The male reproductive system consists of the testis which are the sites of sperm generation and the male reproductive tract which deliver sperm from the testis to the outside world. Along the reproductive tract, several secretory organs deliver nutrients and other factors that facilitate survival of sperm.
The testes are a source of gametes and steroid sex hormones. Each testis is a compound tubular gland contained within a thick connective tissue coat called the tunica albuginea that also contains myofibroblasts that rhythmically contract. The gland is divided into lobules by thin septa that radiate from the dorsal portion of the tunica albuginea. Each lobule contains between one and four seminiferous tubules that are the site of sperm production.
The development of a mature sperm cell takes place in two steps, spermatogenesis and spermiogenesis. Spermatogenesis is the process by which an undifferentiated spermatogonium develops into a spermatid. During spermatogenesis, the number of chromosomes is halved through meiosis. It involves the following stages.
Spermatogenesis occurs within the germinal epithelium of seminiferous tubules. The epithelia is divided into two compartments: basal and adluminal. The basal compartment is closest to the basement membrane whereas the adluminal compartment is closer to the lumen of the seminiferous tubule. The border between the two compartments is defined by Sertoli cells (see below).
Spermatogonia, the stem cell of spermatogenesis, rest on the basement membrane of the seminiferous tubule and divide mitotically to produce more spermatogonia and primary spermatocytes. Primary spermatocytes represent the first differentiation step along the spermatogenesis pathway. Spermatogonia remain in the basal compartment while spermatocytes migrate away from the basement membrane and cross into the adluminal compartment toward the lumen of the seminiferous tubule.
Primary spermatocytes enter meiosis and have a prolonged prophase that facilitates the exchange of genetic material between homologous chromosomes. The first meiotic division gives rise to secondary spermatocytes that have 23 pairs of chromatids. This stage is short-lived and consequently, secondary spermatocytes are rarely seen in histological images. This stage ends with the second meiotic division.
The second meiotic division produces spermatids that are haploid. Despite being the product of meiosis, spermatids remain connected to one another by cytoplasmic bridges. These bridges result from incomplete cytokinesis and allow exchange of material for synchronous maturation.
Spermiogenesis is the process by which a spermatid matures into a spermatozoan. This process involves the following cellular changes to spermatids.
Sertoli cells are located in the germinal epithelium and play a supportive role in the development of spermatozoa. These cells have abundant cytoplasm and extend from the basement membrane to the lumen of seminiferous tubules. Sertoli cells have a characteristic oval nucleus with a dark nucleolus.
Sertoli cells facilitate spermatogenesis by providing structural and chemical support to the developing spermatogonia, spermatocytes and spermatids. In addition, Sertoli cells synthesize androgen-binding protein that keeps testosterone levels high within the seminiferous tubules. Follicle-stimulating hormone produced in the anterior pituitary stimulates production of androgen-binding protein. Sertoli cells also produce inhibin that decreases production of follicle-stimulating hormone in the anterior pituitary.
This electron micrograph allows the viewer to better appreciate the microscopic structure of the Sertoli cell and blood-testis barrier. The basement membrane on which all Sertoli cells rest is visible, as is a narrow myofibroblast which contracts rhythmically. The Sertoli cells are connected to one another via junctional complexes (both tight and adhesion junctions) close to the basement membrane; these complexes divide the germinal epithelium into basal and adluminal comparments. The basal compartment contains diploid spermatogonia that rest upon the basement membrane. These cells develop by migrating into the adluminal compartment, which contains primary spermatocytes, spermatids, and spermatozoa.
The rete testis connects the seminiferous tubules to the ductus efferentes and the rest of the male gentical tract. It is lined by ciliated cuboidal epithelial cells that also contain microvilli. The activity of the cilia helps to move the spermatozoa along the tube, as they are immobile until they reach the epididymis. The microvilli absorb excess materials, including protein and potassium, from the seminal fluid.
Interstitial or Leydig cells are located in the connective tissue surrounding the seminiferous tubules. They produce testosterone, the male sex hormone responsible for the growth and maintenance of the cells of the germinal epithelium and the development of secondary sex characteristics. Leydig cells often display cytoplasmic crystals of Reinke; the function of these crystals is unknown.
The male reproductive tract is a long tube that brings the spermatozoa from the testes to the outside of the body. The tract comprises several segments with different structures and functions. As discussed previously, spermatozoa are produced in the seminiferous tubules in the testis. Spermatozoa flow from the seminiferous tubules into the rete testis and then the ductili efferentes. The ductili efferentes merge to form the epididymis which is the site where spermatozoa gain motility. The epididymis transitions into the ductus deferens which receives secretions from the seminal gland and prostate. The ductus deferens from each side merge with the urethra.
The ductuli efferentes emerge from the dorso-superior margin of each testis. They originate from the rete testis and gradually fuse to form the ductus epididymis. The epithelium has a characteristic scalloped appearance that results from a lining that contains both cuboidal and columnar epithelial cells. A layer of smooth muscle surrounds the walls. The non-ciliated cells reabsorb testicular fluid, while the ciliated cells propel the immobile sperm to the epididymis, where they gain the ability to swim.
The epididymis is a muscular, convoluted tubule that stores spermatozoa and is the site at which they acquire their motility. It is lined by a pseudostratified epithelium whose cells contain non-motile stereocilia. These stereocilia absorb much of the excess fluid containing the spermatozoa. The epithelium of the epididymis also contains mitotic basal cells. In this section, the spermatozoa can be seen in the lumen throughout the epididymis.
The ductus deferens is another muscular tubule that carries sperm downstream from the epididymis. Its wall is thicker than that of the epididymis and contains three muscular layers: inner longitudinal, middle circular, and outer longitudinal. The epithelium of the ductus deferens is similar to that of the epididymis, with pseudostratified cells bearing stereocilia.
The distal portion of the ductus deferens is called the ampulla and receives secretions from the seminal vesicles. The duct is now referred to as the ejaculatory duct and ducts from each side will merge and join the urethra as it runs through prostate gland.
The urethra is lined primarily by stratified or psueodstratified columnar epithelial cells, but its opening displays a stratified squamous epithelium. Erectile tissue surrounds the urethra and contains numerous blood vessels. During an erection, the arteries dilate to fill the sinuses, which obstruct venous outflow and traps blood in the penis.
Seminal vesicles are glandular sacs that produce a secretion that composes 80% of the seminal fluid and contains fructose, fibrinogen, and prostaglandins. The secretion empties via a short duct into the ampulla of the ductus deferens. The seminal vesicles appear as honeycombed saccules with thin, highly branched folds of mucosa, lined by a pseudostratified columnar epithelium. Observe the coat of smooth muscle surrounding the saccular dilation of the gland. Its contraction expels the accumulated secretion during ejaculation.
The prostate is a walnut-sized conglomeration of tubulo-acinar glands that surrounds the initial segment of the urethra. This gland produces a secretory product containing citric acid and proteolytic enzymes that prevent coagulation of semen. The epithelium that lines the glands is usually columnar with numerous flattened basal cells also visible. The lumen of the glands often contain prostatic concretions that accumulate over time. Their significance is unknown but they make a useful marker for identifying the prostate. The glands are surrounded by a stroma that contains connective tissue and smooth muscle.