Structure and Function of the Reproductive Systems

The labia majora (sing., labium majus) is composed of two folds of skin arising at the mons pubis and extending back to the fourchette, forming a cleft. During puberty, the amount of fatty tissue increases, pubic hair grows on lateral surfaces, and sebaceous glands on hairless medial surfaces secrete lubricants. This structure is highly sensitive to temperature, touch, pressure, and pain and protects the inner structures of the vulva. It is homologous to the male scrotum (see Fig. 24.2).

The labia minora (sing., labium minus) is composed of two smaller, thinner, asymmetric folds of skin within the labia majora that form the clitoral hood (prepuce) and frenulum, then split to enclose the vestibule, and converge near the anus to form the fourchette. The labia minora are hairless, pink, and moist; they are well supplied by nerves, blood vessels, and sebaceous glands that secrete bactericidal fluid with a distinctive odor that lubricates and waterproofs vulvar skin. The labia swell with blood during sexual arousal.

The vagina is an elastic, fibromuscular canal that is 9 to 10 cm long in a reproductive-age female. It extends up and back from the introitus to the lower portion of the uterus. As Fig. 24.5 shows, the vagina lies between the urethra (and part of the bladder) and the rectum. Mucosal secretions from the upper genital organs, menstrual fluids, and products of conception leave the body through the vagina. During coitus, the penis enters the vagina. During sexual arousal, the vagina lengthens and widens, and the vaginal wall becomes engorged with blood, much like the labia minora and clitoris. Engorgement pushes some fluid to the surface of the mucosa, enhancing lubrication. The vaginal wall does not contain mucus-secreting glands; rather, secretions drain into the vagina from the endocervical glands or from the Bartholin and Skene glands of the vestibule. The vagina also functions as the birth canal during childbirth. Its elasticity and relatively sparse nerve supply enhance the vagina's function in this role. During childbirth, the pelvic floor muscles and rugae of the vagina stretch to facilitate the passage of the infant.

An illustration of the internal female genitalia and other pelvic organs shows and labels the following structures, clockwise from the top: sacral promontory, fallopian tube, external iliac vessels, ovarian ligament, round ligament, uterus, anterior cul-de-sac, bladder, symphysis pubis, clitoris, urethra, labium minus, labium majus, vagina, urogenital diaphragm, anus, external anus sphincter, fornix of vagina, levator ani muscle, cervix, posterior cul-de-sac, sacrouterine ligament, ureter, and ovary.

The vaginal wall is lined with a mucous membrane of squamous epithelial cells that thickens and thins in response to hormones, particularly estrogen. The squamous epithelial membrane is continuous with the membrane that covers the lower part of the uterus. In females of reproductive age, the mucosal layer is arranged in transverse wrinkles, or folds, called rugae (sing., ruga), that permit stretching during coitus and childbirth. Below the mucosal layer are three more layers: fibrous connective tissue containing numerous blood and lymphatic vessels, smooth muscle and connective tissue, and a rich network of blood vessels.

The upper part of the vagina surrounds the cervix, the lower end of the uterus (see Fig. 24.5). The recessed space around the cervix is called the fornix of the vagina. The posterior fornix is “deeper” than the anterior fornix because of the angle at which the cervix meets the vaginal canal. In most females, this angle is about 90 degrees. A pouch called the cul-de-sac separates the posterior fornix and the rectum.

Two factors help maintain the self-cleansing action of the vagina and defend it from infection, particularly during the reproductive years. They are (1) an acid-base balance that discourages the proliferation of most pathogenic bacteria and (2) the thickness of the vaginal epithelium. Before puberty, vaginal pH is about 7.0 (neutral), and the vaginal epithelium is thin. At puberty, the pH becomes more acidic (4.0 to 5.0), and the squamous epithelial lining thickens. These changes are maintained until menopause (cessation of menstruation) when the pH rises again to more alkaline levels and the epithelium thins. Therefore, protection from infection is greatest during the years when a female is most likely to be sexually active. Both defense factors are greatest when estrogen levels are high, and the vagina contains a normal population of Lactobacillus acidophilus, a harmless resident bacterium that helps maintain pH at acidic levels. Any condition that causes vaginal pH to rise—such as douching or use of vaginal sprays or deodorants, the presence of low estrogen levels, or destruction of L. acidophilus by antibiotics—lowers vaginal defenses against infection.

At puberty, the uterus attains its adult size and proportions and descends from the abdomen to the lower pelvis, between the bladder and the rectum (see Fig. 24.5). The uterus of a mature, nonpregnant female is approximately 7 to 9 cm long and 6.5 cm wide, with muscular walls 3.5 cm thick, enlarging by about 1 cm in all dimensions after pregnancy.¹ It is loosely held in position by ligaments, peritoneal tissue folds, and the pressure of adjacent organs, especially the urinary bladder, sigmoid colon, and rectum. In most females, the uterus is tipped forward (anteverted) so that it rests on the urinary bladder. However, it may be tipped backward (retroverted), and various degrees of forward or backward flexion are normal (Fig. 24.6).

“An illustration of the lateral view of the pelvis, showing the uterus, bladder, vagina, and rectum, highlights the variations in uterine positions. Top panel, midline. The uterus is vertical. Middle-left panel, anteverted. The uterus is bent toward to the bladder. Middle-right panel, anteflexed. The uterus is curled toward the bladder. Bottom-left panel, retroverted. The uterus is bent away from the bladder. Bottom-right panel, retroflexed. The uterus is pressing at the rectal wall.”

The uterus has two major parts: the corpus (body of the uterus) and the cervix (Fig. 24.7). The top of the corpus, above the insertion of the fallopian tubes, is called the fundus. The diameter of the uterine cavity is widest at the fundus and narrowest at the isthmus, just above the cervix (see Fig. 24.5). The cervix, or “neck of the uterus,” extends from the isthmus to the vagina. The passageway between the upper opening (the internal os) and the lower opening (the external os) of the cervix is called the endocervical canal (see Fig. 24.7). The entire uterus, like the upper vagina, is innervated exclusively by motor and sensory fibers of the autonomic nervous system.

An illustration of the cross-section of the uterus, fallopian tube, and ovary, shows and labels the following structures, clockwise from the top: perimetrium, interstitial portion of fallopian tube, ovarian ligament, isthmus of fallopian tube, ampulla of fallopian tube, suspensory ligament, infundibulum of fallopian tube, fimbriae, ovary, broad ligament, uterine artery and vein, reflection of peritoneum, isthmus of uterus, fornix of vagina, vagina, cervix of uterus, and body of uterus. The cervix of uterus includes the following parts, from the top: internal o s of cervix, endocervical canal, and external o s of cervix. The body of uterus includes the following parts, from the top: fundus of uterus, corpus of uterus, endometrium, and myometrium.

The uterine wall is composed of three layers (see Fig. 24.7). The perimetrium (parietal peritoneum) is the outer serous membrane that covers the uterus. The myometrium is the thick, muscular middle layer. It is thickest at the fundus, apparently to facilitate birth. The endometrium, or uterine lining, is composed of a functional layer (superficial compact layer and spongy middle layer) and a basal layer. The functional layer of the endometrium responds to the sex hormones estrogen and progesterone. Between puberty and menopause, this layer proliferates and is shed monthly. The basal layer, which is attached to the myometrium, regenerates the functional layer after shedding (menstruation).

The endocervical canal does not have an endometrial layer but is lined with columnar epithelial cells. It is continuous with the lining of the outer cervix and vagina, which are lined with squamous epithelial cells. The point where the two types of cells meet is called the transformation zone, or squamous-columnar junction (see Fig. 25.20). The transformation zone is vulnerable to the human papillomavirus (HPV), especially HPV types 16 and 18, which can lead to cervical dysplasia or carcinoma in situ. Cells of the transformation zone are removed for examination during a Papanicolaou (Pap test) smear.

The cervix acts as a mechanical barrier, protecting the uterus from infectious microorganisms from the vagina. The external cervical os is a very small opening that contains thick, sticky mucus (the mucous “plug”) during the luteal phase of the menstrual cycle and throughout pregnancy. During ovulation, the mucus changes under the influence of estrogen and forms watery strands, or spinnbarkeit mucus, to facilitate the transport of sperm into the uterus. In addition, the downward flow of cervical secretions moves microorganisms away from the cervix and uterus. In females of reproductive age, the pH of these secretions is inhospitable to many bacteria. Further, mucosal secretions contain enzymes and antibodies (mostly immunoglobulin A [IgA]) of the secretory immune system. Uterine pathophysiologic disorders include infection, displacement of the uterus within the pelvis, benign growths (fibroids) of the uterine wall, hyperplasia of the endometrium, endometriosis, and cancer (see Chapter 25).

The two fallopian tubes (oviducts, uterine tubes) enter the uterus bilaterally just beneath the fundus (see Fig. 24.7). They direct the ova from the spaces around the ovaries to the uterus. From the uterus, the fallopian tubes curve up and over the two ovaries. Each tube is 8 to 12 cm long and about 1 cm in diameter, except at its ovarian end, which resembles the bell of a trumpet and is fringed or fimbriated (infundibulum). The fimbriae (fringes) move, creating a current that draws the ovum into the infundibulum. Once the ovum enters the fallopian tube, cilia (hairlike structures) and peristalsis (muscle contractions) keep it moving toward the uterus.

The ampulla, or distal third, of the fallopian tube is the usual site of fertilization (see Fig. 24.7). Sperm released into the vagina travel upward through the endocervical canal and uterine cavity and enter the fallopian tubes. If an ovum is present in either tube, fertilization can occur. Whether or not the ovum encounters sperm, it continues to travel through the fallopian tube to the uterus. If fertilized, the ovum (then called a blastocyst) implants itself in the endometrial layer of the uterine wall. If not fertilized, the ovum fragments and leaves the uterus with menstrual fluids. Disorders that affect the fallopian tubes (e.g., congenital malformations, infection, and inflammation) can block the path of both sperm and the ovum and may cause infertility or ectopic (tubal) pregnancy.

The ovaries, the female gonads, are the primary female reproductive organs (Fig. 24.8). Their two main functions are secretion of female sex hormones and development and release of female gametes, or ova.

“A series of illustration of the cross-section of the ovary tracks the development of ovarian follicle. Day 1. The primordial follicle is an oocyte surrounded by a layer of granulosa precursors. The follicle undergoes the following changes until Day 12. • The primary follicle has a larger oocyte surrounded by a layer of granulosa cells. • The secondary follicle is filled with oocyte and is surrounded by a zona pellucida, multiple layers of granulosa cells, and an outer layer of theca cells. • The atretic follicle develops. Day 12. The mature follicle (Graafian follicle) shows an oocyte surrounded by a thicker zona pellucida, proliferated granulosa cells, theca interna, and theca externa. Day 14. The follicle ruptures and results in ovulation. A mature corpus luteum develops. Day 20. Geminal epithelium. Mature corpus luteum develops. Day 28. Corpus albicans.”

The almond-shaped ovaries are located on both sides of the uterus and are suspended and supported by a portion of the broad ligament (the mesovarium component), ovarian ligaments, and suspensory ligaments (see Fig. 24.7). The ovaries are smaller than their male homologs, the testes. In females of reproductive age, each ovary is about 3 to 5 cm long, 2.5 cm wide, and 2 cm thick, weighing 4 to 8 g. Size and weight vary slightly during each phase of the menstrual cycle (see the Menstrual (Ovarian) Cycle section).

The central part of the ovary, or medulla, is composed of connective tissue and contains many small arteries, veins, and lymphatics that enter at the hilum. Surrounding the medulla is the cortex. At birth, the cortex of each ovary contains approximately 1 to 2 million ova within primordial (immature) ovarian follicles. By puberty, the number ranges between 300,000 and 500,000, and some of the follicles and the ova within them begin to mature. Follicles grow and undergo atresia continuously and irrevocably throughout a woman's life. Between puberty and menopause, the ovarian cortex always contains follicles and ova in various stages of development (primary and secondary follicles). Once every menstrual cycle (about every 28 days), one of the follicles reaches maturation and discharges its ovum through the ovary's outer covering, the germinal epithelium. During the reproductive years, 400 to 500 ovarian follicles mature completely and release an ovum (ovulation). The remaining follicles either fail to develop at all or degenerate without maturing completely and are known as atretic follicles (see Fig. 24.8).

After the release of the mature ovum (ovulation), the follicle develops into another structure, the corpus luteum (see Fig. 24.8). If fertilization occurs, the corpus luteum enlarges and begins to secrete hormones that maintain and support pregnancy. If fertilization does not occur, the corpus luteum secretes these hormones for approximately 14 days and then degenerates, which triggers the maturation of another follicle. The ovarian cycle—the process of follicular maturation, ovulation, corpus luteum development, and corpus luteum degeneration—is continuous from puberty to menopause, except during pregnancy or hormonal contraceptive use. At menopause, this process ceases, and the ovaries atrophy to the point that they cannot be felt during a pelvic examination.

Sex hormones are secreted by cells present within the ovarian cortex, including two types of cells in the ovarian follicle—theca cells (produce androgens that migrate to granulosa cells) and granulosa cells (convert androgens to estradiol)—and cells of the corpus luteum (secrete primarily progesterone, estrogen, and inhibin) (see Fig. 24.8). These cells all contain receptors for gonadotropins (LH, FSH) or for sex hormones, which are discussed in the next section.

Estrogen has numerous biologic effects, many of which involve interactions with other hormones. It is needed for the maturation of reproductive organs, development of secondary sex characteristics, growth, and maintenance of pregnancy. It also is needed for many nonreproductive effects, including closure of long bones after the pubertal growth spurt (in both males and females), maintenance of bone and skin, and systemic organ function (see Table 24.1 and Box 24.1). After menopause, the ovaries dramatically reduce the production of estradiol, and secretion of estrone is markedly diminished (see the Aging and the Female Reproductive System section). At this time, the majority of estradiol is derived from intracellular synthesis in peripheral tissues. Estradiol acts locally to meet physiologic needs according to cell type and is then inactivated without systemic effects.²

Like other steroid hormones, estrogens are derived from cholesterol in a complex, enzyme-mediated series of reactions. The hypothalamus secretes GnRH in a pulsatile manner that stimulates gonadotropin (LH and FSH) release from the anterior pituitary. Gonadotropins trigger the ovarian production of estrogen. The primary function of LH is to stimulate theca cells of the ovarian follicle to produce androgens, mainly androstenedione. (Androgens are discussed further under the section titled Male Sex and Reproductive Hormones.) Some of these androgens are converted to estrogen by the theca cells themselves, and others diffuse into the granulosa cells. Within the granulosa layer, FSH induces conversion (aromatization) of androgens to estrogens. Estrogens are then released into the bloodstream.

Although androgens are primarily male sex hormones produced in the testes, small amounts are produced in the adrenal cortex in both males and females and in the ovaries in females. Some androgens (dehydroepiandrosterone and its metabolite androstenedione) are precursors of estrogens (estrone, estradiol) (see Table 24.1). At puberty, androgens contribute to the skeletal growth spurt and cause the growth of pubic and axillary hair. Androgens also activate sebaceous glands, accounting for some cases of acne during puberty, and play a role in libido.

Luteinizing hormone from the anterior pituitary stimulates the corpus luteum to secrete progesterone, the second major female sex hormone. With estrogen, progesterone controls the ovarian menstrual cycle. LH surge occurs when there is a peak level of estrogen, about 24 to 36 hours before ovulation. LH promotes luteinization of the granulosa in the dominant follicle, resulting in progesterone production and the development of blood vessels and connective tissue. During the follicular phase, the ovary and adrenal glands each contribute approximately 50% of progesterone production. Conversely, large amounts are cyclically secreted from the ovary while the corpus luteum is active for about 9 to 13 days after ovulation. (The complementary and opposing effects of progesterone and estrogen are listed in Table 24.2.) Progesterone secreted by the corpus luteum stimulates the thickened endometrium to become more complex in preparation for implantation of a blastocyte. If conception and implantation do occur, the corpus luteum persists and secretes progesterone (and estrogen) until the placenta is well established at approximately 8 to 10 weeks’ gestation and undertakes progesterone production.

Progesterone is sometimes called the hormone of pregnancy. Progesterone's effects in pregnancy include:

The menstrual (ovarian) cycle (see Fig. 24.9) is the process of menstruation (menses) in the uterine endometrium and ovulation in the ovary. The cycle consists of the follicular/proliferative phase (postmenstrual) followed by the luteal/secretory phase (premenstrual) and then the ischemic/menstrual phase if conception does not occur. These are named for processes that occur in the ovary (follicular and luteal phases) and the uterine endometrium (proliferative, secretory, and ischemic phases) during the menstrual cycle.

During menstruation (menses), the functional layer of the endometrium disintegrates and is discharged through the vagina. Menstruation is followed by the follicular/proliferative phase. This phase is named for two simultaneous processes: maturation of an ovarian follicle and proliferation of the uterine endometrium (see Fig. 24.9). During this phase, GnRH and a balance between activin and inhibin levels from the granulosa cells contribute to the increase of FSH level, which stimulates a number of follicles. The result is a rescue of a dominant ovarian follicle from normal dissolution by days 5 to 7 of the cycle. Together, estrogen and FSH make the granulosa cells of the primary follicle more sensitive to FSH and promote LH stimulation, which causes a more rapid secretion of follicular estrogen. A surge in the levels of both LH and FSH is then required for final follicular growth and ovulation. Estrogen levels increase, and inhibin B inhibits the secretion of FSH level by the granulosa cells in the dominant follicle. This drop in FSH concentration decreases the growth of less developed follicles (see Fig. 24.8). Estrogen causes cells of the endometrium to proliferate.

Ovulation is the release of an ovum from a mature follicle and marks the beginning of the luteal/secretory phase of the menstrual cycle. The ovarian follicle begins its transformation into a corpus luteum (see Fig. 24.8), hence the name luteal phase. Pulsatile secretion of LH from the anterior pituitary stimulates the corpus luteum to secrete progesterone, estrogen, and inhibin A (suppresses FSH secretion), which in turn initiates the secretory phase of endometrial development. Estrogen maintains the thickness of the endometrium, and progesterone stimulates the growth of glands and blood vessels in the endometrium. The glands begin to secrete a thin, glycogen-containing fluid, hence the name secretory phase. At this point, one of the following two paths occurs:

Ovulatory cycles appear to have a minimum length of 24 to 26.5 days: the ovarian follicle requires 10 to 12.5 days to develop, and the luteal phase appears fixed at 14 days (±3 days). Menstrual blood flow usually lasts 3 to 7 days but may be between 2 and 8 days and still be considered within normal limits. Bleeding is consistently scant to heavy and varies from 30 to 80 ml, with most blood loss occurring during the first 3 days of menses. Menstrual discharge consists of blood, mucus, and desquamated endometrial tissue and does not clot under normal circumstances. It is usually dark and produces a characteristic musty odor on oxidation. Environmental factors such as severe emotional stress, illness, malnutrition, obesity, extreme exercise, and seasonal variation may affect the length of the menstrual cycle.7,8

Cervical mucus also undergoes cyclic changes during the menstrual cycle. During the proliferative phase, the cervical mucus is thin and watery. Peak estrogen levels occur just before ovulation and maximally stimulate the cervical glands to produce mucus. Cervical mucus becomes abundant and more elastic (spinnbarkeit). In the presence of estrogen, tiny channels develop in the mucus, which allows sperm access to the interior of the uterus. Changes in the consistency of cervical mucus can be used to identify fertile intervals. After ovulation, the ovary begins to secrete progesterone under the influence of the corpus luteum. The amount of cervical mucus is reduced, becomes thicker and stickier, and blocks sperm migration.

The vaginal epithelium also responds to the cyclic hormonal changes of the menstrual cycle. Under the influence of estrogen, cells of the vaginal epithelium become thicker during the follicular/proliferative phase. After ovulation, layers of keratinized cells overgrow the basal epithelium (cells become larger and flatter), a process known as cornification. Near the end of the luteal phase, leukocytes invade the vaginal epithelium, removing the outer layers in a process termed decornification with thinning of the epithelium.

Basal body temperature (BBT) undergoes characteristic biphasic changes during menstrual cycles in which ovulation occurs. During the follicular phase, the BBT fluctuates around 98°F (37°C). During the luteal phase, the average temperature increases by 0.4°F to 1.0°F (0.2°C to 0.5°C). At the end of the luteal phase, 1 to 3 days before the onset of menstruation, BBT declines to follicular-phase levels. The shift in temperature is related to ovulation, corpus luteum formation, and increased serum progesterone levels. Progesterone acts on the thermoregulatory center of the hypothalamus to increase body temperature. Changes in BBT are used to estimate ovulatory cycles but when used alone may not be the best method to support fertility awareness.⁹

Hormonal control of the menstrual cycle depends on complex interactions among the hypothalamus, the anterior pituitary, and the ovaries (or hypothalamic-pituitary-ovarian [HPO] axis) (Table 24.3). Hormonal control is dependent on negative and positive ovarian feedback mechanisms. In the hypothalamus, kisspeptin activates the release of GnRH to stimulate the gonadotropin production of FSH and LH. The constant and pulsatile release of GnRH is critical to the timing of the menstrual cycle. GnRH is secreted into the hypophyseal portal system and travels to the anterior pituitary, where it stimulates the secretion of LH and FSH. FSH and LH are released from the anterior pituitary in pulses that correspond to the pulsatile secretion of GnRH.

During the early follicular phase, estrogen levels rise steadily and, through negative feedback, suppress FSH and positively increase the production of LH. During the late follicular phase, the preovulatory rise in progesterone concentration facilitates the positive feedback of estrogen; estrogen levels begin to increase, stimulating a surge of LH secretion from the anterior pituitary. The midcycle surge of LH and FSH induces ovulation. A nonsteroidal ovarian factor, gonadotropin surge–attenuating factor (GnSAF), may antagonize the effect of estrogen on the pituitary and regulate the surge of LH at midcycle.10 Rising estrogen and progesterone levels during the luteal phase may inhibit the anterior pituitary and thus reduce LH and FSH secretion. Just before the onset of menstruation, FSH and LH levels begin to increase slightly, probably because of declining estrogen and progesterone levels (see Fig. 24.9).

A variety of growth factors and autocrine/paracrine peptides influence hormonal control and follicular response. During the early follicular stage, FSH stimulates FSH receptors and LH receptors and the release of insulin-like growth factor one, as well as the production of inhibin and activin in the ovary. Activin from granulosa cells stimulates the secretion of FSH and increases the pituitary response to GnRH, and increases FSH binding in the granulosa cells in the dominant follicle. FSH stimulates inhibin secretion from granulosa cells, and it, in turn, suppresses FSH synthesis. Inhibin B is primarily secreted in the follicular phase of the cycle but sharply spikes when ovulation occurs. Inhibin A is secreted in the luteal phase and further suppresses FSH. Inhibin also restrains prolactin and growth hormone release, interferes with GnRH receptors, and promotes the breakdown of intracellular gonadotropins. In summary, the balance between activin and inhibin regulates FSH secretion. Follistatin inhibits activin and boosts inhibin activity. Inhibin and activin also regulate LH stimulation of androgen synthesis (required for ovarian estrogen biosynthesis) in theca cells.¹¹Fig. 24.9 depicts fluctuating estrogen, progesterone, gonadotropin, and inhibin levels. Research continues to advance understanding of the function and structural complexity of these polypeptides and their interaction with GnRH, gonadotropins, and sex hormones.¹²

During embryonic and fetal life, the testes develop within the abdomen (see Fig. 24.1). About 3 months before birth, the testes start to descend toward the developing scrotum (Fig. 24.13). About 1 month before birth, they enter twin passageways called inguinal canals. Vaginal processes created by outpouchings of the peritoneum (lining of the abdominal cavity) also descend through the inguinal canals. When descent is complete, the abdominal end of each vaginal process closes, and the scrotal end of each process becomes the outer covering of the testis, the tunica vaginalis. (See Fig. 24.16, A for the inguinal canal in a mature adult.) Failure of the testes to descend through the inguinal canal is known as cryptorchidism.

“Three illustrations demonstrate the descent of a testis. Top panel. The illustration shows the scrotal swelling, the bladder, vas deferens, testis, and epididymis. The testis and the epididymis are away from the scrotal swelling and bladder. Middle panel. The testis has descended from the back and into the scrotal swelling. Top panel. The illustration shows the testis and the epididymis in the scrotum. The closed proximal portion of vaginal process is highlighted.”

“Illustration A shows and labels the following structures of the male reproductive organs, clockwise from the top: ampulla of vas deferens, left ureter, left vas deferens, seminal vesicle, ejaculatory duct, prostate, deep transverse perineal muscle, bulbourethral gland, head of epididymis, tunica vaginalis, penis, spermatic cord, bladder, inguinal canal, right vas deferens, right external iliac vein, and right external iliac artery. Illustration B shows and labels the following structures of the male reproductive organs: anterior fibromuscular stroma, transition zone, central zone, peripheral zone, and seminal vesicles.”

Fig. 24.14 shows a sagittal section of a mature testis. The adult testis is oval and varies considerably in length (3 to 6 cm), width (2 to 3.5 cm), depth (3 to 4 cm), and weight (10 to 40 g). The testis is almost entirely surrounded by an outer covering called the tunica vaginalis, which separates the testis from the scrotal wall, and an inner covering called the tunica albuginea. Inward extensions of the tunica albuginea form septa that separate the testis into about 250 compartments, or lobules, each of which contains several tortuously coiled ducts called seminiferous tubules. The seminiferous tubules constitute the bulk (80%) of testicular volume and are the site of sperm production. (Sperm production is described in the Spermatogenesis section.) The tissue surrounding these ducts contains blood and lymphatic vessels, fibroblastic support cells, macrophages, mast cells, andLeydig cells, which occur in clusters and produce androgens, chiefly testosterone.

“Illustration A is an inferior view of the external male genitalia. The following structures on the genitalia are labeled from the top to the bottom: glans penis, body of penis, testicles in scrotum, perineum, bulbospongiosus, and the anus. Illustration B is an anterior view of the male genitalia, with the scrotum. The following structures are labeled, clockwise from the right: pampiniform plexus of veins and testicular artery, genital branch of genitofemoral nerve, vas deferens, parietal layer of tunica vaginalis, cavity of tunica vaginalis, appendix of epididymis, appendix of testis, septum of scrotum, skin of scrotum, dartos fascia, cremaster muscle, external spermatic fascia, corpus spongiosum, corpus cavernosum, deep dorsal vein and dorsal arteries of penis, superficial dorsal vein of penis, deep fascia of penis (buck's fascia), deep external pudendal artery and vein, great saphenous vein, and superficial external pudendal artery and vein. Illustration C is the sagittal view of the testis and surrounding structures. The following structures on the genitalia are labeled, clockwise from the top: vas deferens, head of epididymis, vasa efferentia (efferent tubules), rete testis in mediastinum testis, body of epididymis, seminiferous tubule (straight), tail of epididymis, capsule (tunica albuginea), septum, tunica vaginalis (parietal layer, cavity, visceral layer), lobule, and seminiferous tubule (convoluted).”

The two ends of each seminiferous tubule join and leave the lobule through the tubulus rectus, which leads to the central portion of the testis, the rete testis. The sperm then move through the efferent tubules, or vasa efferentia, to the epididymis, where they mature.

The testes are innervated by adrenergic fibers whose sole function is to regulate blood flow to the Leydig cells. Arterial blood from the internal spermatic and differential arteries flows over the surface of the testes before entering the parenchyma (functional tissues). Surface flow cools the blood to temperatures to approximately 35°C to 36°C, which promotes spermatogenesis.15 Additionally, the testes are suspended outside the pelvic cavity to facilitate cooling.

The epididymis (pl., epididymides) is a comma-shaped structure that curves over the posterior portion of each testis (see Fig. 24.14). It consists of a single, densely packed, and markedly coiled duct measuring 5 to 7 cm in length (but about 6 meters in length when uncoiled). The epididymis has structural and physiologic functions. Its structural function is to conduct sperm from the efferent tubules to the vas deferens, whereas physiologic functions include sperm maturation, mobility, and fertility. When sperm enter the head of the epididymis, they are not fully mature or motile, nor can they fertilize an ovum. During the 12 or more days sperm take to travel the length of the epididymis, they receive nutrients and testosterone, and their capacity for fertilization is enhanced.¹⁶ After traveling the length of the epididymis, sperm are stored in the epididymal tail and vas deferens. The vas deferens is a duct with muscular layers capable of powerful peristalsis that transports sperm toward the urethra (see Fig. 24.14). The vas deferens enters the pelvic cavity through the spermatic cord.

The testes, epididymides, and spermatic cord are enclosed and protected by the scrotum, a skin-covered, fibromuscular sac homologous to the female labia majora (see Fig. 24.2). The skin of the scrotum is thin and has rugae (wrinkles or folds), which enable it to enlarge or relax away from the body. At puberty, the scrotal skin darkens, develops active sebaceous glands, and becomes sparsely covered with hair. Just under the skin lies a layer of connective tissue (fascia) and smooth muscle, the tunica dartos (see Fig. 24.14). The tunica dartos also forms a septum that separates the two testes. Exposure to cold temperatures causes the tunica dartos to contract, pulling the testes close to the warm body. In warm temperatures, the tunica dartos relaxes, suspending the testes away from body heat. These mechanisms promote optimal temperatures for spermatogenesis. In addition, scrotal sensitivity to touch, pressure, temperature, and pain protects the testes from potential harm. During sexual excitement, the scrotal skin and tunica thicken, the scrotum tightens and lifts, and the spermatic cords shorten, partially elevating the testes toward the body. As excitement plateaus, the engorged testes increase 50% in size, rotate anteriorly, and flatten against the body, signaling impending ejaculation.

The penis has two main functions: delivery of sperm to the female vagina and elimination of urine. Embryonically, the penis is homologous to the female clitoris (see Fig. 24.2).

Fig. 24.12 shows a sagittal section of the adult penis and its anatomic relation to other urogenital structures, and Fig. 24.15 shows a cross section of the shaft of the penis. Internally, the penis consists of the urethra and three compartments or sinusoids: two corpora cavernosa (sing., corpus cavernosum) and the corpus spongiosum separated by Buck fascia. Like the testes, these compartments are enclosed by the fibrous tunica albuginea. The urethra passes through the corpus spongiosum and ends at a sagittal slit in the glans.

An illustration of the cross-section of the penis shows and labels the following structures, clockwise from the top: dorsal veins, dorsal artery and nerve, septum penis, tunica albuginea, deep artery, intercavernous septum of Buck fascia, corpus spongiosum, urethra, tunica albuginea, corpus cavernosum, buck fascia, and skin.

Externally, the penis consists of a shaft with a tip (the glans) that contains the opening of the urethra (see Fig. 24.14). The skin of the glans folds over the tip of the penis, forming the prepuce, or foreskin. At birth, the foreskin is adhered to the glans. Penile erections, which commonly occur, cause the adhesions to break so that by age 3 years, the foreskin becomes completely retractable. The skin of the penis is continuous with that of the groin, scrotum, and inner thighs. It is hairless, movable, and darker than the surrounding skin.

Penetration of the female vagina is made possible by the erectile reflex, a process in which erectile tissues within the corpora cavernosa and corpus spongiosum become engorged with blood, generally 20 to 50 mL. The erectile tissues consist of vascular spaces, or chambers, supplied with blood by arterioles (small arteries). Usually, the arterioles are constricted so that not much blood flows through the erectile tissues. Sexual stimulation, however, causes the arterioles to dilate and fill with blood, expanding the erectile tissues and causing an erection. The corpora cavernosa increases in length and width and becomes rigid. Erection apparently is maintained by compression or constriction of veins that drain the corpora cavernosa and corpus spongiosum. When sexual stimulation ceases, or orgasm and ejaculation occur, these veins open, blood flows out of the arterioles, and the penis becomes flaccid (soft and pendulous). Erection is under the control of the autonomic nervous system but can be stimulated or inhibited by CNS input.

Stimulation of the glans, which is endowed with copious sensitive nerve endings, provides maximum erotic sensation. With sexual arousal, skin color deepens, the glans doubles in size, and the urethral meatus dilates. Ejaculation occurs with frequent, strong contractions of the vas deferens, epididymis, seminal vesicles, prostate, urethra, and penis. Erection and ejaculation can occur independently of each other, but it is not common.¹⁷

Erections begin in utero and continue throughout life, but ejaculation does not occur until sperm production begins at puberty. Growth of the penis and scrotal contents continues well past puberty, however, and may not be complete until the late teens or early twenties. Penis size, when flaccid, varies considerably; with an erection, the difference in penis size diminishes.