazed in pastures and consumed wild onions growing there. Chemicals causing the onion flavor pass directly into the milk because they are lipid soluble.

Protein hormones such as prolactin, GnRH, growth hom1one (somatotropin), thyroid hormone (thy- roxine) and their releasing factors have been identified in milk. It should be emphasized that protein hormones have little or no physiological effect on the neonate (or the consumer) because they are hydrolyzed into amino acids in the gash·ointestinal tract and therefore lose their biologic activity.

All steroid hormones can be found in milk. The concentration of estrogen and progesterone in milk reflects cyclic hormone production by the ovaries and is highly conelated with blood concentrations. Such a phenomenon enables progesterone to be easily assayed in milk to detem1ine the reproductive status of the fe- male. Cowside ELISA teclmology enables progesterone concentrations in mille to be determined. Procedures to assay progesterone at each milking through the use of "in-line" assay technology in the milking parlor is a worthy research and development pursuit. The concept would utilize a small sensor in each milking machine that can transduce the progesterone concentration into an electrical signal that could be transferred to the com- puter. The development of such technology would en- able the producer to determine whether a cow is cycling, the stage of the estrous cycle, the pregnancy stah1s and some fom1s of ovarian pathology (e.g. cystic ovarian disease) for each cow on a daily basis. The availability of such technology would revolutionize reproductive management of dairy cows.

The Puerperium and Lactation 329

Figure 15-12. Changes in the Mammary Gland as a Function of Reproductive Stage

(Modified from Mepham. 1987. Phvsio/oqy of Lactation)

' Stromal Tissue

1st Pregnancy

2nd Pregnancy

2nd Lacation

The mammary gland undergoes continuous change from prenatal life through subsequent lactations. During pubertal onset the ductal and secretory tissue of the mammary gland increases. During the first pregnancy these tissues continue to increase but at a faster rate. At the time of parturition, the secretory tissue mass is high and continues to increase until it peaks shortly after parturition during the fi rst lactation. At the conclu- sion of the fi rst lactation (either weaning or drying-off in the dairy cow) the secretory tissue mass decreases significantly (mammary involution, INV). During the second pregnancy and lactation secretory tissue and ductal tissue increases significantly. Following lactation a second involution (INV) takes place.

Growth Factors in Milk May Provide New Insights to Neonate Health

It is known that a number of growth factors are present at high levels in colostrum. Colostrum is the fi rst milk produced after parturition and contains antibodies to provide the neonate with passive immu- nity. These growth factors mirror the profile of immu- noglobulins secreted into the colostrum. Researchers have hypothesized that the accumulation of growth factors in colostrum evolved to promote neonatal growth and development. Examples of growth factors found in colostrum are Insulin Like Growth Factors I & 2 (IGF1&2), Epidermal Growth Factor (EGF) and Transforming Growth Factor a and b (TGF-a, TGF-b). Most of the discoveries related to the presence of these growth factors in milk are relatively recent. Since growth factors are present in milk and have significant biologic activity, two outcomes could be important. First, the discovery that these growth facto rs exist opens a new avenue of study implicating mammary secretions in neonatal health and development that go

beyond simply meeting nutritional needs. Secondly, there must be some molecular protection mechanism for these growth factors that prevents digestion by the gastTointestinal tract. Better understanding in both areas could open doors regarding neonatal health and growth and protection mechanisms for various proteins.

Peptides are Physiologically Derived from Milk Proteins

Over 15 physiologically active peptides are derived from milk proteins. These peptides have been implicated in controlling blood pressure (antihyper- tensive}, prevention of blood clots (antithrombotic) and activating the immune system (immunostimula- tion). Opioid peptides from milk proteins (caseins and lactalbumin) have morphine-like activity. Some of these "casomorphins" are believed to prolong gastrointesti- nal transit time by inhibiting gut moti lity. Such an effect is antidianheal. Further, the dynamics of amino acid transpori and induction of insulin and somatostatin production may be a function of these casomorphins.

[li] I

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328 The Puerperium and Lactation

Involution is the Return to a Nonsecretory State

adrenal cortical hormones and placental lactogen will restimulate alveolar cells to produce milk for another lactation.

Milk Contains Hormones and Growth Factors

The Puerperium and Lactation 329

Figure 15-12. Changes in the Mammary Gland as a Function of Reproductive Stage

(Modified from Mepham. 1987. Phvsio/oqy of Lactation)

' Stromal Tissue

1st Pregnancy

2nd Pregnancy

2nd Lacation

Growth Factors in Milk May Provide New Insights to Neonate Health

Peptides are Physiologically Derived from Milk Proteins

[li] I

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330 The Puerperium and Lactation

One additional proposed function of casomorphins is that they produce an analgesic effect causing drowsi- ness and sleep in infants. While little is known about the physiologic activity of these milk protein derived peptides, the fact that many of these materials have distinct phannacological effects opens new doors for the potential use of milk in a therapeutic sense.

Pharmaceutical Proteins are Secreted in Milk of Transgenic Animals

By employing genetic engineering techniques, it is now possible to "genetically engineer" a mam- mary gland that would secrete materials that can have significant therapeutic effects on the consumer beyond the known nutritional effects of milk. For example, recombinant human antithrombin purified from the milk of transgenic goats has been approved for human use by the U.S. Food and Drug Administration and the European Medicines Agency to treat a rare clotting disorder. The amount of this dmg obtained from one goat in a year is equivalent to that from 90,000 human blood donations. A second product, recombinant human C I inhibitor, purified from rabbit milk and used to treat hereditaty angioedema, has been approved for use in the European Union. Other phammceutical proteins se- creted in milk from transgenic animals and cunently in FDA clinical trials include fibrinogen, malaria antigen, albumin and several other clotting factors. Exploiting the mammary gland of transgenic animals as an organ to synthesize and secrete pharmaceutical proteins holds great promise as health promotion interventions.

Milk Ejection Transfers Milk from the Mammary Alveoli into the Ducts

Milk ejection is the active transferofmilk from the alveoli and alveolar ducts into the larger mammary ducts, the cisterns and into the teat or nipples where it can be removed by the suckling neonate. Milk ejec- tion should not be confused with milk secretion. Milk is synthesized and secreted by the alveolar cell into alveolar lumina. Prior to suckling (or milking) milk is predominately located in alveolar lumina and the fine ducts draining the alveoli. Milk stays in these ana- tomical regions because there is a strong resistance to milk flow in such a small diameter network (a fonn of capillary action causing retention of the milk). Between sucklings (ormilkings) 70% to 80% of all secreted milk is located within the lumina of the alveoli and small ducts of the mammary gland. Therefore, an active mechanism for removal of this large quantity of milk is necessaty so that the neonate can have access to it through suckling.

Milk ejection requires:

• sensory activation (auditory, tactile and visual)

• neural activation of the hypothalamus

• o:A.ytocin release into the blood

• contraction oftlze myoepithelial cells

• mechanical transfer of milk from alveoli into ducts and finally into the teat/nipple

Milk ejection is an active neuroendocrine reflex involving sensmy neurons in the teat or nipple, release of oxytocin from the neurohypophysis and contraction of myoepithelial cells that surround each alveolus and some of the ducts . The ejection process results in a rapid transfer of milk from the alveolus and smaller ducts into the larger ducts and cisterns of the mammary gland. Myoepithelial cells are spindle shaped contractile cells that surround each alveolus in a mesh-like fashion (See Figure 15-13). Myoepithelial cells are very similar in structure to smooth muscle cells. The process of milk ejection is also referred to as " milk letdown." Efficient and timely removal of milk from the mammary gland is important not only for extraction of milk by the neonate, but also is an important part of the milk harvest to pre- vent pressure atrophy. In general, the more frequently milk is removed, the less the pressure atrophy and greater the quantity of milk that can be secreted.

Tactile stimulation of the teat or nipple is the primary sensory "driver" for milk ejection. In addition to direct tactile stimulation of the teat or nipple, sounds of the neonate (or the milking parlor), visual sight of the newbom or a milking faci lity can stimulate release of oxytocin from the neurohypophysis. Release of oxytocin is brought about by afferent nerve fibers that synapse with oxytocin synthesizing neurons in the para- ventricular and the supraoptic nuclei. When sufficient frequency of stimulation has been accomplished, nerves in the two nuclei begin to fire and release oxytocin from their tern1inals located in the neurohypophysis. Oxytocin is then secreted into the blood and enters the systemic circulation of the dam. The physiology of milk ejection is presented in Figure 15- .13.

The myoepithelial cells within the mammary gland have receptors for oxytocin and contract imme- diately upon exposure to it. When myoepithelial cells contract they cause the diameter of the alveolus to be greatly reduced. Thus, milk is ejected into larger ducts and is transferred into the larger spaces and finally into the teat or nipple.

The Puerperium and Lactation 331

Figure 15-13. The Anatomy and Physiology of Milk Ejection

The milk ejection mechanism is initiated by suckling (1 ). The teat contains sensory neurons and impulses from these neurons travel through afferent nerves (2) to the hypothalamus. Nerves in the paraventricular nuclei are stimulated by these afferent neurons and the terminals in the pos- terior lobe of the pituitary (3) release oxytocin. Oxytocin then enters the blood and is delivered to the mammary gland (4 ). The target cells for oxytocin are the myoepithelial cells that surround the alveolus. Contraction of the myoepithelial cells (5) causes milk to be "squeezed" out of each individual alveolus into small ducts and then into larger ducts. The net effect of simultaneous contraction of the myoepithelial cells throughout the entire mammary gland is to deliver milk to the large ducts and the gland cistern so that it is available for removal by the neonate.

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330 The Puerperium and Lactation

Pharmaceutical Proteins are Secreted in Milk of Transgenic Animals

Milk Ejection Transfers Milk from the Mammary Alveoli into the Ducts

Milk ejection requires:

• sensory activation (auditory, tactile and visual)

• neural activation of the hypothalamus

• o:A.ytocin release into the blood

• contraction oftlze myoepithelial cells

• mechanical transfer of milk from alveoli into ducts and finally into the teat/nipple

The Puerperium and Lactation 331

Figure 15-13. The Anatomy and Physiology of Milk Ejection

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332 The Puerperium and Lactation

Further PHENOMENA for Fertility

"Bedroom Talk: R eproductive Physiology Style"

by Ruth Loomis

Hey honey, wake-up and quit your snoring I think I can fee/my E1 /eve/s soaring. My ovary is primed for the LH surge, Come 011, wake-up, I've got tlte urge!

I'm certain this egg is ripe for fertilization, But, in case you '•'e forgottell, that does require i11semination! Ami I recall, it's bee11 nearly a week Your epididymal reserves must be at peak!

· 0/r, I see you need a bit more stimulation. I could continue with some more plwnation? No, don 't close your eyes. Wake-up and take notice. I'm displaying some absolutely fabulous lordo- sis!

What's that you say, you want me to look On page- of my reproductio11 book? So your telling me this passage It as led you to reflect That what would really work for you is the Coolidge Effect?!

Is that so? Well tlo as you wislt, my darling, my sweets But /mow this, you won't be sleeping between these two sheets!

Ruth Loomis was a student inA11imal Reproduc- tive Physiology at Washington State University in the spring of 2002 a11d based the poem above Oil the nomenclature she /eamed i11 the reproduction course. Size graduated from Was!lington State University with a BA in English Literature. She is 11ow in the College of Veteri11ary Medicine at WSU (Class of2006).

The 19th Century British explorer, Sir Richard Burton, developed a recipe that he believed enhanced sperm production am/ viability. Such a recipe would result in an increased probability of conception. He used a mixture of honey, opium, spices and a small lizard. The pmpose of the lizard has not been disclosed. The author beliel'es that Sir Burton knew something about sperm motility and related the rapid crawling mo- tion of a lizard to that of spermatozoa.

Oysters can change from one gender to another and back again.

The tale of the mini ball pregnancy gives new meaning to the term target tissue. A surgeon in the Civil War treated two patients that had been shot near one anothe1: One patient was a soldier who was protecting the treatment ward and suffered a gunshot that passed through his scrotum and took off the left testicle completely on its way out. The other patient was a nurse who received a serious shot to the left side of her abdomen, the bullet lost somewhere inside. Miracu- lously the woman sun,ived the wound but months later she began to notice abnormal swelling of her abdomen. The surgeon was sure the woman was pregnant, but the patient and the villagers all swore to her absolute virginity. Upon examination it was found that the woman was indeed pregnant, with hymen still intact. The child was born without difficulty but soon it was noticed that the young boy had a large, hard mass contained within his right testicle. The doc- tor operated on the young child to remove the lump and was astounded to discover the contents of the testicle was none other than the missing mini ball that wounded the mother nine months befoJ·e. The solu- tion to this mysterious conception? The surgeon thought that this mini ball must have been the same one to have mutilated the soldier's testicle, canying sperm with it into the uterus of the nurse after the bullet left the first victim, where it remained and functioned to fertilize one of her eggs! How could the ball get into the scrotum of the neonate? What parts of this narrative are absolutely false and which could be tme? A great final exam q uestionl

Key Refer ences

Akers, R.M. 2002. Lactation and the Mammarv Gland. Iowa State Press, Ames ISBN 0-8138-2992-5.

Arthur, G.H. D.E. Noakes, H . Pearson, and T.J. Par- kinson. 1996. Veterinarv Reproduction and Obstetrics, 7th Edition. W.B. Saunders, Co. Philadelphia. ISBN 0-7020-1 785-X.

Gier, H.T. and G.B. Marion. 1968. "Uterus of the cow after parturition: involutional changes." Am. J. Vet. Res. 29:83-96.

Larson, B.L. ed. 1985. Lactation. Iowa State Press, Ames. ISBN 0-8138- 1063-9.

McEntee, K. 1990. Reproductive Patholof!Y of'Don!estic Animals. Academic Press, Inc. San Diego.)SJ?N. 012- 483375-6.

Mepham, T.B. 1987. Phvsiologv o(Lactation. Open University Press, Philadelphia. ISBN 0-335-1 5152-3.

Morrow, D.A. 1969. "Postpartum ovarian activity and involution of the uterus and cervix in dairy cattle." Vet- erinm )i Scope. Vol1 4.

Salamonsen, L.A. 2003. "Tissue injury and repair in the female human reproductive h·act." Reprod. 125:30 I.

Salisbury, G.W. , N.L. VanDemark and J.R. Lodge. 1978. Phvsiology o[Reproduction and Artificial Insemination in Cattle. 2nd Edition. W.H. Freeman and Co., San Francisco. ISBN 0-7167-0025-5.

Schmidt, G.H. 1971. Biology o(Lactation. W.H. Free- man, San Francisco. ISBN 07-1670821 -3.

The Puerperium and Lactation 333 Ve tB oo ks .ir