Learning Objectives

Overview of the Male Reproductive System The male reproductive system, like the internet, can be thought of as a series of tubes. These tubes deliver the male gametes from their site of production in the testes to their destination outside the body. The system itself is divided into two distinct units: The testes , located outside the major body cavity and housed in the scrotum. The excretory duct system , which transports the sperm from the testes and whose accessory glands produce and modify the contents of semen .

Testes Like the ovaries, the testes are a source of gametes and steroid sex hormones. Each testis is a compound tubular gland contained within the tunica albuginea , a thick connective tissue coat that contains myofibroblasts that rhythmically contract, and the tunica vascularis , a highly vascular region. The gland is divided into lobules by thin septa that radiate outwards from the dorsal portion of the tunica albuginea. Each of these lobules contains between one and four seminiferous tubules , which are the site of sperm production. The testes contain several important cell types that are important to understand. The first two of these are found in the germinal epithelium , the layer of the seminiferous tubule in which gamete production and development occurs: Sertoli cells extend from the basement membrane of the germinal epithelium to the lumen of the tubule. These cells envelope the developing sperm cells. They are joined to one another by junctional complexes and form the blood-testis barrier . This barrier separates the germinal epithelium into a basal compartment , containing Sertoli cells and diploid germ cell precursors, and an adluminal compartment , which contains the products of meiosis. Spermatogenic cells include each of the stages between the spermatogonium and the mature spermatozoan. These cells appear in layers that represent advancing stages in their differentiation, with the least mature cells near the basal layer of the epithelium and the most mature cells near the luminal layer. Leydig (interstitial) cells are located within the loose connective tissue surrounding the seminiferous tubules. They appear pale due to their high cholesterol content and often contain crystals of Reinke , which have an unknown function. Leydig cells produce testosterone in response to leutinizing hormone released by the pituitary gland. Testosterone is responsible for male secondary sexual characteristics and the maintenance of the germinal epithelium.

Spermatogenesis The development of a mature sperm cell takes place in two steps, spermatogenesis and spermiogenesis. Spermatogenesis is the process by which a spermatogonium develops into a spermatid. During this process, the number of chromosomes is halved through meiosis. It involves the following stages: Spermatogonia rest on the basement membrane of the seminiferous tubule and divide mitotically to produce more spermatogonia and primary spermatocytes. The spermatogonia remain in the basal compartment while the spermatocytes are located in the adluminal compartment. Primary spermatocytes are located in the middle region of the seminiferous tubule, within the adluminal compartment. These cells have a prolonged prophase that gives rise to the first meiotic division. Secondary spermatocytes are the product of the first meiotic division. They have 23 pairs of chromatids. This stage is short-lived and ends with the second meiotic division. Spermatids are the haploid products of meiosis. They remain connected to one another by cytoplasmic bridges. These bridges result from incomplete cytokinesis and allow for synchronous maturation. Spermiogenesis is the process by which a spermatid matures into a spermatozoan . This process involves several steps: The acrosome , containing hydrolytic enzymes, develops and comes to overlie the dense, elongated nucleus. A flagellum grows out of the pole opposite the acrosome, facing the tubular lumen. This flagellum is a modified cilium that develops from the centrioles of the spermatid. Mitochondria become arranged around the flagellum. The bulk of the cytoplasm is cast off as a residual body , leaving only a thin rim of cytoplasm around the nucleus. Sertoli cells consume the residual body. The final maturation of sperm occurs in the epididymis, where the cells gain the ability to move. They gain the ability to fertilize the egg through the process of capacitation , which occurs in the female reproductive tract.

Testicular Duct System Each seiminiferous tubule has a short terminal segment lined by Sertoli cells that transitions to the tubulus rectus and rete testis . These tubes are lined by cuboidal epithelial cells that bear cilia. They turn into the ductus efferentes , which emerge from the dorsal portion of each testis and fuse to form the ductus epididymis . The epithelium of this tube is lined by alternating columnar and cuboidal cells, which give it a scalloped appearance.

Extra-Testicular Duct System These extra-testicular duct system brings the spermatozoa from the testes to the outside of the body. This system is composed of several distinct tubes: 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 . 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 ampulla of the ductus deferens receives secretions from the seminal veiscles. The ejaculatory duct extends from the ampulla of the epididymis to the urethra. The initial portion of the duct has the same pseudostratified columnar epithelium as the previous tubes. Near its termination, it is lined by transitional epithelium. The urethra is lined primarily by stratified or psueodstratified columnar epithelial cells, but its opening displays a stratified squamous epithelium.

Accessory Glands of the Male Reproductive System The male reproductive system contains several accessory glands that contribute to the composition of semen: The seminal vesicles are glandular sacs that empty into the ampulla of the ductus deferens. They produce a secretion that makes up over 80% of the seminal fluid and contains fructose, fibrinogen, and prostaglandins. The pseudostratified epithelial cells appear foamy because of the lipid droplets they contain, and the muscular wall of the vesicles contains inner circular and outer longitudinal layers that contract in response to sympathetic stimulation 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 prostate often contains prostatic concretions in the acinar lumina. The bulbourethral glands , or Cowper’s glands , empty into the urethra and secrete a lubricating fluid. Their secretory cells and duct contain small mucoid droplets.

Penis The penis is composed of two types of erectile tissue: The corpus cavernosa are two columns of erectile tissue located on the dorsal side of the penis. The corpus spongiosum is a single column of erectile tissue located on the ventral side of the penis. It contains the penile urethra and expands distally into the glans penis. The erectile tissue of the penis appears as a vast sponge-like system of irregular vascular spaces intercalated between the arteries and veins. These sinuses receive blood from the helicine arteries , which dilate during erection to engorge the sinuses with blood. This, in turn, restricts venous outflow. The skin covering the penis is thin and contains an abundant subcutaneous layer with smooth muscle but no adipose tissue. The glans penis lacks hair follicles and contains only small sweat glands.