LESSON 14

 

Topography and structure of male reproductive organs

The principal structures of the male reproductive system are shown in Figure. Proceeding from a testis, the sperm cells, or spermatozoa, travel within the epididymis, the ductus deferens, or vas deferens, the ejaculatory duct and the urethra before leaving the body. Accessory organs—the seminal vesicles, the prostate gland, and the bulbourethral glands—secrete into the ejaculatory ducts and urethra. The external genitalia consist of the scrotum, which encloses the testes, and the penis, an erectile organ through which the distal portion of the urethra passes.

The Testes

Each testis has the shape of a flattened egg that is roughly 5 cm (2 in.) long, 3 cm (1.2 in.) wide, and 2.5 cm (1 in.) thick. Each has a weight of 10-15 g (0.35-0.53 oz). The testes hang within the scrotum, a fleshy pouch suspended inferior to the perineum, anterior to the anus and posterior to the base of the penis (Figures 28-1 and 28-2 ).

The Spermatic Cords

The spermatic cords consist of fascia layers, tough connective tissue, and muscle enclosing the blood vessels, nerves, and lymphatics that supply the testes. Each spermatic cord contains a ductus deferens and its deferential artery, a testicular artery, the pampiniform (pampinus, tendril + forma, form) plexus of a testicular vein, and branches of the genitofemoral nerve from the lumbar plexus. Each spermatic cord begins at the deep inguinal ring, the entrance to the inguinal canal (a passageway through the abdominal musculature). After passing through the inguinal canal, the spermatic cord exits at the superficial inguinal ring and descends into the scrotum (Figure 28-2 ).

The inguinal canals form during development as the testes descend into the scrotum; at that time, these canals link the scrotal cavities with the peritoneal cavity. In normal adult males, the inguinal canals are closed, but the presence of the spermatic cords creates weak points in the abdominal wall that remain throughout life. As a result, inguinal hernias, discussed in Chapter 11, are relatively common in males. The inguinal canals in females are very small, containing only the ilioinguinal nerves and the round ligaments of the uterus. The abdominal wall is nearly intact, and inguinal hernias in women are very rare.

Descent of the Testes

During development, the testes form inside the body cavity adjacent to the kidneys (Figure 28-3a b ). A bundle of connective tissue fibers extends from each testis to the posterior wall of a small, anterior and inferior pocket of the peritoneum. These fibers constitute the gubernaculum testis. As growth proceeds, the gubernacula do not get any longer, and they lock the testes in position. As a result, the relative position of each testis changes as the rest of the body enlarges; the testis gradually moves inferiorly and anteriorly toward the anterior abdominal wall (Figure 28-3a ). During the seventh developmental month, fetal growth continues at a rapid pace, and circulating hormones stimulate a contraction of the gubernaculum testis. During this period, each testis moves through the abdominal musculature, accompanied by small pockets of the peritoneal cavity. This process is called the descent of the testes.

As it moves through the body wall, each testis is accompanied by the ductus deferens and the testicular blood vessels, nerves, and lymphatics (Figure 28-3b ). Together, these structures form the body of the spermatic cord. In cryptorchidism (crypto, hidden), one or both of the testes have not descended into the scrotum by the time of birth. Typically, the cryptorchid testes are lodged in the abdominal cavity or within the inguinal canal. Cryptorchidism occurs in about 3 percent of full-term deliveries and in roughly 30 percent of premature births. In most instances, normal descent occurs a few weeks later, but the condition can be surgically corrected if it persists. Corrective measures should be taken before puberty, because cryptorchid (abdominal) testes will not produce sperm, and the individual will be sterile. In most cases, if the testes cannot be moved into the scrotum, they will be removed, because about 10 percent of males with uncorrected cryptorchid testes eventually develop testicular cancer. This surgical procedure is called a bilateral orchiectomy (orchis, testis).

The Scrotum and the Position of the Testes

The scrotum is divided internally into two chambers. The partition between the two is marked by a raised thickening in the scrotal surface known as the perineal raphe (Figures 28-2 and 28-4a ). Each testis lies in a separate compartment, or scrotal cavity. Because the scrotal cavities are separated by a partition, infection or inflammation of one testis does not ordinarily spread to the other. A narrow space separates the inner surface of the scrotum from the outer surface of the testis. The tunica vaginalis, a serous membrane, lines the scrotal cavity and reduces friction between the opposing parietal (scrotal) and visceral (testicular) surfaces. The tunica vaginalis is an isolated portion of the peritoneum that lost its connection with the peritoneal cavity after the testes descended, when the inguinal canal closed.

The scrotum consists of a thin layer of skin and the underlying superficial fascia. The dermis contains a layer of smooth muscle, the dartos. Resting muscle tone in the dartos causes the characteristic wrinkling of the scrotal surface. A layer of skeletal muscle, the cremaster muscle, lies deep to the dermis. Contraction of the cremaster during sexual arousal, or in response to changes in temperature, tenses the scrotum and pulls the testes closer to the body. Normal sperm development in the testes requires temperatures about 1.1°C (2°F) lower than those elsewhere in the body. The cremaster relaxes or contracts to move the testes away from or toward the body as needed to maintain acceptable testicular temperatures. When air or body temperature rises, the cremaster relaxes and the testes move away from the body. Cooling of the scrotum, as occurs when a man enters a cold swimming pool, results in cremasteric contractions that pull the testes closer to the body and keep testicular temperatures from falling.

The scrotum is richly supplied with sensory and motor nerves from the hypogastric plexus and branches of the ilioinguinal nerves, the genitofemoral nerves, and the pudendal nerves (Figure 16-10 ). The vascular supply to the scrotum includes the internal pudendal arteries (from the internal iliac arteries), the external pudendal arteries (from the femoral arteries), and the cremasteric branch of the inferior epigastric arteries (from the external iliac arteries). The names and distributions of the veins follow those of the arteries.

Structure of the Testes

Deep to the tunica vaginalis covering the testis lies the tunica albuginea, a dense layer of connective tissue rich in collagen fibers. The fibers of this network are continuous with those surrounding the adjacent epididymis. The collagen fibers of the tunica albuginea also extend into the substance of the testis, forming fibrous partitions, or septa (Figure 28-4 ). These septa converge toward the area closest to the entrance of the epididymis. This region, located at the superior end of the testis, is called the mediastinum of the testis (or mediastinum testis). The connective tissues in this region support the blood vessels and lymphatics that supply the testis and the efferent ducts, which transport sperm to the epididymis.

Histology of the Testes

The septa subdivide the testis into a series of lobules. Roughly 800 slender, tightly coiled seminiferous tubules are distributed among the lobules (Figure 28-4 ). Each tubule averages about 80 cm (31 in.) in length, and a typical testis contains nearly one-half mile of seminiferous tubules. Sperm production occurs within these tubules.

Each seminiferous tubule forms a loop that is attached to a straight tubule (tubuli recti) at the mediastinum of the testis. The straight tubule is connected to a maze of passageways known as the rete testis (Figure 28-4 ). Fifteen to twenty large efferent ducts (or efferent ductules) connect the rete testis to the epididymis.

Because the seminiferous tubules are tightly coiled, most histological preparations show them in transverse section. Each tubule is surrounded by a delicate capsule, and loose connective tissue fills the spaces between the tubules. Within those spaces are numerous blood vessels and large interstitial cells (cells of Leydig) (Figure 28-5a ). Interstitial cells are responsible for the production of androgens, the dominant sex hormones in males. Testosterone is the most important androgen.

Sperm cells, or spermatozoa, are produced by the process of spermatogenesis. Spermatogenesis begins at the outermost layer of cells in the seminiferous tubules and proceeds toward the tubular lumen (Figure 28-5a , b). Stem cells called spermatogonia divide by mitosis to produce generations of daughter cells, some of which differentiate into spermatocytes. Through meiosis, a specialized form of cell division involved only in the production of gametes (sperm in males, ova in females), spermatocytes give rise to spermatids.

At each step in this process, the daughter cells move closer to the tubular lumen. The spermatids subsequently differentiate into spermatozoa. This differentiation process, called spermiogenesis, ends as the physically mature spermatozoa lose contact with the wall of the seminiferous tubule and enter the fluid in the lumen. Spermiogenesis is the last step in spermatogenesis.

Each seminiferous tubule contains spermatogonia, spermatocytes at various stages of meiosis, spermatids, spermatozoa, and large sustentacular cells (or Sertoli cells). Sustentacular cells are attached to the tubular capsule and extend toward the lumen between the other cell types (Figure 28-5a , b).

Spermatogenesis

Spermatogenesis involves three integrated processes:

1.     Mitosis. Spermatogonia undergo cell divisions throughout adult life. (You may wish to review the description of mitosis and cell division in Chapter 3. ) The cell divisions produce daughter cells that are pushed toward the lumen of the tubule. These cells differentiate into spermatocytes that prepare to begin meiosis.

2.     Meiosis. Meiosis is a special form of cell division involved in gamete production. Gametes contain half the normal chromosome complement. As a result, the fusion of the nuclei of a sperm and an ovum produces a cell that has the normal number of chromosomes (46) rather than twice that number. In the seminiferous tubules, the meiotic divisions of spermatocytes produce spermatids, or undifferentiated male gametes.

3.     Spermiogenesis. Spermatids are small, relatively unspecialized cells. In the process of spermiogenesis, spermatids differentiate into physically mature spermatozoa. Spermatozoa are among the most highly specialized cells in the body. Spermiogenesis involves major changes in a spermatid's internal and external structure.

Mitosis and Meiosis
Mitosis and meiosis differ significantly in terms of the events that take place in the nucleus. Mitosis is part of the process of somatic cell division, which produces two daughter cells, each containing 23 pairs of chromosomes (Figure 28-6a ). Each pair consists of one chromosome provided by the father and another by the mother at the time of fertilization. Because the daughter cells contain both members of each chromosome pair (for a total of 46 chromosomes), they are called diploid (diplo, double) cells. Meiosis involves two cycles of cell division (meiosis I and meiosis II) and produces four cells, each of which contains 23 individual chromosomes (Figure 28-6b ). Because these cells contain only one member of each pair of chromosomes, they are called haploid (haplo, single) cells. The events in the nucleus shown in Figure 28-6b are the same whether you consider the formation of sperm or ova.

As a cell prepares to begin meiosis, DNA replication occurs within the nucleus as if the cell were about to undergo mitosis. As prophase of the first meiotic division, meiosis I, arrives, the chromosomes condense and become visible. As in mitosis, each chromosome consists of two duplicate chromatids.

The corresponding maternal and paternal chromosomes now come together, an event known as synapsis. Synapsis involves 23 pairs of chromosomes; each member of each pair consists of two chromatids. A matched set of four chromatids is called a tetrad (tetras, four). Some exchange of genetic material can occur between the chromatids of a chromosome pair at this stage of meiosis. This exchange, called crossing-over, increases genetic variation among offspring; we shall discuss it in Chapter 29.

The nuclear envelope disappears at the end of prophase I. As metaphase I begins, the tetrads line up along the metaphase plate. As anaphase I begins, the tetrads break up, and the maternal and paternal chromosomes separate. This is a major difference between mitosis and meiosis: In mitosis, each daughter cell receives one of the two copies of every chromosome, maternal and paternal, but in meiosis each daughter cell receives both copies of either the maternal chromosome or the paternal chromosome from each tetrad (compare Figure 28-6a and 28-6b ).

As anaphase proceeds, the maternal and paternal components are randomly distributed. As a result, telophase I ends with the formation of two daughter cells containing unique combinations of maternal and paternal chromosomes. Both cells contain 23 chromosomes. Because the first meiotic division reduces the number of chromosomes from 46 to 23, it is called a reductional division. Each of these chromosomes still consists of two duplicate chromatids. These duplicates will separate during meiosis II.

The interphase separating meiosis I and meiosis II is very brief, and there is no DNA replication over this period. The cell then proceeds through prophase II, metaphase II, and anaphase II. During anaphase II, the duplicate chromatids separate. Telophase II thus yields four cells, each containing 23 chromosomes. Because the number of chromosomes has not changed, meiosis II represents an equational division.

We shall now consider meiosis and the production of spermatozoa. In a later section, we shall deal with oogenesis (the production of ova).

The mitotic divisions of spermatogonia produce primary spermatocytes (Figure 28-7 ). As meiosis begins, each primary spermatocyte contains 46 individual chromosomes. At the end of meiosis I, the daughter cells are called secondary spermatocytes. Every secondary spermatocyte contains 23 chromosomes, each of which consists of a pair of duplicate chromatids. The secondary spermatocytes soon enter prophase II. The completion of metaphase II, anaphase II, and telophase II yields four spermatids, each containing 23 chromosomes.

For each primary spermatocyte that enters meiosis, four spermatids are produced. Because cytokinesis (cytoplasmic division) is not completed in meiosis I or meiosis II, the four spermatids initially remain interconnected by cytoplasmic bridges. These connections assist in the transfer of nutrients and hormonal messages between the cells, thus helping ensure that the cells develop in synchrony. The interconnections are not broken until the last stages of physical maturation.

Spermiogenesis
Each spermatid matures into a single spermatozoon, or sperm cell. This maturation process is called spermiogenesis (Figure 28-7 ). Developing spermatocytes undergoing meiosis and spermatids undergoing spermiogenesis are not free in the seminiferous tubules. Instead, they are surrounded by the cytoplasm of the sustentacular cells. As spermiogenesis proceeds, the spermatids gradually develop the appearance of mature spermatozoa. At spermiation, a spermatozoon loses its attachment to the sustentacular cell and enters the lumen of the seminiferous tubule. The entire process, from spermatogonial division to spermiation, takes approximately 9 weeks.

Spermatogenesis and Sustentacular Cells
Sustentacular cells play a key role in the process of spermatogenesis. These cells have six important functions that directly or indirectly affect mitosis, meiosis, and spermiogenesis within the seminiferous tubules:

1.     Maintenance of the blood-testis barrier. The seminiferous tubules are isolated from the general circulation by a blood-testis barrier comparable in function to the blood-brain barrier. Sustentacular cells are joined by tight junctions, forming a layer that divides the seminiferous tubule into an outer basal compartment that contains the spermatogonia and an inner lumenal compartment (or adlumenal compartment) where meiosis and spermiogenesis occur. Transport across the sustentacular cells is tightly regulated so that conditions in the lumenal compartment remain very stable. The fluid within the lumen of a seminiferous tubule is produced by sustentacular cells, which also regulate the fluid's composition. Tubular fluid is very different from the surrounding interstitial fluid. For example, tubular fluid is high in androgens, estrogens, potassium, and amino acids. The blood-testis barrier is essential to preserving the differences between tubular fluid and interstitial fluid. In addition, developing spermatozoa contain sperm-specific antigens in their cell membranes. These antigens, not found in somatic cell membranes, would be attacked by the immune system if the blood-testis barrier did not prevent their being detected.

2.     Support of mitosis and meiosis. Spermatogenesis depends on the stimulation of sustentacular cells by circulating follicle-stimulating hormone (FSH) and testosterone. Stimulated sustentacular cells then in some way promote the division of spermatogonia and the meiotic divisions of spermatocytes.

3.     Support of spermiogenesis. Spermiogenesis requires the presence of sustentacular cells. These cells surround and enfold the spermatids, providing nutrients and chemical stimuli that promote their development.

4.     Secretion of inhibin. Sustentacular cells secrete inhibin , a peptide hormone, in response to factors released by developing sperm. Inhibin, introduced in Chapter 18, depresses the pituitary production of FSH and perhaps the hypothalamic secretion of gonadotropin-releasing hormone (GnRH). The faster the rate of sperm production, the greater the amount of inhibin secreted. By regulating FSH and GnRH secretion, sustentacular cells provide feedback control of spermatogenesis.

5.     Secretion of androgen-binding protein. Androgen-binding protein (ABP) binds androgens (primarily testosterone) in the fluid contents of the seminiferous tubules. This protein is thought to be important in elevating the concentration of androgens within the tubules and stimulating spermiogenesis. The production of ABP is stimulated by FSH.

6.     Secretion of Müllerian-inhibiting factor. Müllerian-inhibiting factor (MIF) is secreted by sustentacular cells in the developing testes. This hormone causes regression of the fetal Müllerian ducts, passageways that in females participate in the formation of the uterine tubes and the uterus. Inadequate MIF production during development leads to retention of these ducts and failure of the testes to descend into the scrotum.

Anatomy of a Spermatozoon

Each spermatozoon has three distinct regions: (1) the head, (2) the middle piece, and (3) the tail (Figure 28-8 ). The head is a flattened ellipse containing a nucleus with densely packed chromosomes. At the tip of the head is the acrosomal cap, a membranous compartment containing enzymes essential to the process of fertilization. During spermiogenesis, saccules of the Golgi apparatus fuse and flatten into an acrosomal vesicle that ultimately forms the acrosomal cap.

A short neck attaches the head to the middle piece. The neck contains both centrioles of the original spermatid. The microtubules of the distal centriole are continuous with those of the middle piece and tail. Mitochondria in the middle piece are arranged in a spiral around the microtubules. Mitochondrial activity provides the ATP that is needed to move the tail.

The tail is the only flagellum in the human body. A flagellum, an organelle introduced in Chapter 3, moves a cell from one place to another. Whereas cilia beat in a predictable, waving fashion, the flagellum of a spermatozoon has a complex, corkscrew motion.

Unlike other, less specialized cells, a mature spermatozoon lacks an endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, inclusions, and many other intracellular structures. Because the cell does not contain glycogen or other energy reserves, it must absorb nutrients (primarily fructose) from the surrounding fluid.

The Male Reproductive Tract

The testes produce physically mature spermatozoa that are, as yet, incapable of successful fertilization. The other portions of the male reproductive system are responsible for the functional maturation, nourishment, storage, and transport of spermatozoa.

The Epididymis

Late in their development, spermatozoa detach from the sustentacular cells and lie within the lumen of the seminiferous tubule. They have most of the physical characteristics of mature sperm cells but are functionally immature and incapable of coordinated locomotion or fertilization. Fluid currents then transport the cell along the straight tubule, through the rete testis (Figures 28-4 and 28-9a ), and into the epididymis.

The epididymis lies along the posterior border of the testis (Figures 28-1 28-4 and 28-9a ). It is firm and can be felt through the skin of the scrotum. The epididymis consists of a tubule almost 7 meters (23 ft) long, coiled and twisted so as to take up very little space. It has (1) a head, (2) a body, and (3) a tail. The superior head is the portion of the epididymis proximal to the mediastinum of the testis. The head receives spermatozoa from the efferent ducts that connect the rete testis to the epididymis.

The body begins distal to the last efferent duct and extends inferiorly along the posterior margin of the testis. Near the inferior border of the testis, the number of convolutions decreases, marking the start of the tail. The tail recurves and ascends to its connection with the ductus deferens. Sperm are stored primarily within the tail of the epididymis.

The epididymis has the following three functions:

1.     It monitors and adjusts the composition of the tubular fluid. The pseudostratified columnar epithelial lining of the epididymis (Figure 28-9b ) bears distinctive stereocilia that increase the surface area available for absorption and secretion into the fluid in the tubule.

2.     It acts as a recycling center for damaged spermatozoa. Cellular debris and damaged spermatozoa are absorbed in the epididymis, and the products of enzymatic breakdown are released into the surrounding interstitial fluids for pickup by the epididymal circulation.

3.     It stores spermatozoa and facilitates their functional maturation. It takes about 2 weeks for a spermatozoon to pass through the epididymis; during this period, the spermatozoon completes its functional maturation. Although spermatozoa leaving the epididymis are mature, they remain immobile. To become active, motile, and fully functional, spermatozoa must undergo capacitation. Capacitation normally occurs in two steps: (1) Spermatozoa become motile when mixed with secretions of the seminal vesicles, and (2) they become capable of successful fertilization when exposed to conditions inside the female reproductive tract. The epididymis secretes a substance (as yet unidentified) that prevents premature capacitation.

Transport along the epididymis involves some combination of fluid movement and peristaltic contractions of smooth muscle. After passing along the tail of the epididymis, the spermatozoa enter the ductus deferens.

The Ductus Deferens

Each ductus deferens, or vas deferens, is 40-45 cm (16-18 in.) long. It begins at the tail of the epididymis (Figure 28-9a ) and, as part of the spermatic cord, ascends through the inguinal canal (Figures 28-1 and 28-2 ). Inside the abdominal cavity, the ductus deferens passes posteriorly, curving inferiorly along the lateral surface of the urinary bladder toward the superior and posterior margin of the prostate gland. Just before the ductus deferens reaches the prostate gland and seminal vesicles, its lumen enlarges. This expanded portion is known as the ampulla (Figure 28-10a ).

The wall of the ductus deferens contains a thick layer of smooth muscle. Peristaltic contractions in this layer propel spermatozoa and fluid along the duct, which is lined by a pseudostratified ciliated columnar epithelium. In addition to transporting sperm, the ductus deferens can store spermatozoa for several months. During this time, the spermatozoa remain in a state of suspended animation and have low metabolic rates.

The junction of the ampulla with the duct of the seminal vesicle marks the start of the ejaculatory duct. This short passageway (2 cm, or less than 1 in.) penetrates the muscular wall of the prostate gland and empties into the urethra (Figures 28-1 and 28-10 ) near the opening of the ejaculatory duct from the opposite side.

The Urethra

The urethra of the male extends from the urinary bladder to the tip of the penis, a distance of 18-20 cm (7-8 in.). It is divided into prostatic, membranous, and penile regions. We considered those subdivisions in Chapter 26. In males, the urethra is a passageway used by both the urinary and the reproductive systems.

The Accessory Glands

The fluids contributed by the seminiferous tubules and the epididymis account for only about 5 percent of the volume of semen. The fluid component of semen is a mixture of the secretions of many different glands, each with distinctive biochemical characteristics. Important glands include the seminal vesicles, the prostate gland, and the bulbourethral glands (Figures 28-1 and 28-10 ). Major functions of these glandular organs, which occur only in males, include (1) activating the spermatozoa; (2) providing the nutrients spermatozoa need for motility; (3) propelling spermatozoa and fluids along the reproductive tract, mainly by peristaltic contractions; and (4) producing buffers that counteract the acidity of urethral and vaginal contents.

The Seminal Vesicles

The ductus deferens on each side ends at the junction between the ampulla and the duct that drains the seminal vesicle (Figure 28-10a ). The seminal vesicles are embedded in connective tissue on either side of the midline, sandwiched between the posterior wall of the urinary bladder and the rectum. Each seminal vesicle is a tubular gland with a total length of about 15 cm (6 in.). The body of the gland has many short side branches. The entire assemblage is coiled and folded into a compact, tapered mass roughly 5 cm x 2.5 cm (2 in. x 1 in.). You can see the location of the seminal vesicles in Figures 28-1 (lateral view); 28-10 (posterior and inferior views); and 28-11a (anterior view).

The seminal vesicles are extremely active secretory glands with an epithelial lining that contains extensive folds (Figure 28-10 ). The seminal vesicles contribute about 60 percent of the volume of semen. Although the vesicular fluid generally has the same osmotic concentration as blood plasma, the composition of the two fluids is quite different. In particular, the secretion of the seminal vesicles contains (1) relatively high concentrations of fructose, which is easily metabolized by spermatozoa; (2) prostaglandins, which may stimulate smooth muscle contractions along the male and female reproductive tracts; and (3) fibrinogen, which after ejaculation will form a temporary clot within the vagina. The secretions of the seminal vesicles are slightly alkaline. This alkalinity helps neutralize acids in the prostatic secretions and within the vagina. When mixed with the secretions of the seminal vesicles, previously inactive but functional spermatozoa begin beating their flagella, becoming highly mobile.

The secretions of the seminal vesicles are discharged into the ejaculatory duct at emission, when peristaltic contractions are under way in the ductus deferens, seminal vesicles, and prostate gland. These contractions are under control of the sympathetic nervous system.

The Prostate Gland

The prostate gland is a small, muscular, rounded organ with a diameter of about 4 cm (1.6 in.). The prostate gland encircles the proximal portion of the urethra as it leaves the urinary bladder (Figures 28-1 sagittal view; 28-10a , posterior view; and 26-19c and 28-11a anterior views). The glandular tissue of the prostate consists of a cluster of 30-50 compound tubuloalveolar glands (Figure 28-10c and Figure 4-7 ). These glands are surrounded by and wrapped in a thick blanket of smooth muscle fibers.

The prostatic glands produce prostatic fluid, a slightly acidic solution that contributes 20-30 percent of the volume of semen. In addition to several other compounds of uncertain significance, prostatic secretions contain seminalplasmin, an antibiotic that may help prevent urinary tract infections in males. These secretions are ejected into the prostatic urethra by peristaltic contractions of the muscular wall.

The Bulbourethral Glands

The paired bulbourethral glands, or Cowper's glands, are situated at the base of the penis, covered by the fascia of the urogenital diaphragm (Figures 28-1 sagittal view; 28-10a posterior view; and 28-11a anterior view). The bulbourethral glands are round, with diameters approaching 10 mm (less than 0.5 in.). The duct of each gland travels alongside the penile urethra for 3-4 cm (1.2-1.6 in.) before emptying into the urethral lumen. These are compound, tubuloalveolar mucous glands (Figure 28-10d ) that secrete a thick, alkaline mucus. The secretion helps neutralize any urinary acids that may remain in the urethra and provides lubrication for the glans, or tip of the penis.

Semen

A typical ejaculation releases 2-5 ml of semen. This volume of fluid, called an ejaculate, contains:

• Spermatozoa. A normal sperm count ranges from 20 million to 100 million spermatozoa per milliliter.

• Seminal fluid. Seminal fluid, the fluid component of semen, is a mixture of glandular secretions with a distinct ionic and nutrient composition. A typical sample of seminal fluid contains the combined secretions of the seminal vesicles (60 percent), the prostate (30 percent), the sustentacular cells and epididymis (5 percent), and the bulbourethral glands (less than 5 percent).

• Enzymes. Several important enzymes are present in the seminal fluid, including (1) a protease that may help dissolve mucous secretions in the vagina; (2) seminalplasmin, an antibiotic enzyme from the prostate gland that kills a variety of bacteria, including Escherichia coli; (3) a prostatic enzyme that converts fibrinogen to fibrin after ejaculation; and (4) fibrinolysin, which subsequently liquefies the clotted semen.

The Penis

The penis is a tubular organ through which the distal portion of the urethra passes (Figures 28-1 28-2 and 28-11a ). It conducts urine to the exterior and introduces semen into the female vagina during sexual intercourse. The penis is divided into three regions: (1) the root, (2) the body, and (3) the glans (Figure 28-11b ). The root of the penis is the fixed portion that attaches the penis to the body wall. This connection occurs within the urogenital triangle immediately inferior to the pubic symphysis. The body (shaft) of the penis is the tubular, movable portion. Masses of erectile tissue are found within the body. The glans of the penis is the expanded distal end that surrounds the external urethral meatus. The neck is the narrow portion of the penis between the shaft and the glans.

The skin overlying the penis resembles that of the scrotum. The dermis contains a layer of smooth muscle, and the underlying loose connective tissue allows the thin skin to move without distorting underlying structures. The subcutaneous layer also contains superficial arteries, veins, and lymphatics.

A fold of skin called the prepuce, or foreskin, surrounds the tip of the penis. The prepuce attaches to the relatively narrow neck of the penis and continues over the glans. There are no hair follicles on the opposing surfaces, but preputial glands in the skin of the neck and the inner surface of the prepuce secrete a waxy material known as smegma. Unfortunately, smegma can be an excellent nutrient source for bacteria. Mild inflammation and infections in this region are common, especially if the area is not washed thoroughly and frequently. One way of avoiding trouble is to perform a circumcision, surgical removal of the prepuce. In Western societies (especially the United States), this procedure is generally performed shortly after birth. Although the practice of circumcision remains controversial, strong religious and cultural biases and epidemiological evidence suggest that it will continue. (Uncircumcised males have a higher incidence of urinary tract infection and are at a greater risk of developing penile cancer than are circumcised males.)

Beneath the loose connective tissue, a dense network of elastic fibers encircles the internal structures of the penis. Most of the body of the penis consists of three cylindrical columns of erectile tissue (Figure 28-11a,c ). Erectile tissue consists of a three-dimensional maze of vascular channels incompletely separated by partitions of elastic connective tissue and smooth muscle fibers. In the resting state, the arterial branches are constricted, and the muscular partitions are tense. This combination restricts blood flow into the erectile tissue. The parasympathetic innervation of the penile arteries involves neurons that release nitric oxide (NO) at their synaptic knobs. The smooth muscles in the arterial walls relax when NO is released, at which time (1) the vessels dilate, (2) blood flow increases, (3) the vascular channels become engorged with blood, and (4) erection of the penis occurs. The flaccid (nonerect) penis hangs inferior to the pubic symphysis and anterior to the scrotum, but during erection the penis stiffens and assumes a more upright position.

On the anterior surface of the flaccid penis, the two cylindrical corpora cavernosa (singular, cavernosum) are separated by a thin septum and encircled by a dense collagenous sheath. The corpora cavernosa diverge at their bases, forming the crura (crura, legs; singular, crus) of the penis. Each crus is bound to the ramus of the ischium and pubis by tough connective tissue ligaments. The corpora cavernosa extend along the length of the penis as far as the neck. The erectile tissue within each corpus cavernosum surrounds a central artery (Figure 28-11c ).

The relatively slender corpus spongiosum surrounds the penile urethra (Figure 28-11a ). This erectile body extends from the superficial fascia of the urogenital diaphragm to the tip of the penis, where it expands to form the glans. The sheath surrounding the corpus spongiosum contains more elastic fibers than does that of the corpora cavernosa, and the erectile tissue contains a pair of small arteries.

Hormones and Male Reproductive Function

The hormonal interactions in males are diagrammed in Figure 28-12 , and we introduced major reproductive hormones in Chapter 18. The anterior pituitary releases follicle-stimulating hormone (FSH) and luteinizing hormone (LH). The pituitary release of these hormones occurs in the presence of gonadotropin-releasing hormone (GnRH), a peptide synthesized in the hypothalamus and carried to the anterior pituitary by the hypophyseal portal system.

GnRH secretion occurs in pulses rather than continuously. In adult males, small pulses occur at 60-90 minute intervals. As levels of GnRH change, so do the rates of secretion of FSH and LH (and testosterone, which is released in response to LH). Unlike the situation in women, which we will consider later in the chapter, the GnRH pulse frequency in adult males remains relatively steady from hour to hour, day to day, and year to year. As a result, plasma levels of FSH, LH, and testosterone remain within a relatively narrow range throughout adult life.

FSH and Spermatogenesis

In males, FSH targets primarily the sustentacular cells of the seminiferous tubules. Under FSH stimulation, and in the presence of testosterone from the interstitial cells, sustentacular cells (1) promote spermatogenesis and spermiogenesis and (2) secrete androgen-binding protein (ABP).

The rate of spermatogenesis is regulated by a negative-feedback mechanism involving GnRH, FSH, and inhibin. Under GnRH stimulation, FSH promotes spermatogenesis along the seminiferous tubules. As spermatogenesis accelerates, however, so does the rate of inhibin secretion by the sustentacular cells of the testes. Inhibin inhibits FSH production in the anterior pituitary and may also suppress secretion of GnRH at the hypothalamus.

The net effect is that when FSH levels become elevated, inhibin production increases until the FSH levels return to normal. If FSH levels decline, inhibin production falls, and the rate of FSH production accelerates.

LH and Androgen Production

In males, LH causes the secretion of testosterone and other androgens by the interstitial cells of the testes. Testosterone, the most important androgen, has numerous functions, such as (1) stimulating spermatogenesis and promoting the functional maturation of spermatozoa, through its effects on sustentacular cells; (2) affecting CNS function, including the influence of libido (sexual drive) and related behaviors; (3) stimulating metabolism throughout the body, especially pathways concerned with protein synthesis and muscle growth; (4) establishing and maintaining the secondary sex characteristics, such as the distribution of facial hair, increased muscle mass and body size, and the quantity and location of characteristic adipose tissue deposits; and (5) maintaining the accessory glands and organs of the male reproductive tract.

Testosterone functions like other steroid hormones. It circulates while bound to transport proteins; gonadal steroid-binding globulin (GBG) carries roughly two-thirds of circulating testosterone, and the rest binds to albumins. Testosterone diffuses across the cell membrane and binds to an intracellular receptor. The hormone-receptor complex then binds to the DNA in the nucleus. In many target tissues, some of the arriving testosterone is converted to dihydrotestosterone (DHT). A small amount of DHT diffuses back out of the cell and into the circulation, and DHT levels are usually about 10 percent of circulating testosterone levels. DHT can also enter peripheral cells and bind to the same hormone receptors targeted by testosterone. In addition, some tissues, notably those of the external genitalia, respond to DHT rather than to testosterone, and other tissues, including the prostate gland, are more sensitive to DHT than to testosterone.

Testosterone production begins around the seventh week of embryonic development and reaches a peak after roughly 6 months of development. Over this period, secretion of MIF by developing sustentacular cells leads to the regression of the Müllerian ducts. The early surge in testosterone levels stimulates the differentiation of the male duct system and accessory organs and affects CNS development. The best-known CNS effects occur in the developing hypothalamus, where testosterone apparently programs the hypothalamic centers involved with (1) GnRH production and the regulation of pituitary FSH and LH secretion, (2) sexual behaviors, and (3) sexual drive. As a result of this prenatal exposure to testosterone, the hypothalamic centers will respond appropriately when the individual becomes sexually mature. The factors responsible for regulating the fetal production of testosterone are not known.

Testosterone levels are low at birth. Up to puberty, background testosterone levels, although still relatively low, are higher in males than in females. Testosterone secretion accelerates markedly at puberty, initiating sexual maturation and the appearance of secondary sex characteristics. In adult males, negative feedback controls the level of testosterone production. Above-normal testosterone levels inhibit the release of GnRH by the hypothalamus. This inhibition causes a reduction in LH secretion and lowers testosterone levels.

The plasma of adult males also contains relatively small amounts of estradiol (2 ng/dl, versus 525 ng/dl of testosterone). Seventy percent of the estradiol is formed from circulating testosterone. The rest is secreted, primarily by the interstitial and sustentacular cells of the testes. The conversion of testosterone to estradiol is performed by an enzyme called aromatase. For unknown reasons, estradiol production increases in older men.

MASCULINE GENITAL subdivide into internal male sexual organs (testicles, epididymis, spermatic cord, ductus deferens, seminal vesicles, prostate gland and bulbourethral gland) and external genital organs (scrotum and penis). Masculine urethra is not only for passing of urine also for passing of sperm.

The Testicle is a pair parenchymatic organ, which is situated in scrotum and produces sperm and masculine sexual hormones. Each testicle has superior extremity and inferior extremity, medial surface and lateral surface, anterior margin and posterior margin.

Testicle is covered by tunica albuginea which on posterior margin to get in testicle parenchyma and forms testicle mediastinum. Last gives off septula testis, which subdivide organ into 150-200 lobules. In each lobule the tubuli seminiferi contorti are situated (1-2), where masculine sexual cells - spermatozoon produced. Tubuli seminiferi contorti continue into tubuli seminiferi recti [straight], and last run into rete testis in mediastinum. Efferent ductuli (15 - 20) pass from testicle rete transfixing albuginea membrane, continue into head of epididymis and form there the lobules of epididymis. Then spermatozoon runs sufficiently rolled duct of epididymis, which reaches into length 2 m. Duct of epididymis passes down to its tail, where continues into ductus deferens.

The Epididymis adjoins to posterior testicle margin. There are head of epididymis, body and tail of epididymis. Sinus of epididymis is situated between testicle and body of epididymis.

The male genitals include the testes, the ductus deferentes, the vesiculæ seminales, the ejaculatory ducts, and the penis, together with the following accessory structures, viz., the prostate and the bulbourethral glands.

 

VIDEO

 

1. The Testes are two glandular organs, which secrete the semen; they are suspended in the scrotum by the spermatic cords. At an early period of fetal life the testes are contained in the abdominal cavity, behind the peritoneum. Before birth they descend to the inguinal canal, along which they pass with the spermatic cord, and, emerging at the subcutaneous inguinal ring, they descend into the scrotum, becoming invested in their course by coverings derived from the serous, muscular, and fibrous layers of the abdominal parietes, as well as by the scrotum.

  The coverings of the testes are, the

Skin

Scrotum.

Cremaster.

Dartos tunic

Infundibuliform fascia.

Intercrural fascia.

Tunica vaginalis.

The scrotum. On the left side the cavity of the tunica vaginalis has been opened; on the right side only the layers superficial to the Cremaster have been removed.

 

  The Scrotum is a cutaneous pouch which contains the testes and parts of the spermatic cords. It is divided on its surface into two lateral portions by a ridge or raphé, which is continued forward to the under surface of the penis, and backward, along the middle line of the perineum to the anus. Of these two lateral portions the left hangs lower than the right, to correspond with the greater length of the left spermatic cord. Its external aspect varies under different circumstances: thus, under the influence of warmth, and in old and debilitated persons, it becomes elongated and flaccid; but, under the influence of cold, and in the young and robust, it is short, corrugated, and closely applied to the testes.

  The scrotum consists of two layers, the integument and the dartos tunic.

  The Integument is very thin, of a brownish color, and generally thrown into folds or rugæ. It is provided with sebaceous follicles, the secretion of which has a peculiar odor, and is beset with thinly scattered, crisp hairs, the roots of which are seen through the skin.

  The Dartos Tunic (tunica dartos) is a thin layer of non-striped muscular fibers, continuous, around the base of the scrotum, with the two layers of the superficial fascia of the groin and the perineum; it sends inward a septum, which divides the scrotal pouch into two cavities for the testes, and extends between the raphé and the under surface of the penis, as far as its root.

 

 

 

Figure 2

 

(a) Axial T2- and (b) T1-weighted MR images demonstrating homogenous high T2 and homogenous intermediate T1 signal intensity in both testes (arrows). (c) Axial T1-weighted MRI image following administration of IV gadolinium showing decreased enhancement of the left testis (arrow) compared to the right (arrowhead). (d) Coronal T1-weighted MRI image demonstrating torsion of left spermatic cord (arrow).

The scrotum. The penis has been turned upward, and the anterior wall of the scrotum has been removed. On the right side, the spermatic cord, the infundibuliform fascia, and the Cremaster muscle are displayed; on the left side, the infundibuliform fascia has been divided by a longitudinal incision passing along the front of the cord and the testicle, and a portion of the parietal layer of the tunica vaginalis has been removed to display the testicle and a portion of the head of the epididymis, which are covered by the visceral layer of the tunica vaginalis.

 

  The dartos tunic is closely united to the skin externally, but connected with the subjacent parts by delicate areolar tissue, upon which it glides with the greatest facility.

  The Intercrural Fascia (intercolumnar or external spermatic fascia) is a thin membrane, prolonged downward around the surface of the cord and testis (see page 411). It is separated from the dartos tunic by loose areolar tissue.

  The Cremaster consists of scattered bundles of muscular fibers connected together into a continuous covering by intermediate areolar tissue (see page 414).

  The Infundibuliform Fascia (tunica vaginalis communis [testis et funiculi spermatici]) is a thin layer, which loosely invests the cord; it is a continuation downward of the transversalis fascia (see page 418).

  The Tunica Vaginalis is described with the testes.

 

Vessels and Nerves.—The arteries supplying the coverings of the testes are: the superficial and deep external pudendal branches of the femoral, the superficial perineal branch of the internal pudendal, and the cremasteric branch from the inferior epigastric. The veins follow the course of the corresponding arteries. The lymphatics end in the inguinal lymph glands. The nerves are the ilioinguinal and lumboinguinal branches of the lumbar plexus, the two superficial perineal branches of the internal pudendal nerve, and the pudendal branch of the posterior femoral cutaneous nerve.

  The Inguinal Canal (canalis inguinalis) is described on page 418.

  The Spermatic Cord (funiculus spermaticus) extends from the abdominal inguinal ring, where the structures of which it is composed converge, to the back part of the testis. In the abdominal wall the cord passes obliquely along the inguinal canal, lying at first beneath the Obliquus internus, and upon the fascia transversalis; but nearer the pubis, it rests upon the inguinal and lacunar ligaments, having the aponeurosis of the Obliquus externus in front of it, and the inguinal falx behind it. It then escapes at the subcutaneous ring, and descends nearly vertically into the scrotum. The left cord is rather longer than the right, consequently the left testis hangs somewhat lower than its fellow.

 Structure of the Spermatic Cord.—The spermatic cord is composed of arteries, veins, lymphatics, nerves, and the excretory duct of the testis. These structures are connected together by areolar tissue, and invested by the layers brought down by the testis in its descent.

  The arteries of the cord are: the internal and external spermatics; and the artery to the ductus deferens.

  The internal spermatic artery, a branch of the abdominal aorta, escapes from the abdomen at the abdominal inguinal ring, and accompanies the other constituents of the spermatic cord along the inguinal canal and through the subcutaneous inguinal ring into the scrotum. It then descends to the testis, and, becoming tortuous, divides into several branches, two or three of which accompany the ductus deferens and supply the epididymis, anastomosing with the artery of the ductus deferens: the others supply the substance of the testis.

  The external spermatic artery is a branch of the inferior epigastric artery. It accompanies the spermatic cord and supplies the coverings of the cord, anastomosing with the internal spermatic artery.

  The artery of the ductus deferens, a branch of the superior vesical, is a long, slender vessel, which accompanies the ductus deferens, ramifying upon its coats, and anastomosing with the internal spermatic artery near the testis.

 

The spermatic cord in the inguinal canal.

 

  The spermatic veins emerge from the back of the testis, and receive tributaries from the epididymis: they unite and form a convoluted plexus, the plexus pampiniformis, which forms the chief mass of the cord; the vessels composing this plexus are very numerous, and ascend along the cord in front of the ductus deferens; below the subcutaneous inguinal ring they unite to form three or four veins, which pass along the inguinal canal, and, entering the abdomen through the abdominal inguinal ring, coalesce to form two veins. These again unite to form a single vein, which opens on the right side into the inferior vena cava, at an acute angle, and on the left side into the left renal vein, at a right angle.

  The nerves are the spermatic plexus from the sympathetic, joined by filaments from the pelvic plexus which accompany the artery of the ductus deferens.

  The scrotum forms an admirable covering for the protection of the testes. These bodies, lying suspended and loose in the cavity of the scrotum and surrounded by serous membrane, are capable of great mobility, and can therefore easily slip about within the scrotum and thus avoid injuries from blows or squeezes. The skin of the scrotum is very elastic and capable of great distension, and on account of the looseness and amount of subcutaneous tissue, the scrotum becomes greatly enlarged in cases of edema, to which this part is especially liable as a result of its dependent position.

  The Testes are suspended in the scrotum by the spermatic cords, the left testis hanging somewhat lower than its fellow. The average dimensions of the testis are from 4 to 5 cm. in length, 2.5 cm. in breadth, and 3 cm. in the antero-posterior diameter; its weight varies from 10.5 to 14 gm. Each testis is of an oval form compressed laterally, and having an oblique position in the scrotum; the upper extremity is directed forward and a little lateralward; the lower, backward and a little medialward; the anterior convex border looks forward and downward, the posterior or straight border, to which the cord is attached, backward and upward.

  The anterior border and lateral surfaces, as well as both extremities of the organ, are convex, free, smooth, and invested by the visceral layer of the tunica vaginalis. The posterior border, to which the cord is attached, receives only a partial investment from that membrane. Lying upon the lateral edge of this posterior border is a long, narrow, fiattened body, named the epididymis.

 

Spermatic veins.

 

The right testis, exposed by laying open the tunica vaginalis.

 

  The epididymis consists of a central portion or body; an upper enlarged extremity, the head (globus major); and a lower pointed extremity, the tail (globus minor), which is continuous with the ductus deferens, the duct of the testis. The head is intimately connected with the upper end of the testis by means of the efferent ductules of the gland; the tail is connected with the lower end by cellular tissue, and a reflection of the tunica vaginalis. The lateral surface, head and tail of the epididymis are free and covered by the serous membrane; the body is also completely invested by it, excepting along its posterior border; while between the body and the testis is a pouch, named the sinus of the epididymis (digital fossa). The epididymis is connected to the back of the testis by a fold of the serous membrane.

 

Appendages of the Testis and Epididymis.—On the upper extremity of the testis, just beneath the head of the epididymis, is a minute oval, sessile body, the appendix of the testis (hydatid of Morgagni); it is the remnant of the upper end of the Müllerian duct. On the head of the epididymis is a second small stalked appendage (sometimes duplicated); it is named the appendix of the epididymis (pedunculated hydatid), and is usually regarded as a detached efferent duct.

  The testis is invested by three tunics: the tunica vaginalis, tunica albuginea, and tunica vasculosa.

  The Tunica Vaginalis (tunica vaginalis propria testis) is the serous covering of the testis. It is a pouch of serous membrane, derived from the saccus vaginalis of the peritoneum, which in the fetus preceded the descent of the testis from the abdomen into the scrotum. After its descent, that portion of the pouch which extends from the abdominal inguinal ring to near the upper part of the gland becomes obliterated; the lower portion remains as a shut sac, which invests the surface of the testis, and is reflected on to the internal surface of the scrotum; hence it may be described as consisting of a visceral and a parietal lamina.

  The visceral lamina (lamina visceralis) covers the greater part of the testis and epididymis, connecting the latter to the testis by means of a distinct fold. From the posterior border of the gland it is reflected on to the internal surface of the scrotum.

  The parietal lamina (lamina parietalis) is far more extensive than the visceral, extending upward for some distance in front and on the medial side of the cord, and reaching below the testis. The inner surface of the tunica vaginalis is smooth, and covered by a layer of endothelial cells. The interval between the visceral and parietal laminæ constitutes the cavity of the tunica vaginalis.

  The obliterated portion of the saccus vaginalis may generally be seen as a fibrocellular thread lying in the loose areolar tissue around the spermatic cord; sometimes this may be traced as a distinct band from the upper end of the inguinal canal, where it is connected with the peritoneum, down to the tunica vaginalis; sometimes it gradually becomes lost on the spermatic cord. Occasionally no trace of it can be detected. In some cases it happens that the pouch of peritoneum does not become obliterated, but the sac of the peritoneum communicates with the tunica vaginalis. This may give rise to one of the varieties of oblique inguinal hernia (page 1187). In other cases the pouch may contract, but not become entirely obliterated; it then forms a minute canal leading from the peritoneum to the tunica vaginalis.

  The Tunica Albuginea is the fibrous covering of the testis. It is a dense membrane, of a bluish-white color, composed of bundles of white fibrous tissue which interlace in every direction. It is covered by the tunica vaginalis, except at the points of attachment of the epididymis to the testis, and along its posterior border, where the spermatic vessels enter the gland. It is applied to the tunica vasculosa over the glandular substance of the testis, and, at its posterior border, is reflected into the interior of the gland, forming an incomplete vertical septum, called the mediastinum testis (corpus Highmori).

  The mediastinum testis extends from the upper to near the lower extremity of the gland, and is wider above than below. From its front and sides numerous imperfect septa (trabeculæ) are given off, which radiate toward the surface of the organ, and are attached to the tunica albuginea. They divide the interior of the organ into a number of incomplete spaces which are somewhat cone-shaped, being broad at their bases at the surface of the gland, and becoming narrower as they converge to the mediastinum. The mediastinum supports the vessels and duct of the testis in their passage to and from the substance of the gland.

  The Tunica Vasculosa is the vascular layer of the testis, consisting of a plexus of bloodvessels, held together by delicate areolar tissue. It clothes the inner surface of the tunica albuginea and the different septa in the interior of the gland, and therefore forms an internal investment to all the spaces of which the gland is composed.

 

Vertical section of the testis, to show the arrangement of the ducts.

 

Structure.—The glandular structure of the testis consists of numerous lobules. Their number, in a single testis, is estimated by Berres at 250, and by Krause at 400. They differ in size according to their position, those in the middle of the gland being larger and longer. The lobules are conical in shape, the base being directed toward the circumference of the organ, the apex toward the mediastinum. Each lobule is contained in one of the intervals between the fibrous septa which extend between the mediastinum testis and the tunica albuginea, and consists of from one to three, or more, minute convoluted tubes, the tubuli seminiferi. The tubules may be separately unravelled, by careful dissection under water, and may be seen to commence either by free cecal ends or by anastomotic loops. They are supported by loose connective tissue which contains here and there groups of “interstitial cells” containing yellow pigment granules. The total number of tubules is estimated by Lauth at 840, and the average length of each is 70 to 80 cm. Their diameter varies from 0.12 to 0.3 mm. The tubules are pale in color in early life, but in old age they acquire a deep yellow tinge from containing much fatty matter. Each tubule consists of a basement layer formed of laminated connective tissue containing numerous elastic fibers with flattened cells between the layers and covered externally by a layer of flattened epithelioid cells. Within the basement membrane are epithelial cells arranged in several irregular layers, which are not always clearly separated, but which may be arranged in three different groups. Among these cells may be seen the spermatozoa in different stages of development. (1) Lining the basement membrane and forming the outer zone is a layer of cubical cells, with small nuclei; some of these enlarge to become spermatogonia. The nuclei of some of the spermatogonia may be seen to be in process of indirect division (karyokineses, page 37), and in consequence of this daughter cells are formed, which constitute the second zone. (2) Within this first layer is to be seen a number of larger polyhedral cells, with clear nuclei, arranged in two or three layers; these are the intermediate cells or spermatocytes. Most of these cells are in a condition of karyokinetic division, and the cells which result from this division form those of the next layer, the spermatoblasts or spermatids. (3) The third layer of cells consists of the spermatoblasts or spermatids, and each of these, without further subdivision, becomes a spermatozoön. The spermatids are small polyhedral cells, the nucleus of each of which contains half the usual number of chromosomes. In addition to these three layers of cells others are seen, which are termed the supporting cells (cells of Sertoli). They are elongated and columnar, and project inward from the basement membrane toward the lumen of the tube. As development of the spermatozoa proceeds the latter group themselves around the inner extremities of the supporting cells. The nuclear portion of the spermatid, which is partly imbedded in the supporting cell, is differentiated to form the head of the spermatozoön, while part of the cell protoplasm forms the middle piece and the tail is produced by an outgrowth from the double centriole of the cell. Ultimately the heads are liberated and the spermatozoa are set free. The structure of the spermatozoa is described on pages 42, 43.

  In the apices of the lobules, the tubules become less convoluted, assume a nearly straight course, and unite together to form from twenty to thirty larger ducts, of about 0.5 mm. in diameter, and these, from their straight course, are called tubuli recti.

  The tubuli recti enter the fibrous tissue of the mediastinum, and pass upward and backward, forming, in their ascent, a close net-work of anastomosing tubes which are merely channels in the fibrous stroma, lined by flattened epithelium, and having no proper walls; this constitutes the rete testis. At the upper end of the mediastinum, the vessels of the rete testis terminate in from twelve to fifteen or twenty ducts, the ductuli efferentes; they perforate the tunica albuginea, and carry the seminal fluid from the testis to the epididymis. Their course is at first straight; they then become enlarged, and exceedingly convoluted, and form a series of conical masses, the coni vasculosi, which together constitute the head of the epididymis. Each cone consists of a single convoluted duct, from 15 to 20 cm. in length, the diameter of which gradually decreases from the testis to the epididymis. Opposite the bases of the cones the efferent vessels open at narrow intervals into a single duct, which constitutes, by its complex convolutions, the body and tail of the epididymis. When the convolutions of this tube are unravelled, it measures upward of 6 meters in length; it increases in diameter and thickness as it approaches the ductus deferens. The convolutions are held together by fine areolar tissue, and by bands of fibrous tissue.

  The tubuli recti have very thin walls; like the channels of the rete testis they are lined by a single layer of flattened epithelium. The ductuli efferentes and the tube of the epididymis have walls of considerable thickness, on account of the presence in them of muscular tissue, which is principally arranged in a circular manner. These tubes are lined by columnar ciliated epithelium.

 Peculiarities.—The testis, developed in the lumbar region, may be arrested or delayed in its transit to the scrotum (cryptorchism). It may be retained in the abdomen; or it may be arrested at the abdominal inguinal ring, or in the inguinal canal; or it may just pass out of the subcutaneous inguinal ring without finding its way to the bottom of the scrotum. When retained in the abdomen it gives rise to no symptoms, other than the absence of the testis from the scrotum; but when it is retained in the inguinal canal it is subjected to pressure and may become inflamed and painful. The retained testis is probably functionally useless; so that a man in whom both testes are retained (anorchism) is sterile, though he may not be impotent. The absence of one testis is termed monorchism. When a testis is retained in the inguinal canal it is often complicated with a congenital hernia, the funicular process of the peritoneum not being obliterated. In addition to the cases above described, where there is some arrest in the descent of the testis, this organ may descend through the inguinal canal, but may miss the scrotum and assume some abnormal position. The most common form is where the testis, emerging at the subcutaneous inguinal ring, slips down between the scrotum and thigh and comes to rest in the perineum. This is known as perineal ectopia testis. With each variety of abnormality in the position of the testis, it is very common to find concurrently a congenital hernia, or, if a hernia be not actually present, the funicular process is usually patent, and almost invariably so if the testis is in the inguinal canal.

  The testis, finally reaching the scrotum, may occupy an abnormal position in it. It may be inverted, so that its posterior or attached border is directed forward and the tunica vaginalis is situated behind.

  Fluid collections of a serous character are very frequently found in the scrotum. To these the term hydrocele is applied. The most common form is the ordinary vaginal hydrocele, in which the fluid is contained in the sac of the tunica vaginalis, which is separated, in its normal condition, from the peritoneal cavity by the whole extent of the inguinal canal. In another form, the congenital hydrocele, the fluid is in the sac of the tunica vaginalis, but this cavity communicates with the general peritoneal cavity, its tubular process remaining pervious. A third variety known as an infantile hydrocele, occurs in those cases where the tubular process becomes obliterated only at its upper part, at or near the abdominal inguinal ring. It resembles the vaginal hydrocele, except as regards its shape, the collection of fluid extending up the cord into the inguinal canal. Fourthly, the funicular process may become obliterated both at the abdominal inguinal ring and above the epididymis, leaving a central unobliterated portion, which may become distended with fluid, giving rise to a condition known as the encysted hydrocele of the cord.

 

The Ductus deferens has scrotal part, funicular part, inguinal part and pelvic part. It enters to composition of spermatic cord, which passes in inguinal canal to internal ring. Here ductus deferens separates from seminal funiculus, then it runs under fundus of urinary bladder. Pelvic part joins with excretorial duct of seminal vesicles, forming ampoule of ductus deferens. Attaching ducts generate ejaculatory duct (length 2 cm), which passes over prostata and opens into prostatic part of urethra on top of seminal tubercle.

The Spermatic cord is a formation, which consists of arteries and testicle veins, arteries and veins of ductus deferens, pampiniform venous plexus, cremaster muscle, vaginal processes, nerves, lymphatic vessels and ductus deferens.

The Prostate is a musculо-secretory organ, for shape reminds the chestnut, has a base of prostate, which adjoins to urinary bladder, and top of prostate, which is contact with urogenital diaphragm. It has an anterior surface and posterior surface, right and left lobes of prostate and isthmus of prostate, that envelops a urethra. Prostate gland consists of 36 alveolar-tubular glandules, which produce prostate juice and open by numerous ductuli into prostate part of urethra on base of seminal tubercle. Muscular apparatus contributes to extrusion of secret from prostate gland during ejaculation and is as additional (involuntary) urethral sphincter, which withholds the urine in bladder. Gland in old age atrophies and its mass diminishes.

 

Fundus of the bladder with the vesiculæ seminales.

 

The Seminal vesicles produces a seminal liquid, it communicate with ductus deferens. Seminal liquid together with secret of prostate composes part of sperm.

The Bulbourethral gland is a pair alveolar-tubular gland, which is situated in thickness of urogenital diaphragm. It has a duct of bulbourethral gland, which passes over bulb of penis and opens into spongy part of masculine urethra. Gland produces a secret, which protects mucous membrane of the urethra from irritation by urine.

 

External reproductive organs. Spermatic cord. Layers of scrotal wall. Structure of masculine urethra. Topography of small pelvis organs on male

 

The Scrotum is external organ, muscular and fascial sac which contains testicles and epididymis. Scrotal septum separates right and left halves. Scrotum is physiological thermostat, which keep temperature of testis at lower level then temperature of body (necessary for normal spermatogenesis).

Scrotal wall contains 7 membranes, which cover a testicle and derive from layers of anterior abdominal wall, namely:

1. Skin - has scrotal raphe, numerous folds, pigmented, with hair and contains specific sweat and sebaceous glands.

2. Under skin is situated a tunica dartos, which derives from hypodermic adipose tissue and grows together with skin.

3. External seminal fascia derives from superficial fascia of anterior abdominal wall.

4. Cremasteric fascia derives from proper abdominal fascia.

5. Musculus cremaster derives from internal oblique abdominis and transversal abdominal muscles.

6. Internal seminal fascia derives from transversal fascia of abdominal wall.

7. Vaginal tunica is serous membrane (derives from peritoneum) and consists of visceral plate and parietal plate. Last grows together with albuginea membrane and continues on epididymis. There is furrow-shaped space between both plates is a vaginal cavity, which is filled by small amount of serous liquid.

Transverse section through the left side of the scrotum and the left testis. The sac of the tunica vaginalis is represented in a distended condition.

 

The Penis serves removal of the urine and ejaculation. It has a radix, corpus and head. Skin which covers the penis in base of head forms the fold – preputium. Last thank to frenulum connects with skin of head. Penis formed by two cavernous bodies and spongious body. All bodies of penis covered by tunica albuginea. Spongious body contains male urethra.

The penis is a pendulous organ suspended from the front and sides of the pubic arch and containing the greater part of the urethra. In the flaccid condition it is cylindrical in shape, but when erect assumes the form of a triangular prism with rounded angles, one side of the prism forming the dorsum. It is composed of three cylindrical masses of cavernous tissue bound together by fibrous tissue and covered with skin. Two of the masses are lateral, and are known as the corpora cavernosa penis; the third is median, and is termed the corpus cavernosum urethræ.

  The Corpora Cavernosa Penis form the greater part of the substance of the penis. For their anterior three-fourths they lie in intimate apposition with one another, but behind they diverge in the form of two tapering processes, known as the crura, which are firmly connected to the rami of the pubic arch. Traced from behind forward, each crus begins by a blunt-pointed process in front of the tuberosity of the ischium. Just before it meets its fellow it presents a slight enlargement, named by Kobelt the bulb of the corpus cavernosum penis. Beyond this point the crus undergoes a constriction and merges into the corpus cavernosum proper, which retains a uniform diameter to its anterior end. Each corpus cavernosum penis ends abruptly in a rounded extremity some distance from the point of the penis.

  The corpora cavernosa penis are surrounded by a strong fibrous envelope consisting of superficial and deep fibers. The superficial fibers are longitudinal in direction, and form a single tube which encloses both corpora; the deep fibers are arranged circularly around each corpus, and form by their junction in the median plane the septum of the penis. This is thick and complete behind, but is imperfect in front, where it consists of a series of vertical bands arranged like the teeth of a comb; it is therefore named the septum pectiniforme.

  The Corpus Cavernosum Urethræ (corpus spongiosum) contains the urethra. Behind, it is expanded to form the urethral bulb, and lies in apposition with the inferior fascia of the urogenital diaphragm, from which it receives a fibrous investment. The urethra enters the bulb nearer to the upper than to the lower surface. On the latter there is a median sulcus, from which a thin fibrous septum projects into the substance of the bulb and divides it imperfectly into two lateral lobes or hemispheres.

 

The constituent cavernous cylinders of the penis.

The glans and anterior part of the corpus cavernosum urethræ are detached from the corpora cavernosa penis and turned to one side.

 

Transverse section of the penis.

  

             The portion of the corpus cavernosum urethræ in front of the bulb lies in a groove on the under surface of the conjoined corpora cavernosa penis. It is cylindrical in form and tapers slightly from behind forward. Its anterior end is expanded in the form of an obtuse cone, flattened from above downward. This expansion, termed the glans penis, is moulded on the rounded ends of the corpora cavernosa penis, extending farther on their upper than on their lower surfaces. At the summit of the glans is the slit-like vertical external urethral orifice. The circumference of the base of the glans forms a rounded projecting border, the corona glandis, overhanging a deep retroglandular sulcus, behind which is the neck of the penis.

 

VIDEO

 

  For descriptive purposes it is convenient to divide the penis into three regions: the root, the body, and the extremity.

 

Vertical section of bladder, penis, and urethra.

 

  The root (radix penis) of the penis is triradiate in form, consisting of the diverging crura, one on either side, and the median urethral bulb. Each crus is covered by the Ischiocavernosus, while the bulb is surrounded by the Bulbocavernosus. The root of the penis lies in the perineum between the inferior fascia of the urogenital diaphragm and the fascia of Colles. In addition to being attached to the fasciæ and the pubic rami, it is bound to the front of the symphysis pubis by the fundiform and suspensory ligaments. The fundiform ligament springs from the front of the sheath of the Rectus abdominis and the linea alba; it splits into two fasciculi which encircle the root of the penis. The upper fibers of the suspensory ligament pass downward from the lower end of the linea alba, and the lower fibers from the symphysis pubis; together they form a strong fibrous band, which extends to the upper surface of the root, where it blends with the fascial sheath of the organ.

  The body (corpus penis) extends from the root to the ends of the corpora cavernosa penis, and in it these corpora cavernosa are intimately bound to one another. A shallow groove which marks their junction on the upper surface lodges the deep dorsal vein of the penis, while a deeper and wider groove between them on the under surface contains the corpus cavernosum urethræ. The body is ensheathed by fascia, which is continuous above with the fascia of Scarpa, and below with the dartos tunic of the scrotum and the fascia of Colles.

  The extremity is formed by the glans penis, the expanded anterior end of the corpus cavernosum urethræ. It is separated from the body by the constricted neck, which is overhung by the corona glandis.

  The integument covering the penis is remarkable for its thinness, its dark color, its looseness of connection with the deeper parts of the organ, and its absence of adipose tissue. At the root of the penis it is continuous with that over the pubes, scrotum, and perineum. At the neck it leaves the surface and becomes folded upon itself to form the prepuce or foreskin. The internal layer of the prepuce is directly continuous, along the line of the neck, with the integument over the glans. Immediately behind the external urethral orifice it forms a small secondary reduplication, attached along the bottom of a depressed median raphé, which extends from the meatus to the neck; this fold is termed the frenulum of the prepuce. The integument covering the glans is continuous with the urethral mucous membrane at the orifice; it is devoid of haris, but projecting from its free surface are a number of small, highly sensitive papillæ. Scattered glands on the corona, neck, glans and inner layer of the prepuce, the preputial glands, have been described. They secrete a sebaceous material of very peculiar odor, which probably contains casein, and readily undergoes decomposition; when mixed with discarded epithelial cells it is called smegma.

  The prepuce covers a variable amount of the glans, and is separated from it by a potential sac—the preputial sac—which presents two shallow fossæ, one on either side of the frenulum.

 

Structure of the Penis.

  —From the internal surface of the fibrous envelope of the corpora cavernosa penis, as well as from the sides of the septum, numerous bands or cords are given off, which cross the interior of these corpora cavernosa in all directions, subdividing them into a number of separate compartments, and giving the entire structure a spongy appearance. These bands and cords are called trabeculæ, and consist of white fibrous tissue, elastic fibers, and plain muscular fibers. In them are contained numerous arteries and nerves. The component fibers which form the trabeculæ are larger and stronger around the circumference than at the centers of the corpora cavernosa; they are also thicker behind than in front. The interspaces (cavernous spaces), on the contrary, are larger at the center than at the circumference, their long diameters being directed transversely. They are filled with blood, and are lined by a layer of flattened cells similar to the endothelial lining of veins.

  The fibrous envelope of the corpus cavernosum urethræ is thinner, whiter in color, and more elastic than that of the corpora cavernosa penis. The trabeculæ are more delicate, nearly uniform in size, and the meshes between them smaller than in the corpora cavernosa penis: their long diameters, for the most part, corresponding with that of the penis. The external envelope or outer coat of the corpus cavernosum urethræ is formed partly of unstriped muscular fibers, and a layer of the same tissue immediately surrounds the canal of the urethra.

Vessels and Nerves.—The arteries bringing the blood to the cavernous spaces are the deep arteries of the penis and branches from the dorsal arteries of the penis, which perforate the fibrous capsule, along the upper surface, especially near the forepart of the organ. On entering the cavernous structure the arteries divide into branches, which are supported and enclosed by the trabeculæ. Some of these arteries end in a capillary net-work, the branches of which open directly into the cavernous spaces; others assume a tendril-like appearance, and form convoluted and somewhat dilated vessels, which were named by Müller helicine arteries. They open into the spaces, and from them are also given off small capillary branches to supply the trabecular structure. They are bound down in the spaces by fine fibrous processes, and are most abundant in the back part of the corpora cavernosa.

 

Diagram of the arteries of the penis.

 

  The blood from the cavernous spaces is returned by a series of vessels, some of which emerge in considerable numbers from the base of the glans penis and converge on the dorsum of the organ to form the deep dorsal vein; others pass out on the upper surface of the corpora cavernosa and join the same vein; some emerge from the under surface of the corpora cavernosa penis and receiving branches from the corpus cavernosum urethræ, wind around the sides of the penis to end in the deep dorsal vein; but the greater number pass out at the root of the penis and join the prostatic plexus.

  The nerves are derived from the pudendal nerve and the pelvic plexuses. On the glans and bulb some filaments of the cutaneous nerves have Pacinian bodies connected with them, and, according to Krause, many of them end in peculiar endbulbs (see page 1060).

 

Veins of the penis.

 

Masculine  urethra is a tube of length 16-22 cm, in which there distinguish prostatic part, membranous part and spongious part. On its tract a urethra makes a superior (fixed) bend and inferior (free) bend.

Prostatic part passes through the prostate. In this part on the urethral wall is situated seminal colliculus, on top of which prostatic utriculus disposed. Ejaculatory duct opens at last and prostatic ductuli opens on tubercle base. Intermediate (membranous) part of urethra shorter, it passes through urogenital diaphragm. Described two parts have to fixed position within pelvis and perineum. Spongy part of urethra lies in spongious body of penis and opens by external urethral ostium on head top.

Male urethra has following constrictions: external urethral ostium on head of penis; membranous part of urethra; internal urethral ostium. Also a urethra has such expansions: all prostatic part; expansion in bulb of penis; scaphoid fossa in head of penis.

 

CONCEPT CHECK QUESTIONS

1.     On a warm day, would the cremaster muscle be contracted or relaxed? Why?

2.     What will occur if the arteries within the penis dilate?

3.     What effect would low levels of FSH have on sperm production?