Gross anatomies of representative species are illustrated in Figures 3-1 (human), 3-2 (sheep), and 3-3 (mouse).
Testis. The testes of most mammals are suspended into a scrotum through adjacent inguinal canals (Figure 3-1). The spermatic cord is the link between the testis and abdominal cavity. Each spermatic cord is circumscribed by muscle and contains blood vessels, nerves, and a duct (vas deferens). The tunica vaginalis and albuginea form the dense connective tissue coverings of the testis. The innermost tunica albuginea gives way to septa that segment the testis into lobules.
The testis is divided into two tissue compartments - seminiferous and interstitial (Figure 3-4). The seminiferous compartment is tubular; it houses Sertoli (sustentacular) cells, which constitute the epithelial lining of the tube, and developing sperm cells (Figure 3-5). Sertoli cells are versatile - they closely interact with sperm cells and are secretory (Table 3-1). The highly convoluted nature of the tubular compartment bolsters a large surface-to-volume area for spermatogenesis (one tube is 1-3 feet long; the total length of all tubes is several hundred yards). The interstitial (connective tissue) compartment is dispersed between and supports the seminiferous tubules. Steroidogenic Leydig cells are located within the interstitium. Blood/lymph vessels and nerves terminate within the interstitial compartment; they do not penetrate the basement membrane of seminiferous tubules. Seminiferous tubules are surrounded by a thin layer of muscular (myoid) cells (Figure 3-6).
The mature sperm cell. The head and tail are the basic morphological components of a developed sperm cell (Figure 3-7). The sperm head contains condensed chromatin (genetic material) and is covered on its anterior surface by the acrosomal cap. There are species variations in the shape of the head and length of the tail of sperm (eg., spermatozoa of rodents have a sickle-shaped head and are quite long [rat @ 0.2 mm, Figure 3-8; human @ 0.06 mm]). Lytic enzymes are released from the acrosome when the ovum is penetrated by sperm cells.
The sperm tail is designed for motility; it is subdivided into the neck (connecting piece), middle piece, annulus, principal piece, and end piece (Figure 3-7). Contractile machinery of the sperm cell extends longitudinally within the core (axoneme) from the neck distally. Internal structures of sperm cell tails and of flagella and cilia of plant and animal cells are similar. Microtubules are oriented as nine radial doublets surrounding a central tubular pair. Nine dense filaments are associated with each outer pair of tubules ("9 + 9 + 2" arrangement). Cytoskeletal components are functionally linked by proteins. A sliding motion between adjacent microtubules provides the bending force within the tail necessary for cellular motility. A mitochondrial sheath surrounds the axoneme along the middle piece. Mitochondria generate the energy required for motility. A stabilizing fibrous sheath surrounds the axoneme of the principal piece. The end piece is covered only by plasma membrane and is void of dense fibers (Figures 3-9 and 3-10).
Duct system. The excurrent duct system of the male transports sperm cells from the testes. Convergence of ducts from each testis is progressive. Hundreds of seminiferous tubules give rise to the tubuli (canaliculi) recti. Tubuli recti empty into a testicular collecting network, the rete testis. Sperm cells are next moved into the head of the epididymis (caput) via the efferent ducts (vasa efferentia). In addition to its role in transport and storage of sperm, the epididymis participates in sperm cell maturation and in resorption of dead and abnormal cells. At ejaculation, motile sperm cells are propelled from the cauda (tail) of the epididymides (the walls are enriched with nerve endings and muscular cells) onward through the vas (ductus) deferens (Figure 3-11) and into a common pelvic and penile urethra (Figure 3-1).
Ducts are surrounded by an inner mucosal layer of epithelium supported by a matrix of submucosal connective tissue (lamina propria), intermediate muscle (muscularis), and epithelial covering (serosa) (Figure 3-12). Continual passage of spermatozoa through the system of male ducts is executed by rhythmic basal contractions of the ductal muscularis. Cilia of epithelial cells (Figure 3-13) help to keep sperm cells moving along. Mucosal fluid secretions function as a lubricant. Many unejaculated spermatozoa are voided in the urine.
Surgical sterilization in the human male involves ligation and removal of a small section from each vas deferens (vasectomy); this disrupts the pathways by which spermatozoa are ejaculated. Ducts are isolated individually just beyond the tail of the epididymis - before they make their way into the body cavity. Vasectomy can be carried out using a local anesthetic in an outpatient facility. An uncomplicated procedure does not interfere with testicular production of testosterone (an interstitial attribute) or sexual drive. An alternative method which does not require cutting/suturing or cautery, the Vasclip, is now (FDA approval 2003) available to men.
Penis. The mammalian penis contains cavernous sinuses (corpus spongiosum and cavernosum); these structures are especially prominent in man (Figure 3-14) and stallions. During sexual arousal the corpora cavernosa are filled with blood (arteriole dilation) causing erection; autonomic nervous inputs promote release of nitric oxide, which increases cGMP, causing smooth muscle relaxation (Viagra [sildenafil citrate], Levitra [vardenafil], and Cialis [tadalafil], drugs used to treat erectile dysfunction in men, are inhibitors cGMP phosphodiesterase). Corpora cavernosa can become partially ossified (os penis or baculum) in many carnivores, rodents, nonhuman primates, and bats. Blood usually drains from the penis soon after ejaculation (sustained vascular engorgement after intromission [eg., in dogs] can result in "locking" of the mating pair).
The penis of ruminants, swine, and cetaceans is fibroelastic and moderately rigid when not erect - erection in these species is the result primarily of straightening of the sigmoid flexure (Figure 3-2). The sigmoid flexure retracts (autonomic muscular control) following ejaculation.
(Urethral calculi are common in feedlot steers and wethers on high mineral diets; they usually become trapped in the sigmoid flexure and can obstruct the flow of urine leading to rupture of the bladder, a ureter, or urethra - water belly. Blockade of the urethra can also occur in castrate dogs and cats in which the penis remains in an infantile state.)
The prepuce is the foreskin that covers the glans (head) of the penis (part of this skin is removed during circumcision). Penile spines extend from the epithelial folds of the glans in some species, such as the cat and rat.
Accessory glands. Male accessory organs include the prostate gland, seminal vesicles, and bulbourethral (Cowper's) glands. Highly branched tubular matrices are embedded in interstitial tissue containing smooth muscle (Figure 3-15).
Structural integrity and function of male accessory organs is dependent upon testicular androgens (Figure 3-16). Accessory epithelial cells die when deprived of testosterone, a process called programmed physiological cell death or apoptosis (from the ancient Greek for "falling off of tree leaves or flower petals"); this involves DNA fragmentation by endogenous nucleases.
Accessory glands produce the bulk of fluid (seminal plasma) in which ejaculated spermatozoa are suspended. Semen is a composite of sperm and seminal plasma (Table 3-2). Volume and biochemical composition of seminal plasma contributed by any one accessory organ can vary widely between species. Seminal plasma has a pH of 7.6.
Spermatic fluid derived from the seminiferous epithelium and epididymides (containing spermatozoa) moves to the ejaculatory ducts during emission. Accessory fluids are added to the spermatic mixture within the pelvic urethra (Figure 3-17). Fluid movements are caused by strong contractions of the musculature of the duct system and accessory glands induced by impulses propagated by the autonomic nervous system (penile receptors sense temperature and pressure). The neck of the bladder normally closes at ejaculation (however, some reflux of semen into the bladder usually occurs). (Rhythmic contractions of the striated musculature of the bulbocavernosum [located at the base of the penis/clitoris] and perineum [area between the scrotum/vulva and anus] manifest the sensation of orgasm.)
In man and boars the ejaculate can be split into distinct fractions. Contents of the bulbourethral glands are released first and clear the urethra, followed by contents of the epididymides, vas deferens and prostate gland, and finally by vesicular fluid. The presence of blood cells in seminal plasma (hemospermia) may be indicative of a pathology involving the accessory glands.
Seminal plasma is unique in several respects. High levels of fructose in seminal plasma provide a readily-available source of energy to sperm. Seminal plasma also has significant buffering (bicarbonate) capacity; this protects spermatozoa from the detrimental effects of the acidic environment of the vagina. Chemicals that inhibit acrosomal enzyme release (decapacitation factors) and enhance motility of sperm cells and contractility of the female reproductive tract (eg., relaxin and prostaglandins) are present in seminal plasma. Nonetheless, insemination of epididymal spermatozoa can produce a pregnancy.
The prostate gland is common to all mammals; it is the only male accessory organ of cetaceans and some carnivores. In man and dogs the prostate gland is prone to disease. Disorders of the prostate gland rise steadily with age. Accumulation of prostatic concretions (Figure 3-18) is least threatening; these lamellated deposits are composed of condensed, sometimes calcified, secretory materials (larger concretions can become lodged in the ductal system of the gland). Approximately one-third of American males over the age of sixty have benign prostatic hyperplasia (BPH) or evidence of latent malignancy. Prostatic carcinoma is the second leading cause of death from cancer (behind lung) among men. Clinical incidence of prostatic disease is proportionally higher among Americans than in most other societies; blacks are more susceptible than whites (of additional concern are recent reports that carcinoma is more prevalent in vasectomized men). Benign hyperplasia generally affects the central portion of the prostate gland; carcinoma usually involves the peripheral zone (Figure 3-19). Growth of the prostatic tumor causes obstruction of the urethra and difficult urination. Levels of plasma acid phosphatase (originating from the prostate gland) and a prostate-specific antigen (PSA) are elevated in diseased patients. Castration causes tumor regression (and is still used in the management of malignancy). Surgical removal of part or all the prostate gland is more conventional, particularly in cases of BPH. Transurethral prostatectomy can result in neural damage to the neck of the bladder (and therefore patients often experience retrograde ejaculation, but generally do remain continent). Inhibition of the enzymatic conversion of testosterone to DHT with Proscar (finasteride) is helpful in suppressing prostatic overgrowth in some individuals.
Other accessory tissues of lesser notoriety are the ampullae, coagulating, urethral, and preputial glands. The ampullae are specialized thickened areas of the vas deferens near their junctions with the urethra (Figure 3-2). Secretory products (a transglutaminase, which catalyzes protein cross-linkages) of the coagulating gland of some species (Figure 3-3) are responsible for formation of a copulatory vaginal plug - preventing backflow of deposited semen. Urethral glands are disseminate prostate-like tissues encircling the pelvic urethra. Preputial glands are sebaceous glands that secrete pheromones (bioactive substances released into the external environment) that attract sexual partners and mark breeding territories. Preputial pheromones are transmitted in the urine. The active ingredient of the preputial "boar odor" is a steroid (5a-androst-16-ene-3-one).