Ronald A. Bergman, Ph.D., Adel K. Afifi, M.D., Paul M. Heidger,
Peer Review Status: Externally Peer Reviewed
The male reproductive organs include the primary sex glands, the testes, the various excretory ducts, the accessory glands, and the penis.
The testes, contained within serous cavities within the scrotum, are compound tubular glands serving both exocrine and endocrine functions. The exocrine function is the formation of the mature male germ cells (the spermatozoa), and the endocrine function is the production of the male sex hormone, testosterone. The testes are ovoid glands approximately 4.5 cm in length and are covered by a thick capsule (tunica albuginea) composed primarily of collagenous connective tissue with some elastic fibers. The tunica albuginea thickens posteriorly to form the mediastinum testis, which is an area where ducts, blood vessels, and nerves leave or enter. From the mediastinum, thin, incomplete, and branching fibrous septa and associated blood vessels radiate into the testis and divide it into about 250 lobules. These incomplete pyramidal spaces contain the seminiferous tubules surrounded by a stroma rich in vascular and cellular elements. Of particular interest in the stroma are the interstitial cells (of Leydig), which secrete the male sex hormone, testosterone. These cells lie within the intertubular or interstitial space, in close proximity to vascular and lymph vascular channels.
The lymphatic endothelium, fibroblasts, and, in many mammalian species, several layers of epithelioid cells possessing many of the fine structural features of smooth muscle ensheath the seminiferous tubules and constitute their tunica propria (boundary) tissue. Apical to the basement membrane lies the seminiferous epithelium, which consists of cells of the germ line that will by means of meiotic divisions and a complex process of cytodifferentiation give rise to the haploid male gamete, the spermatozoon. The production of sperm by the epithelium proceeds in an orderly, cyclic fashion from the base of the epithelium' to its apex, as well as in a coordinated, wave-like manner along the length of the tubule. The mitotic and meiotic divisions in the more basal aspects of the epithelium displace the developing sex cells progressively farther away from the basement membrane, with the more mature sex cells occupying, therefore, the most apical (luminal) position.
Cytoplasmic processes of a second population of cells, termed Sertoli cells (also sustentacular or nurse cells), extend from the basement membrane to the most apical portion of the seminiferous epithelium. Sertoli cells are somatic cells that establish an intimate morphological and physiological relationship with each of the developing germ cells, from the earliest, most primitive gonial cell at the basal lamina, to the apically situated mature spermatids ready to be shed into the tubular lumen. Because of the cyclic nature of the process of spermatogenesis occurring in all seminiferous tubules and the microtome knife's random entry into this cycle, the student must be reconciled to studying many sections of seminiferous tubules in order to appreciate and correctly identify the named germ cells of the epithelium and their place in the process of spermatogenesis.
As in other renewing epithelia with which the student is already familiar, the most primitive, regenerative cells lie at the basement membrane of the epithelium. In the testis, these cells are termed the spermatogonia (and are diploid). They divide mitotically and renew the gonial stem cell population. However, a subpopulation of spermatogonia is programmed to enter meiotic prophase and thus become primary spermatocytes. As daughter cells of spermatogonia, the primary spermatocytes reside just apical to the layer of spermatogonia. These cells, the largest germ cells, are recognized by their distinct meiotic nucleus, as well as by their position near the spermatogonia at the base of the epithelium. Primary spermatocytes undergo first meiotic division, giving rise to secondary spermatocytes; these cells occupy a place within the seminiferous epithelium just apical to the primary spermatocytes. The secondaries are short-lived (and thus not frequently observed within sections of the germinal epithelium); secondaries complete second meiotic division and give rise to round spermatids, which occupy an adluminal site within the epithelium. A complex process of cytodifferentiation ensues, whereby these round cells are transformed into elongate spermatids and, finally, spermatozoa.
There exists no "free space" within the seminiferous epithelium, all interstices being filled with Sertoli cell cytoplasm. Because of the attenuated nature of the Sertoli cell processes within the seminiferous epithelium, it is not possible to distinguish cell-to-cell boundaries between Sertoli cells. For this reason, the Sertoli cells were considered prior to the advent of electron microscopy to be syncytial. Indeed, the occluding tight junctions between Sertoli cells are now recognized as the morphological basis for the "blood-testis" (blood epithelial) barrier. Because of the placement of the tight junctions just apical to the gonial stem cells, the seminiferous epithelium is segregated into a basal compartment, open to the interstitium, and an apical compartment containing the meiotic cell line (considered non-self by the body's immune surveillance system). Thus, the haploid spermatids develop within an immunologically privileged compartment maintained by the Sertoli-Sertoli cell occluding junctions.
Within the seminiferous tubules (also termed tubuli contorti in recognition of the convoluted course that they describe with in lobules), sperm are transported along their lengths by means of a remarkably high fluid flux across the epithelium. Upon reaching the epididymis, the spermatozoa undergo additional physiological maturation (capacitation) and gain fertilizing capacity and motility; there, also, the testicular fluid is resorbed.
Leading from the seminiferous tubules are the tubuli recti or straight tubules lined only with columnar cells apparently derived from Sertoli cells. The straight tubules open into the rete testis, a network of irregular, anastomosing channels lined with a simple cuboidal or columnar epithelium, which may possess a single flagellum. The efferent ductules (10 to 15 in man) emerge from the rete testis and join to form a portion of the head of the epididymis. Histologically, each efferent duct presents a stellate luminal profile, reflecting an epithelium in which ciliated columnar cells alternate with non-ciliated cuboidal cells. The epididymal duct in man is a highly coiled tube about 5 m in length. It is lined with a pseudostratified columnar epithelium with tall columnar cells and rounded basal cells. The columnar cells possess long non-motile stereocilia. The ductus deferens is continuous with the epididymis. The lumen of the ductus deferens increases in size, and the wall thickens as it extends distally. Near the prostate gland, the ductus is enlarged to form the ampulla; immediately thereafter, it is joined by the duct from the seminal vesicle. The duct formed by this confluens, the ejaculatory duct, courses through the substance of the prostate gland and empties into the prostatic urethra. It is lined with a pseudostratified or simple columnar epithelium.
Three major accessory sex glands are associated with the male reproductive system: the seminal vesicles, the prostate gland, and the bulbourethral (Cowper's) glands. The seminal vesicles develop as outgrowths of the ductus deferens. Each is a glandular sac honey-combed by thin branching folds of the mucosa lined with a pseudostratified columnar or cuboidal epithelium. The prostate gland encircles the urethra adjacent to the neck of the bladder and is formed of 30 to 50 tubuloalveolar glands grouped into lobes. The glandular epithelium consists of simple cuboidal or columnar cells. Prostatic concretions (corpora amylacea) are prominent constituents of the alveoli. The bulbourethral glands are compound tubuloalveolar glands that secrete a clear, viscous mucoid product. The secretory epithelium is cuboidal to columnar. The ducts of the gland enter the cavernous urethra. The secretions of each of these glands are important physiological components of semen.
The penis serves as an outlet for urine and semen and as a copulatory organ. The penis is made up of three cylinders of erectile tissue. These include the corpus spongiosum, which enlarges distally into the glans penis; parallel with, and dorsal to, the corpus spongiosum are the paired corpora cavernosa, which extend distally to the glans. The corpora cavernosa are united distally by a median partition, the pectiniform septum. All three structures are surrounded by a thick fibrous tunica albuginea and a subcutaneous connective tissue layer covered by a thin skin.
Some names associated with the male reproductive system follow: Cowper was a seventeenthcentury English anatomist, von Leydig, a nineteenth-century German anatomist, and Sertoli, a nineteenthcentury Italian histologist.
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