Questions To Be Answered 8

STUDENT LEARNING OBJECTIVES

At the completion of this chapter, you should be able to do the following:

1.Explain how sexual reproduction and asexual reproduction differ.

2.Briefly outline the male reproductive organs.

3.Discuss the structure and function of the testes.

4.Describe how testosterone works in the male body.

5.Outline the roles of FSH and LH in the male reproductive system.

6.Discuss the functions of the various reproductive ducts (epididymis, vas deferens, ejaculatory duct, and urethra)

7.Outline the role of the accessory reproductive glands.

8.List some factors that affect male fertility.

LANGUAGE OF SCIENCE AND MEDICINE

Before reading the chapter, say each of these terms out loud. This will help you avoid stumbling over them as you read.

accessory organ (ak-SES-oh-ree OR-gan)

[access- extra, -ory relating to, organ instrument]

acrosome (AK-roh-sohm)

[acro- top or tip, -some body]

anal triangle (AY-nal)

[an- ring (anus), -al relating to]

androgen (AN-droh-jen)

[andro- male, -gen produce]

androgen-binding protein (ABP) (AN-droh-jen-BYND-ing PRO-teen)

[andro- male, -gen produce, prote- first rank, -in substance]

asexual reproduction (ay-SEK-shoo-al re-proh-DUK-shun)

[a- without, sexu- sex, -al relating to, re- again, -produc- bring forth, -tion process]

benign prostatic hypertrophy (BPH) (be-NYNE proh-STAT-ik hye-PER-troh-fee)

[benign kind, pro- before, -stat- set or place, -ic relating to, hyper- excessive or above, -troph- nourishment, -y state]

bulbourethral gland (BUL-boh-yoo-REE-thral)

[bulb- swollen root, -ure- urine, -thr- agent or channel (urethra), -al relating to]

capacitation (kah-pass-ih-TAY-shun)

clone (klohn)

[clon a plant cutting]

corpus cavernosum (KOHR-pus kav-er-NO-sum)

[corpus body, cavern- large hollow, -os- relating to, -um thing] pl., corpora cavernosa (KOHR-poh-rah kav-er-NO-sah)

corpus spongiosum (KOHR-pus spun-jee-OH-sum)

[corpus body, spong- sponge, -os- relating to, -um thing] pl., corpora spongiosa (KOHR-poh-rah spun-jee-OH-sah)

cremaster muscle (kreh-MASS-ter MUSS-el)

[cremastos- hanging, mus- mouse, -cle little]

ejaculation (ee-jak-yoo-LAY-shun)

[e- out or away, -jacula- throw, -ation process]

ejaculatory duct (ee-JAK-yoo-lah-toh-ree)

[e- out or away, -jacula throw, -ory relating to, ducere- lead]

emission (ee-MISH-un)

[e- out or away, -mis- send, -sion process]

epididymis (ep-ih-DID-ih-mis)

[epi- upon, -didymis pair] pl., epididymides (ep-ih-DID-ih-mih-deez)

erection (ee-REK-shun)

essential organ (OR-gan)

[organ instrument]

external genitalia (eks-TER-nal jen-ih-TAIL-yah)

[extern- outside, -al relating to, gen- produce, -al relating to]

gamete (GAM-eet)

[gamete marriage partner]

glans penis (glans PEE-nis)

[glans acorn, penis male sex organ] pl., glandes penes (GLAN-deez PEE-neez)

gonad (GO-nad)

[gon- offspring, -ad relating to]

head

hyaluronidase (hye-al-yoo-RAHN-id-ayz)

[hyal- glass, -uron- urine, -id- relating to, -ase enzyme)

inhibin (in-HIB-in)

[inhib- inhibit, -in substance]

interstitial cell (in-ter-STISH-al sell)

[inter- between, -stit- stand, -al relating to, cell storeroom]

midpiece (MID-pees)

[mid- middle, -piece portion]

orgasm (OR-gaz-um)

[orgasm excitement]

penis (PEE-nis)

[penis male sex organ] pl., penes or penises (PEE-neez, PEE-nis-ez)

perineum (pair-ih-NEE-um)

[peri- around, -ine- excrete, -um thing] pl., perinea (pair-ih-NEE-ah)

prepuce (PREE-pus)

[pre- before, -puc- penis]

prostate (PROSS-tayt)

[pro- before, -stat- set or place]

scrotum (SKROH-tum)

[scrotum bag] pl., scrota or scrotums (SKROH-tah, SKROH-tumz)

semen (SEE-men)

[semen seed]

seminal vesicle (SEM-ih-nal VES-ih-kul)

[semen- seed, -al relating to, vesic- blister, -cle little]

sexual reproduction (SEK-shoo-al re-proh-DUKshun)

[sexu- sex, -al relating to, re- again, -produce bring forth, -tion process]

spermatic cord (sper-MAT-ik kord)

[sperma- seed, -ic relating to]

spermatogenesis (sper-mah-toh-JEN-eh-sis)

[sperma- seed, -gen- produce, -esis process]

spermatozoon (sper-mah-tah-ZOH-on)

[sperma- seed, -zoon animal] pl., spermatozoa (sper-mah-tah-ZOH-ah)

sustentacular cell (sus-ten-TAK-yoo-lar sell)

[sustent- support, -acular relating to, cell storeroom]

testis (TES-tis)

[testis witness (male gonad)] pl., testes (TES-teez)

testosterone (tes-TOS-teh-rohn)

[test- witness (testis), -stero- solid or steroid derivative, -one chemical]

tunica albuginea (TOO-nih-kah al-byoo-JIN-ee-ah)

[tunica tunic or coat, albuginea white] pl., tunicae albuginea (TOO-nih-kee al-byoo-JIN-ee-ah)

urethra (yoo-REE-thrah)

[ure- urine, -thr- agent or channel]

urogenital triangle (yoor-oh-GEN-ih-tal)

[uro- urine, -gen- produce, -al relating to]

vas deferens (vas DEF-er-enz)

[vas duct or vessel, deferens carrying away] pl., vasa deferentia (VAS-ah def-er-EN-shee-ah)

vasectomy (vah-SEK-toh-mee)

[vas- duct or vessel (vas deferens), -ec- out, -tom- cut, -y action]

CARLOS and his wife had been trying for years to have a baby with no success. Carlos had always assumed they just had bad timing. But recently they had started tracking Maria's cycle and found everything seemed to be on schedule. Finally, at Maria's request, they made an appointment with an infertility specialist. Carlos was expecting them to order expensive tests. But after the introductions, one of the first things the doctor asked about was what kind of underwear and pants Carlos typically wore. “What business is that of yours?” Carlos thought. Then the specialist added, “…because that may affect the average temperature of the testes.”

It may seem odd to you, but Carlos wearing tight underwear and tight pants may really affect Maria's chance of getting pregnant. You may already know something about testes and temperature, but in this chapter you'll get “the rest of the story.”

Remember Carlos and Maria from the Introductory Story? See if you can answer the following questions about Carlos' fertility now that you have read this chapter.

1.Sperm production occurs optimally at what temperature?

a.3° C above body temperature

b.At body temperature

c.3° C below body temperature

d.Optimal temperature changes with the seasons

Next, Carlos was asked to provide a sperm sample. “We're going to analyze the sperm count and morphology,” said the doctor.

2.What number should Carlos' sperm count be above for that factor to be ruled out as a cause of the couple's infertility?

a.250 million/ml

b.25 million/ml

c.2500/ml

d.250/ml

3.Which is the correct pathway the sperm would take during ejaculation?

a.Seminiferous tubules, rete testis, efferent ductules, epididymis, vas deferens, ejaculatory duct, urethra

b.Rete testis, seminiferous tubules, efferent ductules, epididymis, vas deferens, urethra, ejaculatory duct

c.Epididymis, vas deferens, seminiferous tubules, rete testis, ejaculatory duct, urethra

d.Seminiferous tubules, rete testis, epididymis, vas deferens, efferent ductules, urethra, ejaculatory duct

4.What hormone directly stimulates sperm production?

a.Estrogen

b.Progesterone

c.LH

d.Testosterone

To solve these questions, you may have to refer to the glossary or index, other chapters in this textbook, A&P Connect, Mechanisms of Disease, and other resources.

The importance of reproductive system function is notably different from that of any other organ system of the body. Ordinarily, body systems function to maintain the relative stability and survival of the individual organism. The reproductive system, on the other hand, ensures survival not of the individual but of the genes that characterize the human species. In both sexes, organs of the reproductive system are adapted for the specific sequence of functions that are concerned primarily with transferring genes to a new generation of offspring. A male reproductive system in one parent and a female reproductive system in another parent are needed to reproduce.

This chapter begins with a brief description of the male reproductive system. Chapter 26 then follows with the story of the female reproductive system.

SEXUAL REPRODUCTION

During sexual reproduction, a male and female each contribute half the number of chromosomes required to create the next generation of children. (Asexual reproduction requires just one parent who produces an offspring identical to it—a clone.) One advantage of sexual reproduction is that the process allows for the exchange and mixing of genes as sex cells are made and then recombined. Mixing the genetic deck of cards, so to speak, allows us tremendous, almost infinite variability in our children. This is vitally important to the survival and success of our species. Why is this important? Because such natural variation makes it more likely that at least some individuals will be able to survive new and evolving pathogens or other life-threatening changes that may occur over time in our internal or external environments.

Our reproductive systems also produce hormones that regulate the development of secondary sex characteristics that promote successful reproduction. For example, a variety of hormones creates structural and behavioral differences in the sexes. These differences permit adults to form sexual attractions with the opposite sex. In fact, reproductive hormones and other regulatory mechanisms provide us with the urge to have sex. Our sex drives are thus essential to successful reproduction.

FIGURE 24-1 Male reproductive organs. Sagittal section of inferior abdominopelvic cavity showing placement of male reproductive organs.

MALE REPRODUCTIVE ORGANS

The male reproductive system consists of organs whose functions are to produce, transfer, and introduce mature sperm into the female reproductive tract. Here, the genes from each parent join to form a new individual.

Organs of the male reproductive system (Figure 24-1) are classified as (1) essential organs (primary organs) for the production of gametes (sex cells or sperm) and (2) accessory organs (secondary organs) that support gamete formation and viability.

The essential organs or gonads of a male are the testes. The accessory organs of male reproduction include the genital ducts, glands, and other supportive structures. Reproductive ducts (genital ducts) together are responsible for delivering sperm outside the body. The ducts include a pair of epididymides (singular, epididymis), the paired vasa deferentia (singular, vas deferens), a pair of ejaculatory ducts, and the urethra. Accessory glands in the reproductive system produce secretions that serve to nourish, transport, and mature sperm. The glands include a pair of seminal vesicles, a prostate, and a pair of bulbourethral glands. Supporting structures include the scrotum, the penis, and a pair of spermatic cords. You may be familiar with a number of these structures, at least in name, but we will go over each in some detail.

Perineum

The perineum in the male is an area between the thighs, shaped roughly like a diamond (Figure 24-2). It extends from the pubic symphysis anteriorly to the coccyx posteriorly. Its most lateral boundary on either side is the

FIGURE 24-2 Male perineum. Sketch showing outline of the urogenital triangle (red) and anal triangle (blue).

ischial tuberosity (see Chapter 9, page 168). A line drawn between the two ischial tuberosities divides the perineal area into a larger urogenital triangle and a smaller anal triangle. The urogenital triangle contains the external genitals (penis and scrotum), and the anal triangle surrounds the anus.

1. What is the most significant difference between the reproductive system and other systems of your body?

2. Identify the essential and accessory organs of the male reproductive system.

3. Describe the perineum and its triangles.

TESTES

Structure and Location

The testes (singular, testis) are small, egg-shaped glands. They are about 4 to 5 cm in length and weigh 10 to 15 grams each. In a normal male, both testes are enclosed in a supporting sac, the scrotum. Both testes are suspended in the scrotum by attachments to the scrotal wall and by the spermatic cords (Figure 24-3). In addition to the vas deferens, note that the nerves, blood vessels, and lymphatics to the testis pass and are contained within the spermatic cord.

FIGURE 24-3 Tubules of the testis and epididymis. Illustration showing epididymis lifted free of testis. The ducts and tubules are exaggerated in size.

FIGURE 24-4 Testis. Low-power view showing several seminiferous tubules surrounded by septa containing interstitial (Leydig) cells.

A dense, white, fibrous capsule called the tunica albuginea encases each testis and then enters each gland. It sends dividing walls called septa that extend into the interior of the testis, dividing the gland into 200 or more cone-shaped lobules. Each lobule contains scattered interstitial cells and one to three tiny, coiled seminiferous tubules. Unraveled, each of these minute tubules would stretch more than 75 cm (2 feet) in length! The tubules from each lobule come together to form a network called the rete testis. Sperm ducts called efferent ductules drain the rete testis. The tubes then pass through the tunica albuginea to enter the head of the epididymis.

Microscopic Anatomy of the Testis

Figure 24-4 shows a low-power view of testicular tissue. Note that a number of seminiferous tubules have been cut. This reveals numerous interstitial cells (Leydig cells) in the surrounding connective septa. Maturing sperm appear as dense nuclei; their flagella or “tails” project into the lumen of the tubule. The wall of each seminiferous tubule may contain five or more layers of these cells.

At puberty, when sexual maturity begins, sperm-forming cells in different stages of development appear. At this time, the hormone-producing interstitial cells become much more prominent in the surrounding septa.

The sustentacular cells (Sertoli or nurse cells) are long, irregular cells. They provide mechanical support and protection for the developing sperm attached to their surface. Sustentacular cells also secrete the hormone inhibin, which inhibits follicle-stimulating hormone (FSH) production in the anterior pituitary (see Chapter 15, p. 333). A drop in FSH lowers the rate of sperm production. This starts a negative feedback mechanism in which the supportive sustentacular cells can slow down sperm production, if conditions require.

At sexual maturity, sustentacular cells begin to secrete androgen-binding protein (ABP). This protein adheres to the steroid hormone testosterone, making it more water soluble. The testosterone-ABP complex increases the testosterone concentration within the seminiferous tubules. This is important because high concentrations of testosterone are required for normal germ cell maturation. Thus sustentacular cells play an important role in spermatogenesis (the process of sperm formation, discussed later).

Sustentacular cells extend from the basement membrane all the way to the surface facing the lumen of the seminiferous tubules (Figure 24-5). Tight junctions (see Chapter 3, p. 56)

FIGURE 24-5 Seminiferous tubule. Wedge from a cross section of the tubule, showing spermatogenesis and the relationship of the developing spermatozoa (sperm cells) to the sustentacular (Sertoli) cells. Mitotic cell division was explained in Chapter 5. Meiotic cell division, which reduces the number of chromosomes by half, will be explained further in Chapter 26.

exist between the sustentacular cells. These junctions divide the wall of the tubule into two compartments. The compartment near the basement membrane houses sperm-producing cells called spermatogonia. The compartment near the surface facing the lumen houses meiotically active cells.

Function of Testes and Testosterone

The testes perform two primary functions: spermatogenesis and secretion of hormones.

Spermatogenesis is the production of spermatozoa (sperm)—the male reproductive cell. The sperm are produced in the seminiferous tubules. The cross section of a seminiferous tubule in Figure 24-5 shows two cell divisions that result in a reduction of chromosomes from 46 in a normal body cell to 23 in a normal sperm. You'll find a complete discussion of this special type of division—called meiosis—in Chapter 26.

As you probably know, testosterone is the major androgen (masculinizing hormone) of males. This steroid hormone is produced by interstitial cells. Actually, testosterone has a number of important functions. First, it promotes “maleness.” By this we mean the development and maintenance of male secondary sexual characteristics and accessory organs such as the prostate and seminal vesicles. Testosterone also develops and maintains adult male sexual behavior.

Testosterone also helps regulate metabolism. In fact, it stimulates protein anabolism (see Chapter 2, page 26), which in turn promotes growth of skeletal muscle. This, of course, is responsible for greater male muscular development and strength. Unfortunately, various synthetic versions of testosterone are sometimes used by athletes in ill-advised attempts to enhance muscular strength.

Testosterone also stimulates bone growth and promotes closure of the epiphyses in long bones (see Chapter 8, p. 144). Early sexual maturation leads to early epiphyseal closure. The opposite is also true: Late sexual maturation delays epiphyseal closure. As a result, tallness tends to be enhanced by late epiphyseal closure.

Testosterone also affects fluid and electrolyte balance. It has a mild stimulating effect on kidney tubule reabsorption of sodium and water, and promotes kidney tubule excretion of potassium.

Hormonal Control of Reproduction

The anterior pituitary gland controls the testes by means of its gonadotropin-releasing hormone (GnRH). As we've seen in Chapter 15 (p. 341), there are two major hormones involved, specifically follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FSH stimulates the seminiferous tubules to produce sperm more rapidly. In the male, LH stimulates interstitial cells to increase their secretions of testosterone.

Note the negative feedback mechanism in Figure 24-6. If the blood concentration level of testosterone reaches a high level, it will inhibit secretion of GnRH from the hypothalamus. As a

FIGURE 24-6 Negative feedback loop controlling testosterone secretion. Diagram shows the negative feedback mechanism that controls anterior pituitary gland secretion of LH and interstitial cell secretion of testosterone. A similar negative feedback loop exists between inhibin-secreting sustentacular cells in the testis and FSH-secreting cells in the anterior pituitary gland.

result, the anterior pituitary secretion of LH will decrease and testosterone levels will return to the normal set point value.

Increasing blood levels of inhibin, produced by the sustentacular cells, will selectively decrease FSH secretion by the anterior pituitary and decrease the rate of sperm production. However, if sperm counts decrease below the normal set point, inhibin secretion will decrease, FSH secretion will increase, and sperm numbers will increase back to normal levels.

The negative feedback loops regulating testosterone secretion involve the hypothalamus (GnRH), the anterior pituitary gland (FSH and LH), and the hormone-producing cells of the testes (testosterone and inhibin).

Small but measurable amounts of estrogen are present in healthy adult males. In fact, much of the estrogen, a steroid hormone derived from testosterone, is made in the interstitial cells. However, estrogen in males is also made in the liver and other tissues. Possible roles for estrogen in men include (1) regulation of spermatogenesis, (2) feedback inhibition of FSH and LH, and (3) promotion of normal male sexual behavior. We are sure to learn much more about the role of estrogen in males. Recent research suggests that, in addition to gonadotrophins and testosterone, estrogens are likely playing a relevant role in spermatogenesis and human male gamete maturation. Take a moment to review the male reproductive hormones and their actions listed for you in Table 24-1.

TABLE 24-1 Male Reproductive Hormones

HORMONE

SOURCE

TARGET

ACTION

Dehydroepiandrosterone (DHEA)

Adrenal gland, testis, other tissues

Converted to other hormones

Eventually converted to estrogens, testosterone, or both

Estrogen

Testis (interstitial cells), liver, other tissues

Testis (spermatogenic tissue), other tissues

Role of estrogen in men is still uncertain; may play role in spermatogenesis, inhibition of gonadotropins, male sexual behavior and partner preference

Follicle-stimulating hormone (FSH)

Anterior pituitary (gonadotroph cells)

Testis (spermatogenic tissue)

Gonadotropin; promotes development of testes and stimulates spermatogenesis

Gonadotropin-releasing hormone (CnRH)

Hypothalamus (neuroendocrine cells)

Anterior pituitary (gonadotroph cells)

Stimulates production and release of gonadotropins (FSH and LH) from anterior pituitary

Inhibin

Testis (interstitial cells)

Anterior pituitary (gonadotroph cells)

Inhibits FSH production in the anterior pituitary

Luteinizing hormone (LH)

Anterior pituitary (gonadotroph cells)

Testis (interstitial cells)

Conadotropin; stimulates production of testosterone by interstitial cells of testis

Testosterone

Testis (interstitial cells)

Spermatogenic cells, skeletal muscle, bone, other tissues

Stimulates spermatogenesis, stimulates development of primary and secondary sexual characteristics, promotes growth of muscle and bone (anabolic effect)

Structure of Spermatozoa

The long, “tailed” spermatozoa you see in the seminiferous tubules of Figure 24-7, A, may appear fully formed. However, they undergo further maturation as they pass through the genital ducts before ejaculation. Even then the process is not complete. After ejaculation, sperm must undergo a process called capacitation, which takes place in the vagina. Only after this process is complete is a sperm cell capable of fertizing an ovum.

You can see the basic features of a normal spermatozoon in Figure 24-7, B and C. Each is composed of a head, middle piece, and lashlike “tail.” The head is a compact package of 23 chromosomes. The head has no organelles and virtually no cytoplasm.

An acrosome containing hydrolytic enzymes forms the cap over the head of the sperm. These hydrolytic enzymes first break down the cervical mucus, allowing sperm to pass into the uterus and uterine tubes.

The cylindrical midpiece of the sperm consists of a helix-like arrangement of mitochondria joined end-to-end. The “tail” is actually a flagellum capable of propelling a sperm cell great distances.

A&P CONNECT

Many physicians encourage male patients to perform regular selfexamination of their testes, especially if they are at a high risk for a getting a disorder. Check out Male Genital Self-Examination online at A&P Connect.

4. Describe the basic features of the testis.

5. List the two primary functions of the testes. What are the different cell types involved in these activities?

6. List several important functions of testosterone outside those of reproduction.

7. Identify the structural components of a mature sperm.

FIGURE 24-7 Development and structure of sperm. A, Spermatid cells in a seminiferous tubule. B, Mature sperm. C, Enlarged view of head and midpiece. D, Micrograph of sperm. Note glowing nuclear material in sperm heads caused by uptake with a fluorescent dye.

FIGURE 24-8 The male reproductive system. A, Illustration shows the testes, epididymis, vas (ductus) deferens, and glands of the male reproductive system in an isolation/dissection format. B, Cross section of the shaft of the penis. Note the urethra within the substance of the corpus spongiosum.

REPRODUCTIVE DUCTS

Epididymis

Each epididymis is a single, tightly coiled tube enclosed in a fibrous casing. Although its diameter is just barely visible with the naked eye, the tube measures 6 meters (20 feet) when uncoiled! It lies along the top of and behind the testis (see Figure 24-3). Shaped roughly like a comma, the epididymis is divided into several sections. The head is connected to the testis by the efferent ductules from the testis. A central body separates the head from the tail—a tapered portion that is continuous with the vas deferens.

Sperm must pass through the epididymis from the testis to the vas deferens. Each epididymis stores sperm, nourishing them with nutrients, from 1 to 3 weeks. The epididymal secretions also eventually become a small portion of the seminal fluid (semen) that is ejaculated during intercourse. After about 3 weeks, any unused sperm break down and are reabsorbed by the body.

Vas Deferens

The vas deferens (plural, vasa deferentia) is also a tube but, unlike the epididymis, it has a thick, muscular wall (see Figure 24-3). It can be felt (palpated) in the scrotal sac as a smooth, movable cord.

The vas deferens has a layered, muscular wall. Contractions of the muscles in the wall of the vasa deferentia help propel sperm through the duct system. The vas deferens from each testis ascends from the scrotum and passes through the inguinal canal as part of the spermatic cord. This cord, enclosed by fibrous connective tissue, contains muscle, blood vessels, nerves, and lymphatics, as we've seen (see Figure 24-3). The vas deferens continues into the abdominal cavity, where it extends over the top and down the posterior surface of the bladder. Here an enlarged and winding portion called the ampulla joins the duct from the seminal vesicle to form the ejaculatory duct (Figures 24-1 and 24-8, A).

Functionally, the vas deferens connects the epididymis with the ejaculatory duct. Sperm may remain in the vas deferens for varying periods of time, depending on the degree of sexual activity and the frequency of ejaculations. Storage time may exceed 1 month with no loss of fertility. A vasectomy (severing or clamping off of the vas deferens) makes a man sterile because it effectively stops the flow of sperm to the urethra.

Ejaculatory Duct and Urethra

The two ejaculatory ducts are short tubes about 1 cm long that pass through the prostate gland and terminate in the urethra. As you can see in Figure 24-8, these ducts are formed by the union of the vas deferens with the ducts from the seminal vesicle.

The male urethra serves a double function in males. It transfers both urine from the bladder and semen with sperm from the reproductive ducts.

ACCESSORY REPRODUCTIVE GLANDS

Seminal Vesicles

The seminal vesicles are highly convoluted pouches nearly 15 cm in length when extended. They lie along the lower part of the posterior surface of the bladder, directly in front of the rectum (see Figures 24-1 and 24-8, A). The secretory epithelium of the seminal vesicles is highly branched and convoluted.

The seminal vesicles secrete an alkaline, viscous, creamy-yellow liquid that makes up about 60% of the semen volume. The alkalinity helps neutralize the acid pH environment of the terminal male urethra and of the vagina. Fructose in the semen serves as an energy source for sperm motility after ejaculation. Other important components include prostaglandins.

Prostate Gland

The prostate lies just below the bladder and is shaped roughly like a doughnut. The fact that the urethra passes through the small hole in the center of the prostate is clinically important. This is because many older men suffer from a noncancerous enlargement of this gland known as benign prostatic hypertrophy (BPH). As the prostate enlarges, it squeezes the urethra, frequently closing it so completely that urination becomes nearly impossible. Urinary retention results, which can be uncomfortable and even painful. Surgical removal of all or part of the prostate is required if other less invasive methods fail.

The prostate secretes a watery, milky-looking, and slightly acidic fluid that constitutes about 30% of the seminal fluid volume. Citrate in the prostatic fluid provides additional nutrients for sperm. Other constituents include enzymes such as hyaluronidase and prostate-specific antigen (PSA). The functions of these components are discussed later in this chapter. Box 24-1 discusses different methods for prostate cancer screening. Overall, prostatic fluid with its many components plays an important role in sperm activation, viability, and motility.

BOX 24-1 Diagnostic Study

Prostate Cancer Screening

Many of the 32,000 men who die each year from prostate cancer—the most common nonskin type of cancer in American men—could be saved if the cancer were detected early enough for effective treatment. Several screening tests are available for the detection of prostate cancer once it develops. Cancerous growths in the gland can often be palpated through the wall of the rectum (see figure).

Sometimes, rectal examinations are performed in conjunction with a screening test called the PSA test. This test is a type of blood analysis that screens for prostate-specific antigen (PSA), a substance sometimes found to be elevated in the blood of men with prostate cancer. Unfortunately, PSA levels may not be elevated with prostate cancer and may be high in some men without prostate cancer. Thus the PSA test is most useful when used with other screening methods.

A nuclear medicine bone scan is often used either to exclude metastatic spread of prostate cancer or to locate areas of the body where secondary prostate cancer tumors have already developed.

Palpation of the prostate gland. A physician inserts a lubricated, gloved finger through the anus to feel the prostate through the thin anterior wall of the rectum.

Bulbourethral Glands

The two bulbourethral glands (Cowper's glands) resemble peas in size and shape. You can see the location of these compound glands in Figure 24-8, A. A duct approximately 2.5 cm (1 inch) long connects each gland with the penile portion of the urethra.

Like the seminal vesicles, the bulbourethral glands secrete an alkaline fluid. This fluid is important for counteracting the acid environment of the male urethra and the female vagina. Mucus produced in these glands serves to lubricate the urethra and helps protect sperm from damage due to friction during ejaculation.

A&P CONNECT

Infections of the reproductive tract, often acquired through sexual contact with infected individuals, can progress into conditions that may cause sterility—or even death. These sexually transmitted diseases (STDs) are discussed in Sexually Transmitted Diseases online at A&P Connect.

8. List, in order, the reproductive ducts that sperm must pass through from their formation to ejaculation.

9. Describe the problems associated with the relationship of the prostate gland and the urethra.

10. Briefly compare the pH and composition of the secretions produced by the accessory reproductive glands.

SUPPORTING STRUCTURES

Scrotum

The scrotum is a skin-covered pouch suspended from the perineal region. Internally, it is divided into two sacs by a septum. Each sac contains a testis, epididymis, and lower part of a spermatic cord. Just below the skin lie the dartos fascia and dartos muscles. Contraction of the dartos muscle wrinkles the scrotal skin and can elevate or move the testes slightly. However, it is the cremaster muscles that are primarily responsible for testicular movement within the scrotal pouch. These two bands of skeletal muscle extend through the inguinal canal on either side as part of the spermatic cord (see Figure 24-8, A), and then attach to the posterior aspect of the testes. When contracted these “suspender” muscles, which arise from the internal oblique muscles of the lower abdominal wall, can dramatically elevate the testes during sexual arousal, exposure to cold, or threat of injury.

The temperature required for optimal sperm formation is about 3° C below normal body temperature. This is the “functional” reason that justifies placement of the testes outside the body cavity (where they are constantly exposed to potential environmental shock and traumatic injury). In a warm environment, the scrotum becomes elongated and its skin appears loose. This permits the testes to descend in the sac away from the body, thereby keeping them cool. However, in the cold, the scrotum elevates and becomes heavily wrinkled. Contraction of the cremaster muscles effectively pulls the testes upward toward the body wall, keeping them warmer. Both actions help maintain the temperature of the testes at a more constant level. Of course, factors other than temperature, including blood flow dynamics and even sexual selection, are also cited as reasons for the scrotal placement of the testes.

Penis and Spermatic Cords

The penis (see Figure 24-8) is composed of three cylindrical masses of erectile tissues. These cavernous tissues are enclosed in a separate fibrous covering and held together by a covering of skin. The two larger and uppermost of these cylinders are the corpora cavernosa. The smaller, lower cylinder, which contains the urethra, is called the corpus spongiosum (see Figure 24-8, B). The distal part of the corpus spongiosum overlaps the terminal end of the two corpora cavernosa. Here it forms a slightly bulging structure, the glans penis. A loose-fitting, retractable prepuce (foreskin) encloses most of the glans penis but leaves the urethral opening unobstructed for urination.

The penis contains the urethra—the terminal duct for both urinary and reproductive tracts. During sexual arousal, the erectile tissue of the penis fills with blood. This causes the organ to become rigid and enlarged in both diameter and length. The end result is called an erection, which allows the penis to penetrate the vagina during intercourse. The scrotum and penis together constitute the external genitalia of males.

The spermatic cords are cylindrical casings of white, fibrous tissue located in the inguinal canals between the scrotum and the abdominal cavity. They enclose the vasa deferentia, blood vessels, lymphatics, and nerves (see Figure 24-8, A).

COMPOSITION OF SEMINAL FLUID

Let's summarize the components of the semen (seminal fluid) we discussed earlier:

1.The testes and epididymis secretions comprise less than 5% of the seminal fluid volume.

2.Seminal vesicles secrete approximately 60% of the seminal fluid volume.

3.The prostate gland secretes about 30% of the seminal fluid volume.

4.The bulbourethral glands secrete less than 5% of the seminal fluid volume.

The seminal fluid serves to lubricate, protect, provide nourishment, and aid in the process of maturing sperm for ejaculation and survival. Note that sperm originate in the testes (glands located outside the body), travel inside the abdominal cavity, and then are expelled outside. In Box 24-2, we outline the basic neural controls of the male sexual response.

BOX 24-2 FYI

Neural Control of the Male Sexual Response

Recall that all body functions but one have for their ultimate goal survival of the individual. Only the function of reproduction serves a different, longer range, and (in nature's scheme) more important purpose—survival of the human species.

Male functions in reproduction consist of the production of male sex cells (spermatogenesis) and introduction of these cells into the female body (coitus or sexual intercourse). For coitus to take place, erection of the penis must first occur, and for sperm to enter the female body, both the sex cells and secretions from the accessory glands must be introduced into the urethra (emission) and semen must be ejaculated from the penis.

Erection is a parasympathetic reflex initiated mainly by certain tactile, visual, and mental stimuli. It consists of dilation of the arteries and arterioles of the penis, which in turn floods and distends spaces in its erectile tissue and compresses its veins. Therefore, more blood enters the penis through the dilated arteries than leaves it through the constricted veins. As a result, the penis becomes larger and rigid: erection occurs.

Emission is the reflex movement of sex cells, or spermatozoa, and secretions from the genital ducts and accessory glands into the prostatic urethra. Once emission has occurred, ejaculation will follow.

Ejaculation of semen is also a reflex response. It is the usual outcome of the same stimuli that initiate erection. Ejaculation and various other responses—notably accelerated heart rate, increased blood pressure, hyperventilation, dilated skin blood vessels, and intense sexual excitement—characterize the male orgasm, or sexual climax.

MALE FERTILITY

Male fertility depends on many factors, but primarily on the number of sperm ejaculated as well as their size, shape, and motility (activity). In fact, fertile sperm typically have a uniform size and shape and are highly motile. Although it takes just one sperm (and only one sperm) to fertilize an egg, it appears now that millions of sperm must be ejaculated for this to occur. According to one recent estimate, when the sperm count falls below about 25 million/ml of semen, functional sterility can result.

One hypothesis that may explain why so many sperm must be ejaculated is that enough sperm must be present to secrete sufficient hyaluronidase and other hydrolytic enzymes. These enzymes liquefy the intercellular substance between the cells that encase each ovum. Without this, a single sperm cannot penetrate the layer and thus cannot fertilize the egg. Apparently it takes a large number of sperm to ensure fertilization. In effect, fertilization is a community effort!

If an ovum (egg) is present in the female reproductive tract when semen is introduced, then the release of additional capacitation enzymes from the multitude of sperm come into play. This mass release of hydrolytic enzymes is vital to fertilization because it allows the first sperm contacting the plasma membrane of the egg to actually enter the egg. Once the plasma membrane of the egg is penetrated, a series of events take place that eventually culminates in fertilization.

Infertility can also be caused by the production of antibodies some men make against their own sperm. This is called immune infertility and is caused by an antigenantibody reaction.

Figure 24-9 shows that, as average plasma testosterone levels increase during puberty, sperm production begins.

FIGURE 24-9 Testosterone levels and sperm production. Plasma testosterone levels (red line) rise during fetal development, when testosterone stimulates early development of male sexual organs. Testosterone rises again briefly around the time of birth, which facilitates descent of the testes into the scrotum. Then at puberty, testosterone rises enough to support sperm production (blue line) and later tapers off in advanced old age.

Testosterone levels—and thus sperm production—reach a peak in early adulthood and remain high into old age. In advanced old age, testosterone production tapers off, causing a drop in sperm count and fertility.

11. What are the structures that compose the external genitals of the male?

12. Name the three cylindrical masses of erectile tissue in the penis.

13. What is the function of the dartos muscle? The cremaster muscle? How do their actions potentially influence fertility?

14. What factors may influence male fertility?

Cycle of LIFE

Our reproductive systems are unlike other systems in our bodies with regard to normal changes that occur throughout our life spans. All other systems perform their functions from the time they develop in utero until advanced old age. However, both male and female reproductive systems are “delayed” in that they cannot function until puberty.

Initial development of the male reproductive organs begins before birth. At about the seventh week of embryonic development, genes in the Y chromosome trigger the production of enough testosterone to stimulate the development of male reproductive organs from the undifferentiated reproductive tissues. Without the early secretion of testosterone, the organs would instead develop into their female counterparts.

Several months before birth, the immature testes descend from behind the parietal peritoneum and down into the scrotum (Figure 24-10). At this point, each testis is guided in its descent by the threadlike, fibrous gubernaculum. It is not uncommon for the testes to be late in their descent. And sometimes, they fail to descend until several weeks after birth. There is a spurt of testosterone levels around the time of birth—this stimulates the descent of the testes.

The testes and other reproductive organs remain in an immature state until puberty, when high levels of reproductive hormones stimulate the final stages of their development. From puberty until advanced old age, the male reproductive system continues to operate successfully. In fact, men can sire children until the time of death!

MECHANISMS OF DISEASE

Disorders of the male reproductive system include a variety of conditions that cause infertility and even sterility. In addition, there are occasional disorders resulting from reproductive tract infections that cause decreased sperm production. Older males may suffer from benign prostatic hypertrophy, an enlargement of the prostate, and many older males exhibit various stages of prostate cancers. Beyond these conditions and diseases are disorders of the penis and scrotum, including erectile dysfunction, hydrocele, and hernias.

Find out more about these diseases and disorders of the male reproductive system online at Mechanisms of Disease: Male Reproductive System.

FIGURE 24-10 Descent of the testes. Prior to birth, the testes move from their location near the kidneys and through the inguinal canal to the scrotum.

The BIG Picture

Reproduction of genes by individual humans provides the potential contribution of genes to the gene pool of the next generation of humans—truly a “big picture!” In males, the reproductive and urinary tracts converge terminally so that sometimes they are referred to as the genitourinary tract (or urogenital tract). This “sharing” also means functional sharing as well. For example, the urethra conducts urine during micturition but conducts semen during ejaculation. Nervous regulation of the muscles controlling the bladder, urethra, and ejaculatory duct prevents the flow of urine from the bladder and backflow of semen into the bladder during sexual activity.

As we've seen, both the primary and secondary sexual functions in males depend on complex interrelationships involving nervous, endocrine, muscular, urinary, and circulatory system structures. Even the skin can be perceived as a sexual organ—it receives many of the stimuli needed to produce the sexual response.

CHAPTER SUMMARY

To download an MP3 version of the chapter summary for use with your iPod or other portable media player, access the Audio Chapter Summaries online at http://evolve.elsevier.com.

Scan this summary after reading the chapter to help you reinforce the key concepts. Later, use the summary as a quick review before your class or before a test.

SEXUAL REPRODUCTION

A. The reproductive system is an important part of our individual homeostasis

1. Vital part of our continuing survival and evolution as humans

2. Organs of the reproductive system are adapted to transferring genes from parents to their offspring

3. Reproductive systems produce hormones that regulate the development of secondary sex characteristics that promote successful reproduction

B. Sexual reproduction—male and female each contribute half the number of chromosomes required to create the next generation of children

1. Advantage of sexual reproduction is that the process allows for the exchange and mixing of genes as sex cells are made and then recombined

MALE REPRODUCTIVE ORGANS

A. Functions are to produce, transfer, and introduce mature sperm into the female reproductive tract (Figure 24-1)

1. Classified as essential organs (primary organs) and accessory organs (secondary organs)

a. Essential organs or gonads of a male are the testes

b. Accessory organs of male reproduction include the genital ducts, glands, and other supportive structures

B. Perineum—in males, it is an area between the thighs, shaped roughly like a diamond; extends from the pubic symphysis anteriorly to the coccyx posteriorly (Figure 24-2)

1. Urogenital triangle—contains the external genitals (penis and scrotum)

2. Anal triangle—surrounds the anus

TESTES

A. Structure and location

1. Small, egg-shaped glands enclosed in a supporting sac called the scrotum

2. Suspended in the scrotum by attachments to the scrotal wall and by the spermatic cords (Figure 24-3)

3. Dense, white, fibrous capsule called the tunica albuginea encases each testis and then enters each gland

4. Seminiferous tubules in testis open into a plexus called the rete testis, which is drained by a series of efferent ductules that emerge from the top of the organ and enter the head of epididymis

B. Microscopic anatomy of the testis

1. Interstitial (Leydig) cells—hormone-producing cells between the seminiferous tubules

2. Sustentacular cells (Sertoli or nurse cells)—provide mechanical support and protection for the developing sperm attached to their surface

a. Secrete inhibin—inhibits follicle-stimulating hormone (FSH) production in the anterior pituitary

b. Produce androgen-binding protein that adheres to the steroid hormone testosterone; makes it more water soluble

c. Sustentacular cells play an important role in spermatogenesis

d. Tight junctions exist between sustentacular cells to divide the wall of the tubule into two compartments

C. Functions of testis and testosterone (Figure 24-5)

1. Spermatogenesis

a. Production of spermatozoa (sperm)

b. Involves meiosis—a special type of cell division that halves the number of chromosomes (see Chapter 26)

2. Secretion of hormones by interstitial cells

a. Testosterone—major androgen (masculinizing hormone)

b. Functions of testosterone include: develops and maintains male secondary sexual characteristics and accessory organs; develops and maintains adult male sexual behavior; stimulates protein anabolism; affects fluid and electrolyte balance

D. Hormonal control of reproduction

1. Anterior pituitary gland controls the testes by means of its gonadotropin-releasing hormone (GnRH)

2. Two major hormones

a. Follicle-stimulating hormone (FSH)—stimulates the seminiferous tubules to produce sperm more rapidly

b. Luteinizing hormone (LH)—stimulates interstitial cells to increase their secretions of testosterone

E. Structure of spermatozoa (Figure 24-7)

1. Consists of a head (covered by acrosome), neck, midpiece, and tail (Figure 24-7, B and C)

REPRODUCTIVE DUCTS

A. Epididymis—single, tightly coiled tube enclosed in a fibrous casing

1. Lies along the top of and behind the testis (Figure 24-3)

2. Anatomical divisions include head, central body, and tail

3. Each epididymis stores sperm, nourishing them with nutrients from 1 to 3 weeks

4. Epididymal secretions also eventually become a small portion of the seminal fluid (semen)

B. Vas deferens—tube but, unlike the epididymis, it has a thick, muscular wall

1. Contractions of the muscles in the wall of the vasa deferentia help propel sperm through the duct system

2. Functionally, the vas deferens connects the epididymis with the ejaculatory duct

C. Ejaculatory duct and urethra

1. Formed by the union of the vas deferens with the ducts from the seminal vesicle (Figure 24-8)

2. Urethra serves a double function in males; transfers both urine from the bladder and semen with sperm from the reproductive ducts

ACCESSORY REPRODUCTIVE GLANDS

A. Seminal vesicles

1. Convoluted pouches nearly 15 cm in length when extended

2. Lie along the lower part of the posterior surface of the bladder, directly in front of the rectum (Figures 24-1 and 24-8)

3. Secrete an alkaline, viscous, creamy-yellow liquid that makes up about 60% of the semen volume

B. Prostate gland

1. Lies just below the bladder; shaped roughly like a doughnut

2. Secretes a watery, milky-looking, and slightly acidic fluid that constitutes about 30% of the seminal fluid volume

C. Bulbourethral glands (Cowper's glands) (Figure 24-8)

1. Resemble peas in size and shape

2. A duct approximately 2.5 cm (1 inch) long connects each gland with the penile portion of the urethra

3. Secrete an alkaline fluid; important for counteracting the acid environment of the male urethra and the female vagina

SUPPORTING STRUCTURES

A. Scrotum

1. Skin-covered pouch suspended from the perineal region (Figure 24-8)

2. Divided internally into two sacs by a septum

3. Each sac contains a testis, epididymis, and lower part of a spermatic cord

4. Dartos wrinkles the scrotal skin and cremaster muscles elevate the scrotal pouch

B. Penis and spermatic cords

1. Penis is composed of three cylindrical masses of erectile tissues (Figure 24-8)

2. Functions—contains the urethra, the terminal duct for both urinary and reproductive tracts; during sexual arousal, penis becomes erect, serving as a penetrating copulatory organ during sexual intercourse

3. Spermatic cords—cylindrical casings of white, fibrous tissue located in the inguinal canals between the scrotum and the abdominal cavity

a. Enclose the vasa deferentia, blood vessels, lymphatics, and nerves (Figure 24-8)

COMPOSITION OF SEMINAL FLUID

A. Seminal fluid serves to lubricate, protect, provide nourishment, and aid in the process of maturing sperm for ejaculation and survival

MALE FERTILITY

A. Depends on many factors, but primarily on the number of sperm ejaculated, size, shape, and motility

B. Functional sterility—when the sperm count falls below about 25 million/ml of semen

C. Sufficient numbers of sperm must be present to secrete enough hyaluronidase (enzymes that liquefy the substance that encases an ovum) so that one sperm can penetrate the ovum

D. Infertility can also be caused by the production of antibodies some men make against their own sperm—immune infertility

E. Male fertility begins at puberty and extends into old age (Figure 24-9)

REVIEW QUESTIONS

Write out the answers to these questions after reading the chapter and reviewing the Chapter Summary. If you simply think through the answer without writing it down, you won't retain much of your new learning.

1.Name the accessory glands of the male reproductive system.

2.List the genital ducts in the male.

3.List the supporting structures of the male reproductive system.

4.What is the tunica albuginea? How does it aid in dividing the testis into lobules?

5.What are the two primary functions of the testes?

6.What are the general functions of testosterone?

7.Discuss the structure of a mature spermatozoon.

8.What is meant by the term capacitation?

9.List the three functions of the epididymis.

10.List the anatomical divisions of the epididymis.

11.Discuss the formation of the ejaculatory ducts.

12.Discuss the type of secretion typical of the prostate gland and seminal vesicles.

13.What and where are the bulbourethral glands?

14.Describe the structure, location, and function or functions of the scrotum.

15.Name the three cylindrical masses of erectile, or cavernous, tissue in the penis.

16.What and where is the glans penis? The prepuce, or foreskin?

17.What is the spermatic cord? From what does it extend, and what does it contain?

CRITICAL THINKING QUESTIONS

After finishing the Review Questions, write out the answers to these items to help you apply your new knowledge. Go back to sections of the chapter that relate to items that you find difficult.

1.How does the function of reproduction differ from all other body functions?

2.Can you identify the functions of the male reproductive system?

3.What is the relationship between the rete testis, seminiferous tubules, and efferent ductules?

4.How is the prostate gland related to the urethra? What problems can result from this relationship?

5.Can you list the structures in the reproductive system that contribute to the formation of seminal fluid?

6.Trace the course of seminal fluid from its formation to ejaculation.

7.What is the chemical in seminal fluid that is important to fertility? What is its function?

8.How is the structure of the spermatozoon related to its function?

CHAPTER 25 Female Reproductive System

STUDENT LEARNING OBJECTIVES

At the completion of this chapter, you should be able to do the following:

1.Briefly describe the functions of the female reproductive system.

2.Differentiate between essential organs and accessory organs of the female reproductive system.

3.Describe the structure of the ovaries and list their functions.

4.Make an outline of oogenesis, listing the major structures involved.

5.Discuss the layers comprising the walls of the uterus, and the functions of these layers.

6.Describe the basic functions of the following: uterus, uterine tubes, vagina.

7.Outline the major components of the external genitalia and describe their basic functions.

8.Outline in general the recurring cycles of the female reproductive system.

9.Discuss the roles of hormones in the recurring cycles of female reproduction.

10.Identify the factors that affect female fertility.

11.Describe the structures involved in breast milk production and identify the hormones that affect its production.

LANGUAGE OF SCIENCE AND MEDICINE

Before reading the chapter, say each of these terms out loud. This will help you avoid stumbling over them as you read.

accessory organ (ak-SES-oh-ree OR-gan)

[access- extra, -ory relating to, organ instrument]

alveolus (al-VEE-oh-lus)

[alve- hollow, -olus little] pl., alveoli (al-VEE-oh-lye)

ampulla (am-PUL-ah)

[ampu- flask, -ulla little] pl., ampullae (am-PUL-ee)

anal triangle (AY-nal)

[an- ring (anus), -al relating to]

anterior fornix (an-TEER-ee-or FOR-niks)

[ante- front, -er- more, -or quality, fornix arch] pl., fornices (FOR-nih-seez)

areola (ah-REE-oh-lah)

[are- area or space, -ola little] pl., areolae, areoles, or areolas (ah-REE-oh-lee, ah-REE-ohlz, ah-REE-oh-lahz)

body [of the uterus]

(BOD-ee)

cervix (SER-viks)

[cervix neck] pl., cervices or cervixes (SER-veh-seez, SER-viks-ehz)

clitoris

(KLIT-oh-ris) pl., clitorides (klit-OH-rih-deez)

corpus albicans (KOHR-pus AL-bih-kanz)

[corpus body, albicans whitening] pl., corpora albicantia (KOHR-pohr-ah al-bih-KAN-shee-ah)

corpus luteum (KOHR-pus LOO-tee-um)

[corpus body, lute- yellow, -um thing] pl., corpora lutea (KOHR-pohr-ah LOO-tee-ah)

cortex (KOHR-teks)

[cortex bark] pl., cortices (KOR-tih-sees)

ectopic pregnancy (ek-TOP-ik)

[ec- out of, -top- place, -ic relating to]

endometrium (en-doh-MEE-tree-um)

[endo- within, -metr- womb, -um thing] pl., endometria (en-doh-MEE-tree-ah)

episiotomy (eh-piz-ee-OT-oh-mee)

[episi- vulva, -tom- cut, -y action]

essential organ (OR-gan)

[organ instrument]

estrogen (ES-troh-jen)

[estr- frenzy, -gen produce]

fimbria (FIM-bree-ah)

[fimbria fringe] pl., fimbriae (FIM-bree-ee)

follicle-stimulating hormone (FSH) (FOL-ih-kul-STIM-yoo-lay-ting HOR-mohn)

[foll- bag, -icle little, hormon- excite]

follicular phase (foh-LIK-yoo-lar fayz)

[foll- bag, -icul- little, -ar relating to]

fornix (FOR-niks)

[fornix arch] pl., fornices (FOR-nih-seez)

fundus (FUN-duss)

[fundus bottom] pl., fundi (FUN-dye)

glans clitoris (glans KLIT-oh-ris)

[glans acorn]

granulosa cell (gran-yoo-LOH-sah sell)

[gran- grain, -ul- little, -osa relating to, cell storeroom]

greater vestibular gland (ves-TIB-yoo-lar)

[vestibul- entrance hall, -ar relating to, gland acorn]

hymen (HYE-men)

[hymen Greek god of marriage]

imperforate hymen (im-PER-fah-rayt HYE-men)

[im- not, -perfor- pierce, -ate state, hymen Greek god of marriage]

infertility (in-fer-TIL-ih-tee)

[in- not, -fertil- fruitful, -ity state]

infundibulum (in-fun-DIB-yoo-lum)

[infundibulum funnel]

isthmus (iSS-muss)

[ithmus narrow connection or passage]

labia majora (LAY-bee-ah mah-JOH-rah)

[labia lips, majora large] sing., labium majus (LAY-bee-um MAY-jus)

labia minora (LAY-bee-ah mih-NO-rah)

[labia lips, minora small] sing., labium minor (LAY-bee-um MYE-nor)

labor

(LAY-bor)

lactation (lak-TAY-shun)

[lact- milk, -ation process]

lactiferous duct (lak-TIF-er-us)

[lact- milk, -fer- bear or carry, -ous relating to, duct lead]

luteal phase (LOO-tee-al fayz)

[lute- yellow, -al relating to]

luteinization (loo-tee-in-ih-ZAY-shun)

[lute- yellow, -ization process]

luteinizing hormone (LH) (loo-tee-in-EYE-zing HOR-mohn)

[lute- yellow, -izing process, hormon- excite]

mammary gland (MAM-er-ee)

[mamma- breast, -ry relating to, gland acorn]

mature follicle

medulla (meh-DUL-ah)

[medulla middle] pl., medullae or medullas (meh-DUL-ee, meh-DUL-ahz)

menarche (meh-NAR-kee)

[men- month, -arche beginning]

menopause (MEN-oh-pawz)

[men- month, -paus- cease]

menses (MEN-seez)

[menses months] sing., mensis (MEN-sis)

menstrual period (MEN-stroo-al)

[mens- month, -al relating to]

menstruation (men-stroo-AY-shun)

[mens- month, -ation process]

mons pubis (monz PYOO-bis)

[mons mountain, pubis groin] pl., montes pubis (MON-teez PYOO-bis)

myometrium (my-oh-MEE-tree-um)

[myo- muscle, -metr- womb, -um thing]

nipple (NIP-el)

[nip- beak, -le small]

oogonium (oh-oh-GO-nee-um)

[oo- egg, -gon- offspring, -um thing] pl., oogonia (oh-oh-GO-nee-ah)

oral contraceptive (OR-al kon-tra-SEP-tiv)

[contra- against, -cept- take or receive (conception), -ive agent]

ovarian follicle (oh-VAIR-ee-an FOL-ih-kul)

[ov- egg, -arian relating to, foll- bag, -icle little]

ovarian medulla (oh-VAIR-ee-an meh-DUL-ah)

[ov- egg, -arian relating to, medulla middle] pl., medullae or medullas (meh-DUL-ee, meh-DUL-ahz)

ovary (OH-var-ee)

[ov- egg, -ar- relating to, -y location of process]

ovulation (ov-yoo-LAY-shun)

[ov- egg, -ation process]

ovum (OH-vum)

[ovum egg] pl., ova (OH-vah)

perimetrium (pair-ih-MEE-tree-um)

[peri- around, -metr- womb, -um thing]

perineal body (pair-ih-NEE-al BOD-ee)

[peri- around, -ine- excrete (perineum), -al relating to]

perineum (pair-ih-NEE-um)

[peri- around, -ine- excrete, -um thing] pl., perinea (pair-ih-NEE-ah)

peritonitis (pair-ih-toh-NYE-tis)

[peri- around, -ton- stretch (peritoneum), -itis inflammation]

placenta (plah-SEN-tah)

[placenta flat cake] pl., placentae or placentas (plah-SEN-tee, plah-SEN-tahz)

posterior fornix (pohs-teer-ee-or FOR-niks)

[poster- behind, -or quality, fornix arch]

prepuce (PREE-pus)

[pre- before, -puc- penis]

primary follicle (PRY-mair-ee FOL-ih-kul)

[prim- first, -ary state, folli- bag, -cle small]

progesterone (pro-JES-ter-ohn)

[pro- provide for, -gester- bearing (pregnancy), -stero- solid or steroid derivative, -one chemical]

proliferative phase (PROH-lif-er-eh-tiv fayz)

[proli- offspring, -fer- bear or carry, -at- process, -ive relating to]

retroflexion (ret-roh-FLEK-shen)

[retro- backward, -flex- bend, -ion process]

salpingitis (sal-pin-JYE-tis)

[salping- tube, -itis inflammation]

secretory phase (SEEK-reh-toh-ree fayz)

[secret- separate, -ory relating to]

urogenital triangle (yoor-oh-GEN-ih-tal)

[uro- urine, -gen- produce, -al relating to]

uterus (YOO-ter-us)

[uterus womb]

vagina (vah-JYE-nah)

[vagina sheath]

vaginal orifice (VAH-jih-nal OR-ih-fis)

[vagina- sheath, -al relating to, ori- mouth, -fice- something made]

vulva (VUL-vah)

[vulva wrapper]

CARLOS and his wife had been trying for years to have a baby with no success. After a visit to an infertility specialist, they found that Carlos had a low sperm count. Before suggesting a solution, the physicians will also check Maria's reproductive system to confirm that there is an open pathway for the egg.

Finding no blockage in Maria's reproductive tract, the physicians recommended an intrauterine insemination (IUI). To increase the chances of a sperm encountering an egg, a medication called Clomid (clomiphene) was prescribed for Maria. Clomid works as an ovulatory stimulant and acts as an antiestrogen agent, causing the body to perceive low estrogen levels. It is given on about days 5 to 10 of the menstrual cycle.

As you read the rest of this chapter, keep Carlos and Maria in mind, and see if you can answer questions about their situation at the end of the chapter.

Now that you have read this chapter, see if you can answer these questions about Carlos and Maria from the Introductory Story of this chapter and Chapter 24.

1. What effect will Clomid have on FSH production?

a. Increase FSH production

b. Decrease FSH production

c. No change in FSH production

d. Slight decrease in FSH production followed by a sharp increase

2. After ovulation, the follicular cells first transform into what?

a. Corpus lucidum

b. Corpus luteum

c. Corpus rubrum

d. Corpus albicans

3. Where in the female reproductive tract should the sperm and the oocyte meet (hopefully completing the process of fertilization)?

a. In the cervix

b. In the uterus

c. In the uterine tubes

d. In the ovaries

To solve these questions, you may have to refer to the glossary or index, other chapters in this textbook, A&P Connect, Mechanisms of Disease, and other resources.

OVERVIEW OF THE FEMALE REPRODUCTIVE SYSTEM

Function of the Female Reproductive System

The female reproductive system produces gametes called ova (eggs). The haploid nucleus of the egg must combine with the haploid nucleus of the sperm if successful fertilization is to occur. The process and function of sex cell formation emphasizes the similarity between the male and female reproductive systems. However, this is where the similarity ends. Unlike the male system, the female reproductive system also provides protection and nutrition to the developing offspring for up to several years after conception, as we shall see.

Structural Plan of the Female Reproductive System

A number of organs make up the female reproductive system, making it somewhat complex. For this reason, we need to look first at the structural plan of the system as a whole (Figure 25-1). As we stated in the previous chapter, reproductive organs can be classified as essential organs or accessory organs, depending on how directly they are involved in producing offspring. The gonads of women are the paired ovaries (the essential organs), which produce the ova. The accessory organs of reproduction in women consist of the following structures:

▪A series of ducts or modified duct structures that includes the uterine tubes, uterus, and vagina

▪The vulva, or external reproductive organs

▪Additional glands, including the mammary glands (highly modified sebaceous glands), which secrete milk for the nourishment of newborn children

FIGURE 25-1 Female reproductive organs. Diagram (sagittal section) of pelvis showing location of female reproductive organs.

You can see most of the essential and accessory organs of the female reproductive system in Figures 25-1, 25-2, and 25-3. Refer to these figures often as you read through the following pages.

FIGURE 25-2 Female perineum. Sketch showing outline of the urogenital triangle (red) and anal triangle (blue).

Perineum

The female perineum is a muscular region within a diamond-shaped area between the thighs and the vaginal orifice and the anus (see Figure 25-2). It extends from the pubic symphysis anteriorly to the coccyx posteriorly. Its lateral boundary on either side is the ischial tuberosity. A line drawn between the ischial tuberosities divides the area into two triangles. The larger urogenital triangle contains the external genitals (labia, vaginal orifice, clitoris) and urinary opening; the smaller anal triangle surrounds the anus.

The perineum has great clinical importance because it may be torn during childbirth. Such tears are often deep, have irregular edges, and may extend all the way through the perineum, through the muscular perineal body, and even through the anal sphincter. Such damage may result in seepage from the rectum until the laceration is repaired. To avoid these possibilities in a woman prone to such injuries, a surgical incision known as an episiotomy may be made in the perineum, particularly at the birth of a first baby. In current medical practice, episiotomy procedures are decreasing in frequency and are no longer performed on a routine basis preceding vaginal delivery of a baby.

1. What are the essential organs of the female reproductive system?

2. List the major accessory organs of the female reproductive system.

3. What is the purpose of an episiotomy?

FIGURE 25-3 Internal female reproductive organs. Posterior view. Diagram shows left side of uterus and upper portion of the vagina and the left uterine tube and ovary in a frontal section. The broad ligament has been removed from the posterior surface of the uterus and adjacent structures.

OVARIES

Location of the Ovaries

The ovaries are homologous (share the same embryonic origin) to the testes of the male. They are nodular oval glands with a puckered, uneven surface. After puberty, they resemble large almonds in size and shape. One ovary lies on each side of the uterus, below and behind the uterine tubes.

Each ovary weighs about 3 grams and is attached to the posterior surface of the broad ligament by the mesovarian ligament (mesovarium). This structure carries blood vessels, nerves, and lymphatics. The ovarian ligament anchors the ovary to the uterus. The distal portion of the uterine tube has fimbriae that form a cup of fingerlike extensions around the ovary. Note, however, that most of the fimbriae do not actually attach to it (see Figure 25-3). Only one of these, the ovarian fimbria, actually attaches directly with the ovary. Unfortunately, this configuration makes it possible for a pregnancy to begin in the pelvic cavity instead of in the uterus, as is normal. Development of the fetus in a location other than the uterus is referred to as an ectopic pregnancy (from the Greek ektopos, “displaced”).

Structure and Function of the Ovaries

The ovary, like many organs in the body, consists of two major layers of tissue: an outer cortex and an inner medulla. Covering the outer cortex is a layer of flattened epithelial cells called the germinal epithelium. Deep to the surface layer of germinal epithelium is a tough layer of connective tissue called the tunica albuginea. This tough layer covers the ovarian cortex. Hundreds of thousands of microscopic ovarian follicles are embedded in the connective tissue matrix of the cortex. Each follicle contains an immature female sex cell, or oocyte, as well as its surrounding cells. After puberty, the oocytes and the specialized cells that surround them are present in varying stages of development. The ovarian medulla contains supportive connective tissue cells, blood vessels, nerves, and lymphatics.

Overview of Oogenesis

Now look at Figure 25-4 for a moment and follow the development of a female sex cell from its origin through its release (ovulation).

Throughout the process of ovarian development the oocyte grows in size. So, too, does the number of cell layers surrounding it. Initially, there is a single layer of flat epithelial cells that originate from the surface epithelium covering

FIGURE 25-4 Stages of ovarian follicle development. Artist's rendition shows the successive stages of ovarian follicle and oocyte development. Begin with the first stage (primary follicle) and follow clockwise to the final stage (degenerating corpus luteum). Note that all the stages shown occur over time to a single follicle. The presence of all these stages at a single point in time is an artificial arrangement for learning purposes only.

the ovary. These epithelial cells then change from flat to cuboidal to produce a layer of stratified cuboidal epithelial cells called granulosa cells. The multiple layers of granulosa cells completely surrounds the primary follicle. As maturation proceeds, the number of granulosa cell layers increases. These cells then begin secreting increasing amounts of an estrogen-rich fluid that pools around the oocyte in an enlarging space called an antrum. The primary follicle matures into a secondary follicle and, eventually, a mature or Graafian follicle. The release of an ovum from the mature follicle at the end of oogenesis is called ovulation. Granulosa cells also secrete the zona pellucida, a clear gel-like shell that surrounds the oocyte.

When ovulation occurs, blood from the modified granulosa cell layer fills the antrum. A small quantity of blood may also enter the peritoneal cavity and irritate its pain-sensitive surface. This causes the transient lower abdominal pain many women experience at the time of ovulation. Proliferating granulosa cells soon replace the blood filling the antrum, forming a yellow body called the corpus luteum. In turn, the corpus luteum secretes the hormones progesterone, inhibin, relaxin, and limited amounts of estrogen. Progesterone and inhibin suppress follicle-stimulating hormone (FSH) secretion. They also prevent the continued development of new follicles during the functional life of the corpus luteum. The small amounts of relaxin secreted by the corpus luteum each month help “quiet” or “calm” uterine contractions. This action improves the chances for successful implantation if fertilization should occur. If pregnancy does occur, larger amounts of these hormones continue to be produced by the placenta, as we shall see.

In addition to oogenesis, the ovaries are also endocrine organs, secreting the female sex hormones. Estrogens (chiefly estradiol and estrone) and progesterone are secreted by cells of ovarian tissues. These hormones help regulate reproductive function in the female—making the ovaries even more essential to female reproductive function.

We will have a more thorough discussion of oogenesis and fertilization in Chapter 26. Further discussion of hormonal regulation of reproductive functions, as well as associated changes within the ovaries, appears later in this chapter.

4. Briefly describe the location and shape of the ovaries.

5. What is the function of the ovarian follicles?

6. List the major functions of the ovaries.

UTERUS

Location and Support of the Uterus

The uterus is located in the pelvic cavity between the urinary bladder in front and the rectum behind. However, age, pregnancy, and distention of related pelvic viscera such as the bladder may alter the position of the uterus.

Between birth and puberty, the uterus descends gradually from the lower abdomen into the true pelvis. (Recall from Chapter 9 that the true pelvis is the “lesser pelvis” located below the pelvic rim. It houses the urinary and reproductive organs.) At menopause, the uterus decreases in size and assumes a position deep in the pelvis.

Normally the uterus lies over the superior surface of the bladder, pointing forward and slightly upward (see Figure 25-1). The cervix is the lower, narrow part of the uterus: It points downward and backward, joining the vagina at nearly a right angle. Two vault-like recesses, the anterior fornix and posterior fornix, are created where the cervix protrudes into the lumen of the vagina. These corner spaces may help increase the probability of fertilization by pooling seminal fluid for a brief period following intercourse. This in turn helps increase the number of sperm that enter the uterus, and ultimately, the uterine tubes where fertilization occurs.

Several ligaments hold the uterus in place but allow its body considerable movement. In addition, fibers from several muscles that form the pelvic floor converge to form a node called the perineal body (see Figure 25-2). This structure also serves an important role in support of the uterus.

Eight uterine ligaments (three pairs, two single ones) hold the uterus in its normal position by anchoring it in the pelvic cavity. These ligaments include the broad (paired), uterosacral (paired), posterior (single), anterior (single), and round (paired) ligaments. Six of these so-called ligaments are actually extensions of the parietal peritoneum running in different directions. However, the round ligaments are fibromuscular cords. You can see most of these structures in Figures 25-1 and 25-3.

The uterus may lie in any one of several abnormal positions, largely because the ligaments hold it so loosely. A common abnormal position is retroflexion, in which the entire organ is tilted backward. Retroflexion may allow the uterus to prolapse, or descend, into the vaginal canal, which can cause chronic discomfort and pain.

Shape and Structure of the Uterus

In a woman who has never been pregnant, the uterus is pear shaped and measures approximately 7 cm (3 inches) in length, 5 cm (2 inches) in width at its widest part, and 3 cm (1 inch) in thickness. Note in Figure 25-3 that the uterus has two main parts: a wide, upper portion (the body), and a lower, narrow “neck” (the cervix). The body of the uterus rounds into a bulging prominence, the fundus. The dome-shaped fundus is superior to the points of entry of the uterine tubes on both sides.

Three layers comprise the walls of the uterus: (1) the inner endometrium, (2) a middle myometrium, and (3) an outer incomplete layer of parietal peritoneum.

Endometrium

A ciliated mucous membrane called the endometrium lines the uterus. During menstruation and after delivery of a baby, the outer layers of the endometrium slough off. The endometrium varies in thickness from 0.5 mm just after the menstrual flow to about 5 mm near the end of the endometrial cycle.

The endometrium has a rich supply of blood capillaries. It also has numerous exocrine uterine glands that secrete mucus and other substances onto the endometrial surface. The mucous glands in the lining of the cervix produce mucus that changes in consistency during the female reproductive cycle. Most of the time, cervical mucus acts as a barrier to sperm. Around the time of ovulation, however, cervical mucus becomes more slippery and actually facilitates the movement of sperm through the cervix and into the body of the uterus.

Myometrium

The myometrium is the thick, middle layer of the uterine wall. It consists of three layers of smooth muscle fibers. These muscle fibers extend in all directions: longitudinally, transversely, and obliquely—and thus give the uterus great strength. The bundles of smooth muscle fibers are interlaced with elastic and connective tissue components. The result is a blending into the endometrial lining with no sharp boundary between the two layers. The myometrium is thickest in the fundus and thinnest in the cervix—a good example of the principle of structural adaptation to function. The fundus must contract more forcibly than the lower part of the uterine wall to expel the fetus; the cervix must stretch or dilate to accommodate the fetus.

Perimetrium

The outermost serous layer of the uterus (the visceral peritoneal covering), is called the perimetrium. This layer does not completely cover the surface of the uterus. It is absent over the entire cervix and the lower one-fourth of the anterior surface of the uterine body. Look carefully at Figure 25-1 and note that the parietal peritoneum of the anterior pelvic wall folds back on itself and becomes the visceral peritoneum covering the top of the bladder. It then turns upward to cover the upper three fourths of the anterior surface of the uterine body and continues up over the fundus and down the posterior surface of the uterus to the top of the cervix where it is then reflected back to cover the rectum. The perimetrium, although continuous with the peritoneal lining, is incomplete in that it does not cover the entire surface of the uterus. The fact that the entire uterus is not covered with peritoneum may seem silly to point out, but it has clinical significance. It makes it possible to perform operations on this organ without the same risk of infection that occurs in procedures that cut through the peritoneum.

Function of the Uterus

The uterus has many functions important to successful reproduction. It serves as part of the female reproductive tract, permitting sperm from the male to ascend toward the uterine tubes. If fusion of gametes (fertilization, or conception) occurs, the developing offspring implants in the endometrial lining of the uterus and continues its development during the term of pregnancy (gestation). The tiny endometrial glands produce nutrient secretions—sometimes called “uterine milk”—to sustain the developing offspring until a placenta can be produced. The placenta is a unique organ that permits the exchange of materials between the offspring's blood and the maternal blood. A rich network of endometrial capillaries promotes efficiency of this exchange function. Regular contractions of the myometrium, or labor, are inhibited during gestation but become rhythmic and intense as the time of delivery approaches.

If conception or the successful implantation of the embryo fails, then the outer layers of the endometrium are shed during menstruation. Menstruation is a regular event of the female reproductive cycle. It permits the endometrium to renew itself in anticipation of conception and implantation during the next cycle. The myometrial contractions seem to aid menstruation by promoting the complete sloughing of the outer endometrial layers (the “period”). Fatigue of the myometrial muscle tissues may contribute to the abdominal cramping sometimes associated with menstruation.

UTERINE TUBES

Position and Structure of the Uterine Tubes

The uterine tubes are also sometimes called fallopian tubes, or oviducts. They are about 10 cm (4 inches) long and are attached to the uterus at its upper outer angles (see Figures 25-1 and 25-3). You can see that the uterine tubes lie in the upper free margin of the broad ligaments. From here they extend upward and outward toward the sides of the pelvis before curving downward and backward toward the uterus.

The same three layers (mucous, smooth muscle, and serous) of the uterus also comprise the uterine tubes. In fact, the mucosal lining of the tubes is continuous with the peritoneum lining the pelvic cavity. This has great clinical significance because the tubal mucosa is also continuous with that of the uterus and vagina. As a result, the continuous reproductive lining can become infected by gonococci or other organisms introduced into the vagina. Inflammation of the tubes (salpingitis) may readily spread to become inflammation of the peritoneum (peritonitis)—a very serious condition. Inflammation of the uterine tubes may also lead to scarring and partial or complete closure of the lumen. This can happen even if the original infection is cured with antibiotics. (Note: In the male, there is no such direct route by which microorganisms can reach the peritoneum from the exterior.)

Divisions and Tissues of the Uterine Tubes

Each uterine tube consists of three divisions (see Figure 25-3): (1) a connecting part called the isthmus; (2) a dilated portion called the ampulla; and (3) a funnel-shaped end called the infundibulum. The infundibulum lies just above and extends laterally over the ovary. It opens directly into the peritoneal cavity, dividing into fringelike projections called fimbriae.

BOX 25-1 FYI

Tubal Ligation

Tubal ligation literally means “tying a tube.” For this reason, this surgical procedure is often referred to as “having one's tubes tied.” Tubal ligation involves tying a piece of suture material around each uterine tube in two places, then cutting each tube between these two points (see figure). Because sperm and eggs are thus blocked from meeting, fertilization and subsequent pregnancy are prevented. Tubal ligation is also called surgical sterilization and is comparable to vasectomy in the male.

Tubal ligation.

Function of the Uterine Tubes

The uterine tubes are really extensions of the uterus that communicate loosely with the ovaries. This arrangement allows an ovum released from the surface of the ovary to be collected by the fimbriae. From here the ovum is swept along the uterine tube by ciliary action toward the body of the uterus.

However, the uterine tubes serve as more than mere transport channels. The uterine tube is also the site of fertilization. Sperm and ova most often meet, and fertilization occurs, in the ampulla of the uterine tube. A relatively small number of the sperm deposited in the vagina during sexual intercourse move up the uterine tube, where they meet the ovum being swept down toward them. Here is where fertilization usually takes place. Totally blocking the openings into either the distal (abdominal) or proximal (uterine) ends of both uterine tubes, for any reason, results in sterility (Box 25-1).

7. Describe the three principal layers of the uterine wall.

8. Describe the anatomical position of the uterus. How is it held in place?

9. List the major functions of the uterus.

10. What are the functions of the uterine (fallopian) tubes?

VAGINA

Structure of the Vagina

The vagina is a collapsible tube about 8 cm (3 inches) long, situated between the rectum, and the urethra, and the bladder. It is capable of enormous distention during delivery of a baby. It is composed mainly of smooth muscle and is lined with mucous membrane arranged in rugged folds called rugae. The vaginal mucosa contains numerous tiny exocrine mucous glands that secrete lubricating fluid during the female sexual response.

Note that the anterior wall of the vagina is shorter than the posterior wall because of the way the cervix protrudes into the uppermost portion of the tube (see Figure 25-1). In some cases—especially in young girls—a fold of mucous membrane, the hymen, forms a border around the external opening of the vagina, partially closing the orifice. Occasionally, this structure completely covers the vaginal outlet, a condition referred to as imperforate hymen. Perforation must be performed at puberty before the menstrual flow can escape.

Function of the Vagina

The vagina has several important functions. During sexual intercourse, the lining of the vagina lubricates and stimulates the glans penis, which in turn triggers the ejaculation of semen. Thus the vagina also serves as a receptacle for semen, which often pools in the anterior or posterior fornix of the vagina. Here the semen meets the cervix of the uterus. Sperm within the semen may move further into the female reproductive tract by “climbing” along fibrous strands of mucus in the cervical canal.

The vagina also serves as the lower portion of the birth canal. At the time of delivery, the baby is pushed from the body of the uterus, through the cervical canal, and finally through the vagina and out of the mother's body. The placenta, or “afterbirth,” is also expelled through the vagina.

Another important function of the vagina is transport of blood and tissue shed from the lining of the uterus during menstruation.

VULVA

Structure of the Vulva

Figure 25-5 shows you the structures that, together, constitute the female external genitalia. Collectively, these structures are called the vulva. We've summarized the various components and their functions for you in the following paragraphs.

The mons pubis is a skin-covered pad of fat over the pubic symphysis. Coarse pubic hairs appear on this structure at puberty and persist throughout life.

The labia majora (Latin, “large lips”) are covered with pigmented skin and hair on the outer surface and are smooth

FIGURE 25-5 Vulva (pudendum). Sketch showing major features of the external female genitals (genitalia).

and free from hair on the inner surface. Each labium majus is composed mainly of fat and connective tissue with numerous sweat and sebaceous glands on the inner surface. Together the labia majora are homologous to the scrotum in the male.

The labia minora (Latin, “small lips”) are located medially to the labia majora. Each labium minus is covered with hairless skin.

The clitoris is composed of erectile tissue. A small portion of it is visible just behind the junction of the labia minora. Most of the erectile tissue lies buried beneath the skin of the vulva. The structure of this organ is homologous to the penile structure of the male. Like the erectile tissue of the male, the clitoris becomes engorged with blood during the sexual response.

The glans clitoris is the only visible part of the erectile structures of the clitoris. It is equivalent to the glans penis in the male. The glans clitoris is covered with highly sensitive skin that, during sexual stimulation, produces most of the female sexual response.

A clitoral foreskin or prepuce forms a hood over the superior surface of the glans clitoris.

The external urinary meatus (urethral orifice) is the small opening of the urethra, situated between the clitoris and the vaginal orifice. The vaginal orifice has a much larger opening than the urinary meatus. It is located posterior to the meatus.

The greater vestibular glands are two bean-shaped glands, one on each side of the vaginal orifice. Each gland opens by means of a single, long duct into the space between the hymen and the labium minus. These glands, which are also called Bartholin glands, are of clinical importance because they can be infected (bartholinitis or Bartholin abscess), particularly by gonococci. They are homologous to the bulbourethral glands in the male.

Function of the Vulva

The various components of the external genitals of the female operate alone or separately to accomplish several functions important to successful reproduction. For example, the protective features of the mons pubis and labia help prevent injury to the delicate tissues of the clitoris and vestibule. The clitoris becomes erect during sexual stimulation. Like the male glans, it possesses a large number of sensory receptors that feed information back to the sexual response areas of the brain.

A&P CONNECT

There are many sexually transmitted diseases (STDs) that can affect the female reproductive tract. Review examples of important STDs in Sexually Transmitted Diseases online at A&P Connect.

11. List several functions of the vagina.

12. What is another name for the external genitals of the female?

13. List the basic features and functions of the external female genitalia.

14. How are the clitoris of the female and the glans penis of the male similar in structure and function? Can you explain this?

FEMALE REPRODUCTIVE CYCLES

Recurring Cycles

Many changes recur periodically in the female during the years between the onset of the menses (menarche) and their cessation (menopause). Most obvious, of course, is menstruation—the outward sign of changes in the endometrium. Most women also note periodic changes in their breasts. But these are only two of many changes that occur over and over again at fairly uniform intervals during the approximately three decades of female reproductive maturity.

We will first look at the major cyclical changes, and then discuss the mechanisms that produce them.

Ovarian Cycle

Before a female child is born, precursor cells in her ovarian tissue, called oogonia, begin a type of cell division called meiosis, which reduces the number of chromosomes in the daughter cells by half (review Chapter 5, p. 87). By the time the child is born, her ovaries contain about 250,000 primary follicles, each containing an oocyte that has temporarily suspended the meiotic process before it is complete.

Once each month, on about the first day of menstruation, the oocytes within several primary follicles resume meiosis. At the same time, the follicular cells surrounding them increase in number and start to secrete estrogens (and tiny amounts of progesterone). Usually, only one of these developing follicles matures and migrates to the surface of the ovary. Just before ovulation, the meiosis within the oocyte of the mature follicle stops again. It is this cell (which has not quite completed meiosis) that is expelled from the ruptured wall of the mature follicle during ovulation. Meiosis is completed only when, and if, the head of a sperm cell is later drawn into the ovum during the process of fertilization.

When does ovulation occur? This is a question of great practical importance and one that in the past was given many answers. Today it is known that ovulation usually occurs 14 days before the next menstrual period begins. (Only in a 28-day menstrual cycle is this also 14 days after the beginning of the preceding menstrual cycle, as explained later in this chapter.)

Immediately after ovulation, cells of the ruptured follicle enlarge. Because of the appearance of lipid-like substances in them, they are transformed into a golden-colored body, the corpus luteum. The corpus luteum grows for 7 or 8 days. During this time, it secretes progesterone in increasing amounts. Then, provided fertilization of the ovum has not taken place, the size of the corpus luteum and the amount of its secretions gradually diminish. In time, the last components of each nonfunctional corpus luteum are reduced to a white scar called the corpus albicans, which moves into the central portion of the ovary and eventually disappears (see Figure 25-4).

Endometrial (Menstrual) Cycle

During menstruation, parts of the compact and spongy layers of the endometrium slough off. The bleeding that ensues produces a dark menstrual discharge that generally does not clot. Between 30 and 100 ml of blood is expelled, with a majority lost during the first 3 days of the menses. As with the length of the menstrual cycle, considerable variation is normal. After menstruation, the cells of these layers proliferate (increase in size and number), causing the endometrium to reach a thickness of 2 or 3 mm by the time of ovulation. During this period, endometrial glands and arterioles grow longer and more coiled. These two factors contribute to the thickening of the endometrium.

After ovulation, the endometrium grows still thicker, reaching a maximum of about 4 to 6 mm. Most of this increase, however, is probably caused by swelling produced by fluid retention rather than by further proliferation of endometrial cells. The increasingly coiled endometrial glands start to secrete their nutrient fluid during the time between ovulation and the next menses. Then, the day before menstruation starts again, a drop in progesterone causes muscle in the walls of the tightly coiled arterioles to constrict, producing endometrial ischemia. This leads to death of the tissue, sloughing, and once again, menstrual bleeding.

The menstrual cycle is customarily divided into phases, named for major events occurring in each: menses, postmenstrual phase, ovulation, and premenstrual phase.

1. The menses, or menstrual period, occur on days 1 to 5 of a new cycle. There is some individual variation, however.

2. The postmenstrual phase occurs between the end of the menses and ovulation. It is also called the preovulatory phase as well as the proliferative phase. In a 28-day cycle, it usually includes cycle days 6 to 13 or 14. However, the length of this phase varies more than the others. It lasts longer in long cycles and ends sooner in short cycles. This phase is also called the follicular phase, because of the high blood estrogen level resulting from secretion by the developing follicle. Increases in estrogen levels cause predictable changes in the appearance, amount, and consistency of cervical mucus. Collectively, these changes can be used as a fertility sign to predict ovulation (Box 25-2).

3. Ovulation is the rupture of the mature follicle with expulsion of its ovum into the pelvic cavity (Figure 25-6). It occurs most often on cycle day 14 in a 28-day cycle. However, ovulation can occur on different days in cycles of different length, depending on the length of the preovulatory phase. For example, in a 32-day cycle the preovulatory phase probably lasts until cycle day 18. Ovulation would then occur on cycle day 19 instead of 14. Because the majority of women show some month-to-month variation in the length of their cycles, the day of ovulation in a current or future cycle cannot be predicted with accuracy based on the length of previous cycles (see again Box 25-2). However, there is typically a decrease in basal body temperature just before ovulation and a rise in temperature at the time of ovulation. This constitutes yet another “fertility sign” (see Figure 25-9).

BOX 25-2FYI

Fertility Signs Used in Predicting the Time of Ovulation

Many rhythmic and recurring events that a woman may recognize on almost a monthly schedule during her reproductive years are called “fertility signs.” These “signs” represent the body changes required to permit successful reproduction. They include cyclical changes in (1) the ovaries, (2) the amount and consistency of the cervical mucus produced during each cycle, (3) the myometrium, (4) the vagina, (5) gonadotropin secretion, (6) body temperature, and (7) mood or “emotional tone.” Accurately predicting the time of ovulation in any given menstrual cycle by recognizing one or more of these recurring fertility signs would obviously be of help in either avoiding or achieving conception. However, knowing the length of a previous cycle or even a series of cycles cannot ensure with any degree of accuracy the time of appearance of other fertility signs in a current cycle. Nor can it predict how many days the preovulatory phase will last in the next or some future cycle.

Unfortunately, this means that prior cycle length is not an accurate fertility sign. This fact accounts for most of the unreliability of the calendar rhythm method of fertility planning. Other more sophisticated natural family planning (NFP) methods are available that are not based on a knowledge of previous cycle lengths to predict the day of ovulation. Instead, such natural methods base their judgments about fertility at any point in a woman's cycle on other changes. For example, women hopeful of becoming pregnant can predict their general state of receptiveness through the measurement of basal body temperature—body temperature taken after awakening at the same time each day. They can also monitor the cyclical changes in the amount and consistency of cervical mucus during their cycle. Changes in basal body temperature and the amount and consistency of cervical mucus occur in response to changes in circulating hormones that control ovulation. Typically, use of NFP for 1 year to avoid pregnancy will result in approximately 25 of every 100 women becoming pregnant.

The time of ovulation also can be approximated by over-the-counter urine tests that detect the high levels of luteinizing hormone (LH) associated with ovulation (“LH surge”).

4.The premenstrual phase occurs between ovulation and the onset of the menses. This phase is also called the luteal phase or secretory phase, because the corpus luteum secretes progesterone only during this time. The length of the premenstrual phase is fairly constant, lasting usually

FIGURE 25-6Ovulation. The rupture of a mature follicle on the surface of an ovary results in the release of an ovum into the pelvic cavity. This process of ovulation often occurs on day 14 in a 28-day menstrual cycle, but its exact timing depends on the length of the postmenstrual (preovulatory) phase. Notice in this photograph that the ovum released during ovulation is surrounded by a mass of cells.

14 days—or cycle days 15 to 28 in a 28-day cycle. Differences in length of the total menstrual cycle therefore exist mainly because of differences in duration of the postmenstrual rather than of the premenstrual phase.

Gonadotropic Cycle

As we saw in Chapter 15, the anterior pituitary gland secretes two hormones called gonadotropins that influence female reproductive cycles. Their names are follicle-stimulating hormone (FSH) and luteinizing hormone (LH). The amount of each gonadotropin secreted varies with a rhythmic regularity that can be related, as we shall see, to the rhythmic ovarian and uterine changes just described.

15. Define menarche and menopause.

16. What is the function of the corpus luteum?

17. What is the difference between the proliferative phase and the luteal or secretory phase?

18. Briefly describe the four phases of the menstrual cycle.

Control of Female Reproductive Cycles

Hormones play a major role in producing the cyclical changes characteristic of women during their reproductive years. The following paragraphs provide a brief description of the mechanisms that produce cyclical changes in the ovaries and uterus and in the amounts of gonadotropins secreted.

Control of Cyclical Changes in the Ovaries

Cyclical changes in the ovaries result from cyclical changes in the amounts of gonadotropins secreted by the anterior pituitary gland. An increasing FSH blood level has two effects: (1) it stimulates one or more primary follicles and their oocytes to start growing, and (2) it stimulates the follicular cells to secrete estrogens. (Developing follicles also secrete very small amounts of progesterone.)

Because of the influence of FSH on follicle secretion, the level of estrogens in blood increases gradually for a few days during the postmenstrual phase. Then suddenly, on about the twelfth cycle day, it leaps upward to a maximum peak. Scarcely 12 hours after this “estrogen surge,” an “LH surge” occurs and presumably triggers ovulation a day or two later. This hormone surge is the basis of the over-the-counter “ovulation test” (see Box 25-2).

The control of cyclical ovarian changes by the gonadotropins FSH and LH is summarized for you in Figure 25-7. Refer to this diagram as you read the following description of cyclical changes in the ovary.

1. Completion of growth of the follicle and oocyte maturation with increasing secretion of estrogens before ovulation. LH and FSH act as synergists to produce these effects.

2. Rupturing of the mature follicle with expulsion of its ovum (ovulation). Because of this function, LH is sometimes also called “the ovulating hormone.”

3. Formation of a yellowish body, the corpus luteum, in the ruptured follicle (process called luteinization). The name luteinizing hormone refers, obviously, to this LH function—a function to which, experiments have shown, FSH also contributes.

The corpus luteum functions as a temporary endocrine gland. It secretes only during the luteal (postovulatory, or premenstrual) phase of the menstrual cycle. It secretes progesterone and estrogen. The blood level of progesterone rises rapidly after the “LH surge” described earlier. It remains at a high level for about a week, and then decreases to a very low level approximately 3 days before menstruation begins again. This low blood level of progesterone persists during both the menstrual and the postmenstrual phases. What are its sources? Not the corpus luteum, which secretes only during the luteal phase, but the developing follicles and the adrenal cortex. Blood's estrogen content increases during the luteal phase but to a lower level than develops before ovulation.

If pregnancy does not occur, lack of sufficient LH and FSH causes the corpus luteum to regress in about 14 days. The corpus luteum is then replaced by the corpus albicans. To make sure you've understood this process, review again Figure 25-4, which shows the cyclical changes in the ovarian follicles.

Control of Cyclical Changes in the Uterus

Changing blood concentrations of estrogens and progesterone also bring about cyclical changes in the uterus. As blood estrogens increase during the postmenstrual phase of the menstrual cycle, they produce the following changes in the uterus:

▪Thickening of the endometrium

▪Growth of glands and spiral arteries within the endometrium

▪Increase in the water content of the endometrium

▪Increase of myometrial contractions

Increasing blood progesterone concentration during the premenstrual phase of the menstrual cycle produces changes in the uterus due to the actions of progesterones. These changes are favorable for pregnancy—specifically the following:

▪Preparation of the endometrium for the implantation of a fertilized ovum

▪Increase in the water content of the endometrium

▪Decrease of myometrial contractions

As we mentioned earlier, low levels of FSH and LH cause atrophy of the corpus luteum if pregnancy does not occur. This in turn causes a drop in estrogen and progesterone levels. As a result, the maintenance of a thick, vascular endometrium

FIGURE 25-7 The primary effects of gonadotropins on the ovaries. Follicle-stimulating hormone (FSH) gets its name from the fact that it triggers development of primary ovarian follicles and stimulates follicular cells to secrete estrogens. Luteinizing hormone (LH) has several effects on ovaries: (1) LH acts with FSH to enhance its effects on follicular development and secretion; (2) LH presumably triggers ovulation—hence it is called “the ovulating hormone”; and (3) LH has a luteinizing effect (for which the hormone was named); recent evidence shows that FSH is also necessary for luteinization.

ceases. Finally, a drop in estrogen and progesterone levels at the end of the premenstrual phase triggers the endometrial sloughing that characterizes the menstrual phase (the “period”).

Control of Cyclical Changes in Gonadotropin Secretion

Both negative and positive feedback mechanisms help control anterior pituitary secretion of the gonadotropins FSH and LH. These mechanisms involve the ovaries' secretion of inhibin, estrogens, and progesterone. They also involve the secretion of releasing hormones by the hypothalamus. Figure 25-8 shows you the negative feedback mechanism that controls gonadotropin secretion. Examine it carefully. Notice the effects of a sustained high blood concentration of estrogens and progesterone on anterior pituitary gland secretion. Note also the effect of a low blood concentration of FSH on follicular development. Essentially, follicles do not mature and ovulation does not occur under these conditions.

Several observations and animal experiments suggest that sustained high blood levels of estrogens, progesterone, and inhibin decrease pituitary secretion of FSH and LH.

FIGURE 25-8 Control of FSH and estrogen secretion. A negative feedback mechanism controls anterior pituitary secretion of follicle-stimulating hormone (FSH) and ovarian secretion of estrogens. A high blood level of FSH stimulates estrogen secretion, whereas the resulting high estrogen level inhibits FSH secretion.

These ovarian hormones appear to inhibit the hypothalamus from secreting gonadotropin-releasing hormone (GnRH). Without the stimulating effects of these releasing hormones, the pituitary's secretion of FSH and LH decreases.

A positive feedback mechanism may also control LH secretion. There is a sudden and marked increase in blood estrogen level that occurs late in the follicular phase of the menstrual cycle. This is thought to stimulate the hypothalamus to secrete GnRH. In turn, GnRH stimulates the release of LH by the anterior pituitary. This would in turn account for the “LH surge” that triggers ovulation.

FIGURE 25-9 Female reproductive cycles. This diagram illustrates the interrelationships among the cerebral, hypothalamic, pituitary, ovarian, and uterine functions throughout a standard 28-day menstrual cycle. The variations in basal body temperature are also illustrated. The labels at the bottom of the diagram are the phases of the menstrual cycle.

The fact that a part of the brain—the hypothalamus—secretes gonadotropin-releasing hormones has interesting implications. This may be part of the pathway by which changes in a woman's environment or in her emotional state can alter her menstrual cycle. That this occurs is a matter of common observation. Stress, for example—such as intense fear of either becoming or not becoming pregnant—often delays menstruation.

Importance of Female Reproductive Cycles

The female reproductive cycles play several important roles. As Figure 25-9 illustrates for you, the changes associated with the different cycles are all closely interrelated. The primary role of the ovarian cycle, for example, is to produce an ovum at regular enough intervals to make reproductive success likely. The ovarian cycle's secondary role is to regulate the endometrial (menstrual) cycle by means of the sex hormones estrogen and progesterone. The role of the endometrial cycle, in turn, is to ensure that the lining of the uterus is suitable for the implantation of an embryo if fertilization of the ovum occurs. The constant renewal of the endometrium makes successful implantation more likely.

Human fertility is further limited by the fact that sperm usually cannot survive in the female reproductive tract for more than a few days. Such limited fertility increases the likelihood that conception will occur only when the woman's body is at its reproductive peak. Box 25-3 discusses some common methods for managing fertility.

Infertility and Use of Fertility Drugs

Infertility is often defined as failure to conceive after 1 year of regular unprotected intercourse. Infertility may be caused by a wide variety of medical, environmental, and even lifestyle factors, such as smoking or alcohol abuse. Causal factors may be traced to various problems in either the male or female partner, each accounting for about 40% of cases. Of the remaining 20% of affected couples, infertility in about 10% is due to problems shared by both partners and in about 10% the reason is never determined.

TABLE 25-1 Some Female Reproductive Hormones*

HORMONE

SOURCE

TARGET

ACTION

Dehydroepiandrosterone (DHEA)

Adrenal gland, ovary, other tissues

Converted to other hormones

Eventually converted to estrogens, testosterone, or both

Estrogens (including estradiol [E2] and estrone)

Ovary and placenta (small amounts in other tissues)

Uterus, breast, other tissues

Stimulates development of female sexual characteristics, breast development, bone and nervous system maintenance

Follicle-stimulating hormone (FSH)

Anterior pituitary (gonadotroph cells)

Ovary

Gonadotropin; promotes development of ovarian follicle; stimulates estrogen secretion

Conadotropin-releasing hormone (GnRH)

Hypothalamus (neuroendocrine cells)

Anterior pituitary (gonadotroph cells)

Stimulates production and release of gonadotropins (FSH and LH) from anterior pituitary

Human chorionic gonadotropin (hCG)

Placenta

Ovary

Stimulates secretion of estrogen and progesterone during pregnancy

Inhibin

Ovary

Anterior pituitary (gonadotroph cells)

Inhibits FSH production in the anterior pituitary (perhaps by limiting GnRH)

Luteinizing hormone (LH)

Anterior pituitary (gonadotroph cells)

Ovary

Gonadotropin; triggers ovulation; promotes development of corpus luteum

Progesterone

Ovary and placenta

Uterus, mammary glands, other tissues

Helps maintain proper conditions for pregnancy

Relaxin

Placenta

Uterus and joints

Inhibits uterine contractions during pregnancy and softens pelvic joints to facilitate childbirth

Testosterone

Adrenal glands, ovaries

Nervous tissue, bone tissue, other tissues

May affect mood, sex drive, learning, sleep, protein anabolism, other functions

* The role of some hormones related to pregnancy, labor, and delivery are discussed in more detail in the next chapter (Chapter 26, Growth and Development).

BOX 25-3 Health Matters

Methods of Contraception

Hormonal methods of contraception began with establishment of the relationship between sex hormone levels and ovulation. Continuing research in this area led to the development of oral contraceptives—often collectively called “the Pill.” Numerous oral contraceptive products are now available that contain different types, combinations, and dosages of estrogen and/or progesterone. The so-called minipill contains only synthetic progesterone.

Most hormonal contraceptives were developed to prevent pregnancy by initiating negative feedback inhibition of FSH and LH secretion. As a result, mature follicles do not develop, and LH levels required to initiate ovulation do not occur. The next menses, however, does take place if the progesterone and estrogen dosage is stopped in time to allow their blood levels to decrease as they normally do near the end of the cycle to bring on menstruation. For this reason, the Pill can be used to regulate the menstrual cycle, as well as prevent pregnancy. If used correctly and consistently, the pill is an extremely effective contraceptive with an unintended pregnancy rate estimated at between 0.1% and 3%. The higher percentage reflects “typical” rather than “ideal” use, and underscores the impact of human error in using any form of birth control.

In addition to oral contraceptives taken in pill form, other types of hormonal birth control “delivery mechanisms” are available. They include hormone-impregnated vaginal inserts, hormone injections, transcutaneous administration using skin “patches,” and surgical insertion of hormone-containing implants under the skin (see figure).

The effects of hormonal contraceptives—indeed, of estrogens and progesterone—are much more complex than our explanation here indicates. They have widespread effects on the body quite independent of their action on the reproductive and endocrine systems and are still not completely understood. Possible side effects—some extremely serious, including stroke and heart attack—may limit or prohibit use of these birth control methods by some women. Side effects and health risks of hormonal contraceptives are especially troublesome if these products are used for extended periods, by older women, by women who smoke, and by women with blood clotting problems or cardiovascular disease. On the other hand, long-term use has also been shown to have some beneficial health effects such as protection against uterine and ovarian cancer in some groups of women.

In addition to hormonal methods of contraception, many other methods, each with differing rates of effectiveness and unique advantages and disadvantages, are available. For example, spermicidal methods involve use of preparations (foams, jellies, and creams) that act to kill sperm, and mechanical barrier methods use devices such as condoms, diaphragms, and cervical caps to block sperm from entering the uterus. So-called surgical methods such as tubal ligation and male vasectomy result in permanent sterility.

The use of contraceptive methods to regulate reproductive function often involves many personal decisions. For example, the decision to use or avoid contraception—or employ any particular contraceptive method—at any point in time is often influenced by differing medical, social, cultural, ethical, and religious factors as well as by the cost, reliability, safety, or ease of use of a particular method. Informed and thoughtful decision making regarding this human behavior is critically important. It will often be necessary for some individuals to seek out a variety of information—from different but credible and knowledgeable sources—in order to make an informed decision that is “right” for those individuals. Regardless, seeking counsel and advice from a trusted health care provider early in the process is always recommended.

If testing identifies the female member of the couple as infertile, she joins a subset of about 25% of women in the overall population who will experience some period of infertility during their reproductive years. In many cases, infertility results from a failure to ovulate. This is often caused by a medical condition such as polycystic ovary syndrome (PCOS) (see Mechanisms of Disease, available at your Evolve site online). Significant numbers of infertile women who experience ovulatory dysfunction desire to become pregnant. After a sometimes long and complex medical workup and selection process, they may become candidates to receive so-called fertility drugs. These can be used alone or in combination with other “assisted reproductive procedures” such as artificial insemination.

Menarche and Menopause

The menstrual flow first occurs (menarche) at puberty, at about the age of 13 years, although there is individual variation according to race, nutrition, health, and heredity. Normally, it recurs about every 28 days for about three decades, except during pregnancy, and then ceases (menopause). The average age at which menstruation ceases is reported to have increased markedly—from about age 40 years a few decades ago to between ages 45 and 50 years more recently.

The changes just described relate to changes in hormone levels over the life span. Relatively low concentrations of gonadotropins (FSH and LH) sustain a peak of estrogen secretion from menarche to menopause. After menopause, estrogen concentration decreases dramatically—which causes a negative feedback response that increases the gonadotropin levels. Because the follicular cells are no longer sensitive to gonadotropins after menopause, the increased gonadotropin level has no effect on estrogen secretion.

Table 25-1 summarizes some of the hormones important in female reproductive function.

19. A surge in FSH and LH is associated with what major event of the ovarian cycle?

20. How does an increase in estrogen level affect the uterine lining?

21. What is infertility?

BREASTS

Location and Size of the Breasts

Two breasts lie over the pectoral muscles and are attached to them by a layer of connective tissue (Figure 25-10). Breasts are made up of milk-producing mammary glands, which are present in all mammals, along with extensive supporting tissues. They are present in both males and females—but only infrequently develop or produce milk in males.

Estrogens and progesterone both control breast development during puberty. Estrogens stimulate growth of the

FIGURE 25-10 The female breast. A, Sagittal section of a lactating breast. Notice how the glandular structures are anchored to the overlying skin and to the pectoral muscles by suspensory ligaments. Each lobule of glandular tissue is drained by a lactiferous duct that eventually opens through the nipple. B, Anterior view of a lactating breast. Overlying skin and connective tissue have been removed from the medial side to show the internal structure of the breast and underlying skeletal muscle. In nonlactating breasts, the glandular tissue is much less prominent, with adipose tissue making up most of each breast.

ducts of the mammary glands, whereas progesterone stimulates development of the actual milk-secreting cells. Breast size is determined more by the amount of fat around the glandular tissue than by the amount of glandular tissue itself. Hence the size of the breast is not related to its ability to produce milk.

Structure of the Breasts

Each breast consists of several lobes separated by septa (walls) of connective tissue. A lobe consists of several lobules, which, in turn, are composed of connective tissues. Alveoli or “pouches” of milk-secreting cells are imbedded in these tissues. The alveoli are arranged in grapelike clusters around a tiny ductule. Figure 25-11 shows one of the mammary alveoli and the milk-producing cells that form its walls. Modified epithelial cells called myoepithelium surround the outside of the alveolus. This type of cell contracts slightly, as if it were a muscle cell, thus squeezing milk out into the secretory duct.

The ductules from the various lobules unite, forming a single lactiferous “milk-carrying” duct for each lobe. There are between 15 and 20 lobes in each breast. These main lactiferous ducts converge toward the nipple, like the spokes of a wheel. They enlarge slightly into small lactiferous sinuses before reaching the nipple (see Figure 25-10, A). The lactiferous sinuses are positioned so that they will be squeezed by the suckling motion of a baby's jaws during breastfeeding. This allows the sinuses to act as little pumping chambers that help milk flow out of the breast. Each of the main ducts terminates in a tiny opening on the surface of the nipple.

Typically a large amount of adipose tissue is deposited around the surface of the gland, just under the skin, as well as between the lobes. Suspensory ligaments, positioned throughout the connective tissue of the breast, help support the glandular and connective tissues of the entire structure, anchoring them to the underlying pectoral muscles.

The nipples are bordered by a circular pigmented area, the areola (see Figure 25-10, B). It contains numerous sebaceous

FIGURE 25-11 Alveolus of the mammary gland. Notice the contractile myoepithelial cells that surround the milk-producing cells. Milk is released as vesicles of fluid pinched off from the cell.

glands that appear as small nodules under the skin. Sebum produced by these areolar glands helps reduce irritating dryness of the areolar skin associated with nursing. In some lighter-skinned women, the areola and nipple change color from pink to brown early in pregnancy—a fact useful in diagnosing a first pregnancy. The color decreases after lactation has ceased but never entirely returns to the original hue. In some darker-skinned women, no noticeable color change in the areola or nipple heralds the first pregnancy.

Knowledge of the lymphatic drainage of the breast is important in clinical medicine because cancerous cells from malignant breast tumors often spread to other areas of the body through the lymphatics (see Chapter 19, p. 431).

A&P CONNECT

Self-examination of breasts is a recommended routine to monitor breast health. Learn more in Breast Self-Examination online at A&P Connect.

Lactation

The obvious function of the mammary glands is lactation—the secretion of milk for the nourishment of newborn infants. We've summarized the basic mechanism underlying lactation below and we've highlighted the major structures and hormones for you in Figure 25-12. As you read the following, please refer to this figure.

▪ The ovarian hormones, estrogens and progesterone, act on the breasts to make them structurally ready to secrete milk. As we've discussed, estrogens promote development of the ducts of the breasts. Progesterone acts on the estrogen-primed breasts to promote completion of the development of the ducts and development of the alveoli—the secreting cells of the breasts. This is an example of hormonal permissiveness; estrogen permits progesterone to have its full effect. A high blood concentration of estrogens during pregnancy also inhibits anterior pituitary secretion of prolactin.

▪ Shedding of the placenta after delivery of the baby cuts off a major source of estrogens. The resulting rapid drop in the blood concentration of estrogens stimulates anterior pituitary secretion of prolactin. Also, the suckling movements of a nursing baby stimulate anterior pituitary secretion of prolactin and posterior pituitary release of oxytocin.

▪ Prolactin stimulates alveoli of the mammary glands to secrete milk. Lactation starts about the third or fourth day after delivery of a baby. Milk replaces the secretion of a thin, yellowish secretion called colostrum. With repeated stimulation by the suckling infant, plus various favorable mental and physical conditions, lactation may continue for extended periods.

▪ Oxytocin stimulates myoepithelial cells in the alveoli of the breasts to eject milk into the ducts. This makes it accessible to the suckling infant.

This summary highlights only the major hormonal mechanisms that regulate lactation. However, there are many hormones that support the processes needed for successful lactation.

The Importance of Lactation and Breast Milk

The process of lactation plays an important role in the ultimate success of the reproductive system. The biological goal of

FIGURE 25-12 Lactation. The illustration and accompanying flowchart summarize the mechanisms that control the secretion and ejection of milk.

human reproduction does not lie solely in delivering a healthy infant—the infant must also survive until reproductive age. If a child does not survive to reproduce, then the genomes of the mother and father cannot be passed on to successive generations. Thus, the ultimate goal of reproduction will not have been met. Humans and other mammals help ensure the survival of offspring for up to several years by producing nutrient-rich milk. Nursing from the mother's breast provides several advantages for human offspring:

▪Human milk is a rich source of proteins, fat, calcium, vitamins, and other nutrients in proportions needed by a young, developing body.

▪Human milk provides passive immunity to the offspring in the form of maternal antibodies present in both colostrum and the milk.

▪Nursing appears to enhance the emotional bond between mother and child. Such bonding may foster healthy psychological development in the child and strengthen family relationships that contribute to successful human development.

22. Briefly describe the network of ducts and secreting cells that form the mammary glands.

23. List the hormones that prepare the breast structurally for lactation.

24. Which hormone causes milk to be ejected into the lactiferous ducts?

Cycle of LIFE

As we have seen, the reproductive system is unlike any other body system with regard to the normal changes that occur during the life span. For example, the female reproductive system does not begin to perform its functions until the teenage years (puberty). Furthermore, unlike the male reproductive system, the female reproductive system ceases its principal functions in middle adulthood.

The female organs begin their initial stages of development in the womb. As a matter of fact, the first stage of meiotic development of all the ova that will ever be produced by a woman is completed by the time she is born. However, full development of the reproductive organs—and the gametes within the ovaries—does not resume until puberty. At puberty, reproductive hormones stimulate the organs of the reproductive tract to become functional and produce one mature ovum one at a time. Reproductive function then continues in a cyclical fashion until menopause. Menopause is an event that is usually marked by the passage of at least one full year without menstruation. After that time, a woman may continue to enjoy normal sexual activity, but she cannot produce more offspring.

The BIG Picture

Perhaps you can see now how the significance of reproduction lies in the fact that it imparts virtual immortality to our genes. This is important not only to the survival of the human species but also to the survival of life itself. After all, life as we know it could not exist without a genetic code. The “big picture” of human procreation requires two reproductive systems—one reproductive system in each parent. The combined roles of the male and female reproductive systems will be explored as a single topic in the early part of the next chapter.

For now, let's briefly take a closer look at the female reproductive system and its relationships with other systems within a woman's body. As with any system, the female reproductive system cannot function without integrated support from the circulatory, immune, respiratory, digestive, and urinary systems. The female reproductive system shares a special anatomical relationship with the urinary system. These two systems develop in close proximity to each other and thus share a common structure: the vulva. A special anatomical relationship with the skeletal muscular system is evident in the structure known as the perineum. Of course the skeletal and muscular systems both support and protect the internal organs of the female reproductive system.

An even more special relationship with the integumentary system should be noted. The breasts, containing the milk-producing mammary glands, are actually modifications of the skin and their sweat glands. Structurally the breasts can be thought of as belonging to the integumentary system. However, functionally they are best considered as a part of the reproductive system. We have outlined the nervous and endocrine regulation of female reproductive function in this chapter. We will explore these and other connections further in the next chapter.

MECHANISMS OF DISEASE

The intricacies of the female reproductive system make it susceptible to a large number of conditions, disorders, and diseases. Hormonal and menstrual disorders such as dysmenorrhea, amenorrhea, and dysfunctional uterine bleeding are very common. Fibroid cysts and a collection of symptoms called premenstrual syndrome afflict millions of women. There are also numerous infectious diseases that are particularly debilitating, including pelvic inflammatory disease, salpingitis, and vaginitis. The list of afflictions becomes longer when tumors and related conditions of the female reproductive system are considered. These include polycystic ovary syndrome, ovarian cysts, and endometriosis. Sadly, cancers of the breast, cervix, and ovaries are also common and deadly.

Find out more about these diseases and disorders of the female reproductive system online at Mechanisms of Disease: Female Reproductive System.

CHAPTER SUMMARY

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Scan this summary after reading the chapter to help you reinforce the key concepts. Later, use the summary as a quick review before your class or before a test.

OVERVIEW OF THE FEMALE REPRODUCTIVE SYSTEM

A. Function of the female reproductive system

1. Female reproductive system produces gametes called ova (eggs)

2. Female reproductive system also provides protection and nutrition to the developing offspring for up to several years after conception

B. Structural plan of the female reproductive system

1. Reproductive organs are classified as essential or accessory (Figure 25-1)

a. Essential organs—gonads are the paired ovaries

b. Accessory organs—uterine tubes, uterus, vagina, vulva, and mammary glands (Figures 25-1, 25-2, and 25-3)

C. Perineum—female perineum is a muscular region within a diamond-shaped area between the thighs and the vaginal orifice and the anus (Figure 25-2)

1. Extends from the pubic symphysis anteriorly to the coccyx posteriorly

2. A line drawn between the ischial tuberosities divides the area into two triangles

a. Urogenital triangle—contains the external genitals (labia, vaginal orifice, clitoris) and urinary opening

b. Anal triangle—surrounds the anus

3. Area that may be torn during childbirth

OVARIES

A. Location of the ovaries

1. Nodular glands located on each side of the uterus, below and behind the uterine tubes (Figure 25-3)

2. Weigh about 3 grams; attached to the posterior surface of the broad ligament by the mesovarian ligament (mesovarium)

3. Ovarian ligament anchors the ovary to the uterus

B. Structure and function of the ovaries

1. Covering the outer cortex is a layer of flattened epithelial cells; germinal epithelium

2. Ovarian follicles contain the developing female sex cell

C. Overview of oogenesis

1. Oogenesis—process that results in formation of a mature egg (Figure 25-4)

a. Throughout the process of ovarian development, the oocyte grows in size

b. Ovulation—release of an ovum from the mature follicle at the end of oogenesis

c. Ovaries are also endocrine organs, secreting the female sex hormones

UTERUS

A. Location and support of the uterus

1. Uterus is located in the pelvic cavity between the urinary bladder in front and the rectum behind

2. Normally the uterus lies over the superior surface of the bladder, pointing forward and slightly upward (Figure 25-1)

3. Uterus may lie in any one of several abnormal positions, largely because the ligaments hold it so loosely

a. Retroflexion—entire organ is tilted backward; may allow the uterus to prolapse, or descend, into the vaginal canal

B. Shape and structure of the uterus

1. Pear shaped and measures approximately 7 cm (3 inches) in length, 5 cm (2 inches) in width at its widest part, and 3 cm (1 inch) in thickness

2. Uterus has two main parts (Figure 25-3):

a. Body—wide, upper portion,

b. Cervix—lower, narrow “neck”

3. Three layers comprise the walls of the uterus:

a. Inner endometrium

b. Middle myometrium

c. Outer perimetrium, incomplete layer of visceral peritoneum

C. Endometrium—A ciliated mucous membrane that lines the uterus

1. Varies in thickness from 0.5 mm just after the menstrual flow to about 5 mm near the end of the endometrial cycle

2. Endometrium has a rich supply of blood capillaries and numerous uterine glands that secrete mucus and other substances onto the endometrial surface

D. Myometrium—thick, middle layer of the uterine wall; consists of three layers of smooth muscle fibers

1. Thickest in the fundus and thinnest in the cervix

E. Perimetrium—incomplete external layer of visceral peritoneum

1. Location is partially retroperitoneal

F. Functions of the uterus

1. Uterus serves as part of the female reproductive tract, permitting sperm from the male to ascend toward the uterine tubes

2. If conception occurs, an offspring develops in the uterus

3. If conception or the successful implantation of the embryo fails, then the outer layers of the endometrium are shed during menstruation

UTERINE TUBES

A. Uterine tubes are also sometimes called fallopian tubes, or oviducts

B. Uterine tubes are about 10 cm (4 inches) long and are attached to the uterus at its upper outer angles (Figures 25-1 and 25-3)

C. Structure of the uterine tubes

1. Same three layers (mucous, smooth muscle, and serous) of the uterus also comprise the uterine tubes

2. Mucosal lining of the tubes is continuous with the peritoneum lining the pelvic cavity

3. Tubal mucosa is also continuous with that of the uterus and vagina

D. Divisions and tissues of the uterine tubes

1. Each uterine tube consists of three divisions (Figure 25-3)

a. Ampulla

b. Isthmus

c. Infundibulum

E. Functions of the uterine tubes—serve as more than mere transport channels; also site of fertilization

VAGINA

A. Structure of the vagina

1. Collapsible tube about 8 cm (3 inches) long, capable of enormous distention during delivery of a baby

2. Composed mainly of smooth muscle and is lined with mucous membrane arranged in rugged folds called rugae

3. Anterior wall of the vagina is shorter than the posterior wall because the cervix protrudes into the uppermost portion of the tube (Figure 25-1)

4. Hymen—fold of mucous membrane that forms a border around the external opening of the vagina

B. Functions of the vagina

1. Lining of the vagina lubricates and stimulates the glans penis, which in turn triggers the ejaculation of semen

2. Serves as a receptacle for semen

3. Serves as the lower portion of the birth canal

4. Transports blood and tissue shed from the lining of the uterus during menstruation

VULVA

A. Structure of the vulva

1. Mons pubis—skin-covered pad of fat over the pubic symphysis

2. Labia majora—composed mainly of fat and connective tissue with numerous sweat and sebaceous glands on the inner surface

3. Labia minora—located medially to the labia majora; covered with hairless skin

4. Clitoris—composed of erectile tissue

5. Glans clitoris—only visible part of the erectile structures of the clitoris; covered with highly sensitive skin that during sexual stimulation produces most of the female sexual response

6. External urinary meatus—situated between the clitoris and the vaginal orifice

7. Vaginal orifice—much larger opening than the urinary meatus; located posterior to the meatus

8. Greater vestibular glands (Bartholin glands)—two bean-shaped glands, one on each side of the vaginal orifice

B. Functions of the vulva

1. Protective features of the mons pubis and labia help prevent injury to the delicate tissues of the clitoris and vestibule

2. Possesses a large number of sensory receptors that feed information back to the sexual response areas of the brain

FEMALE REPRODUCTIVE CYCLES

A. Recurring cycles—many changes recur periodically in the female during the years between the onset of the menses (menarche) and their cessation (menopause)

1. Menstruation—the outward sign of changes in the endometrium

B. Ovarian cycle—ovaries from birth contain oocytes in primary follicles in which the meiotic process has been suspended; at the beginning of menstruation each month, several of the oocytes resume meiosis; meiosis will stop again just before the cell is released during ovulation

C. Endometrial (menstrual) cycle—divided into four phases

1. Menses (menstrual period)—occur on days 1 to 5 of a new cycle

2. Proliferative phase—occurs between the end of the menses and ovulation

3. Ovulation—rupture of the mature follicle with expulsion of its ovum into the pelvic cavity (Figure 25-6)

4. Luteal phase (secretory phase)—occurs between ovulation and the onset of the menses

D. Gonadotropic cycle—two hormones called gonadotropins that influence female reproductive cycles

1. Follicle-stimulating hormone (FSH)

2. Luteinizing hormone (LH)

E. Control of female reproductive cycles—hormones control cyclical changes

F. Control of cyclical changes in the ovaries

1. Cyclical changes in the ovaries result from cyclical changes in the amounts of gonadotropins secreted by the anterior pituitary gland

2. Increasing FSH blood level has two effects (Figure 25-7):

a. Stimulates one or more primary follicles and their oocytes to start growing

b. Stimulates the follicular cells to secrete estrogens

G. Control of cyclical changes in the uterus

1. Cyclical changes in the uterus are caused by changes in estrogens and progesterone

H. Control of cyclical changes in gonadotropin secretion

1. Both negative and positive feedback mechanisms help control anterior pituitary secretion of the gonadotropins FSH and LH

I. Importance of female reproductive cycles

1. Ovarian cycle's primary function is to produce ova at regular intervals

a. Secondary role is to regulate the endometrial cycle by means of the sex hormones estrogen and progesterone

2. Role of the endometrial cycle is to ensure that the lining of the uterus is suitable for the implantation of an embryo

J. Infertility and use of fertility drugs

1. Infertility—failure to conceive after 1 year of regular unprotected intercourse

a. May be caused by a wide variety of medical, environmental, and even lifestyle factors

b. Fertility drugs and other assisted reproductive procedures are available

K. Menarche and menopause

1. Menstrual flow first occurs (menarche) at puberty, at about the age of 13 years; individual variation according to race, nutrition, health, and heredity

a. Recurs about every 28 days for about three decades, except during pregnancy, and then ceases (menopause)

BREASTS

A. Location and size of the breasts

1. Breasts lie over the pectoral muscles and are attached to them by a layer of connective tissue (Figure 25-10)

2. Made up of milk-producing mammary glands

3. Estrogens and progesterone both control breast development during puberty

4. Breast size is determined more by the amount of fat around the glandular tissue than by the amount of glandular tissue

B. Structure of the breasts

1. Each breast consists of several lobes separated by septa (walls) of connective tissue

a. Each lobe consists of several lobules, which are composed of connective tissues

b. Alveoli or “pouches” of milk-secreting cells are imbedded in these tissues (Figure 25-11)

c. Alveoli are arranged in grapelike clusters around a tiny ductule

d. Ductules from the various lobules unite, forming a single lactiferous duct for each lobe

2. Nipples are bordered by a circular pigmented area, the areola (Figure 25-10, B)

a. Contains numerous sebaceous glands that appear as small nodules under the skin; sebum helps reduce irritating dryness of the areolar skin associated with nursing

C. Lactation—secretion of milk for the nourishment of newborn infants

1. Basic mechanism underlying lactation:

a. The ovarian hormones, estrogens and progesterone, act on the breasts to make them structurally ready to secrete milk

b. Shedding of the placenta after delivery of the baby cuts off a major source of estrogens; stimulates anterior pituitary secretion of prolactin

c. Prolactin stimulates alveoli of the mammary glands to secrete milk

d. Oxytocin stimulates myoepithelial cells in the alveoli of the breasts to eject milk into the ducts

D. The importance of lactation and breast milk

1. The process of lactation plays an important role in the ultimate success of the reproductive system

2. Nursing from the mother's breast provides several advantages for human offspring

a. Human milk is a rich source of proteins, fat, calcium, vitamins, and other nutrients in proportions needed by a young, developing body

b. Human milk provides passive immunity to the offspring in the form of maternal antibodies present in both colostrum and the milk

c. Nursing appears to enhance the emotional bond between mother and child

REVIEW QUESTIONS

Write out the answers to these questions after reading the chapter and reviewing the Chapter Summary. If you simply think through the answer without writing it down, you won't retain much of your new learning.

1. Identify the essential and accessory organs in the female reproductive system.

2. Describe the three layers that compose the wall of the uterus.

3. Identify the vessels that supply blood to the uterus.

4. List the eight ligaments that hold the uterus in a normal position.

5. How does the uterus serve as part of the female reproductive tract?

6. What and where are the uterine tubes? Approximately how long are they? What lines the uterine tubes? Their lining is continuous on their distal ends with what? With what on their proximal ends?

7. What hormones are secreted by the cells in ovarian tissue?

8. Identify all vaginal functions.

9. List all the structures that make up the female external genitals.

10. Define the term episiotomy.

11. Identify the advantages that nursing from the mother's breast provides offspring.

12. Describe the hormonal changes during menopause.

CRITICAL THINKING QUESTIONS

After finishing the Review Questions, write out the answers to these items to help you apply your new knowledge. Go back to sections of the chapter that relate to items that you find difficult.

1. Name and explain the function of the various hormones that regulate lactation. Where are they produced, and how would you summarize their function and their influence on lactation?

2. List the phases of the menstrual cycle. Which of these phases shows the most variance in length of time? How do the events in each phase contribute to the overall function of the reproductive system?

3. How would you explain the interaction of the hormones that result in ovulation? From what is the name “luteinizing” hormone derived?

4. State in your own words the control of cyclical ovarian changes brought on by FSH and LH.

5. Explain, in your own words, the control of cyclical uterine changes brought on by the ovarian hormones. The drop in the level of these hormones triggers what event?

6. How would you correlate the events of the ovarian cycle with the events of the uterine cycle?