Male Genital Tract

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The male genital tract consists of the testes with epididymides, vas deferentia, seminal vesicles, prostate, and penis.

Testes

Each testis consists of a body and an epididymis. The testis is covered by a thin but tough connective tissue layer known as the tunica albuginea. This is covered by a mesothelium. There is another mesothelial layer lining the surrounding scrotum that is composed of loose connective tissue. Between the mesothelial layers is a potential space in which fluid may collect in pathologic conditions.

The body of the testis consists of seminiferous tubules. In the fetal testis or prepubertal testis, the tubules are small and filled with primitive germ cells and surrounded by prominent Leydig cells. In the adult testis, the seminiferous tubules are larger and coiled. The tubules are surrounded by peritubular connective tissue. Between the tubules is a loose stroma in which small nodules of Leydig cells (which make androgenic hormones, mainly testosterone) and loose connective tissue are found.<.P>

The seminiferous tubules have long thin sustentacular (Sertoli) cells that ordinarily are hard to see because there are many germ cells and their progeny. The spermatogonia at the outer edge of each tubule give rise to larger primary spermatogonia with large prominent nuclei containing vesicular chromatin. From these cells are derived the smaller secondary spermatogonia which quickly divide to produce the spermatozoa (spermatids). There are four cycles of 16 days each which occur in the seminiferous tubules to produce spermatozoa. Millions of spermatozoa are produced each day. If a sperm count is performed on ejaculate, there are typically 90 to 150 million sperm per milliliter. Each spermatid has a head with the nucleus containing the haploid set of chromosomes (22 autosomes and one sex chromosome) and a tail containing microtubules with a 9+2 arrangement of doublets and singlets similar to cilia elsewhere. There are mitochondria to power movement of the tail.

The seminiferous tubules uncoil into tubuli recti and coalesce toward the upper posterior portion of the testis where they drain into the rete testis lined by a simple cuboidal or columnar epithelium. From here the rete drains to the ductuli efferentes that connects to the epididymis. These portions of the testis do not occupy a large area.

The epididymis consists of a single convoluted tubule 4 to 6 meters long. In cross section, the epididymis appears as multiple channels, but these are all portions of the same tubule. The epididymis is lined by a pseudostratifed columnar epithelium with small basal cells and tall columnar cells with cilia (principal cells). The principal cells secrete glycerophosphocholine which inhibits sperm capacitation until the sperm are capacitated by secretions in oviducts. Beneath the epithelium is a thin lamina propria and a thin outer circular layer of smooth muscle.

Within the epididymis, spermatozoa complete their development in the 2 to 3 months it takes them to pass through. While in the epididymis, the fluid between the sperm is resorbed, making the sperm more compacted.

Vas Deferens

The vas deferens (ductus deferens) is a narrow tube that conducts the sperm from the epididymis to the prostatic urethra. There is an epithelial lining that is a pseudostratified columnar epithelium with tall ciliated and short basal cells similar to the epididymis, but this can change to a cuboidal epithelium distally. Beneath the epithelium is a lamina propria surrounded by both inner circular and outer longitudinal layers of smooth muscle. Contraction of these muscles provides most of the propulsive force for ejaculation.

Seminal Vesicles

The paired seminal vesicles are located beneath and behind the prostate. The ducts from the seminal vesicles join the vas deferentia to form the ejaculatory ducts. The mucosa is extensively folded to produce a maze-like appearance that seems multi-channeled on cross section. The columnar to cuboidal mucosa is supported by lamina propria and surrounded by inner circular and outer longitudinal smooth muscle layers. The epithelial cells may contain a small amount of light brownish-yellow cytoplasmic pigment that imparts a slightly yellowish color to the secretions. The viscous fluid secreted by the seminal vesicles forms the majority of the ejaculate, about 70%. This seminal fluid has a high fructose content to nourish the sperm carried within it. The ejaculatory ducts lead to the prostatic urethra.

Prostate

The prostate is a spherical organ located below the bladder, through which the urethra runs. The prostate is composed of complex branching glands in a fibromuscular stroma. The glands are lined by tall columnar epithelium that is folded. Prostatic secretions include proteolytic enzymes that aid in liquefaction of the ejaculate to release the sperm deposited in the upper vagina during intercourse. With aging, prostatic secretions tend to form glycoprotein and calcium concretions in the gland lumina known as corpora amylacea.

Bulbourethral (Cowper's) Glands

The paired Cowper's (bulbourethral) glands are only about 5 to 7 mm in size and produce a secretion that may lubricate the ejaculate. These organs have a tubuloalveolar structure with a lining of cuboidal to columnar mucinous epithelium. They are located below the prostate and connect to the urethra just beyond the ejaculatory ducts. No pathologic process is known to originate in them.

External Male Genitalia

The external genitalia of the male consist of the penis, which has the distal penile urethra surrounded by erectile tissue. The penis is covered by keratined stratified squamous epithelium. The tip of the penis has a central opening for the urethra known as the urethral meatus surrounded by skin known as the glans penis that is rich in nerve endings and is very sensitive to tactile stimulation. The penile urethra is lined by stratified squamous epithelium in the distal portion contiguous with skin, while the upper portion may be transitional merging to membranous urethra, and the major portion is pseudostratified columnar. There is a foreskin of loose skin and connective tissue that covers the glans (this is what is removed with circumcision).

Beneath the outer skin of the penis, there is dermal connective tissue with vessels. Deep to this is connective tissue and a thin layer of smooth muscle bundles known as the dartos tunic. The central portion of the penis is organized into three ovoid zones of erectile tissue: a corpus cavernosum is present on each side laterally, and there is a ventral corpus spongiosum through which the penile urethra extends. The corpora cavernosa and the corpus spongiosum are bounded by connective tissue and are composed of connective tissue trabeculae and smooth muscle bundles surrounding cavernous veins (lacunae). Arteries run directly into these lacunae, but blood normally drains out via arteriovenous anastomoses that bypass the lacunae.

The physiologic mechanism of erection of the penis involves nerve impulses that lead to release of nitric oxide (NO) in the corpus cavernosum during sexual stimulation. NO then activates the enzyme guanylate cyclase, which results in increased levels of cyclic guanosine monophosphate (cGMP), producing smooth muscle relaxation in the corpus cavernosum and allowing inflow of blood. Phosphodiesterase type 5 (PDE5) is responsible for degradation of cGMP in the corpus cavernosum. With an erection, there is constriction of arteriovenous anastomoses that ordinarily shunt blood past the lacunae. Now blood fills the lacunae, expanding the erectile tissue. This expansion compresses superficial veins, aiding expansion. The result is a rigid, erect penis, which averages 12.5 to 18 cm in length, with a mean of 15 cm. Mythology imparts greater size, probably aided by a darkened room or an elevated blood ethanol.

Male Breast

The male breast holds about as much interest as watching water evaporate. Unlike the female breast, there is no hormonal millieu to drive development, so only small ducts in a fibrous stroma are seen. No lobules and no secretions are present. The condition called gynecomastia (abnormal male breast enlargement) can occur, with proliferation of the ductular elements.