FUNCTIONAL ANATOMY

The Male Reproductive System Consists of Many Individual Organs Acting in Concert to Produce Spermatozoa and Deliver Them to the Female's Reproductive Tract

The male reproductive system is made up of a number of individual organs acting in concert to produce spermatozoa and deliver them to the reproductive tract of the female.

The scrotum, along with the cremaster muscles and the vascular anatomy of the testicular arteries and veins, protects and thermoregulates the testes. Present in all domestic animals, the scrotum essentially is a skin pouch with a subcutaneous fibroelastic and muscular layer called the tunica dartos. The vascular arrangement of the testicular artery surrounded by the plexus of testicular veins (pampiniform plexus) provides a countercurrent heat exchange mechanism that is vital to testicular thermoregulation. Contraction and relaxation of the tunica dartos and cremaster muscles occur with changes in ambient temperature as well as in response to other tactile stimuli. The scrotum of some species, such as the horse, contains numerous sweat and sebaceous glands that further contribute to the thermoregulatory mechanism.

The testis is the pivotal organ of the male reproductive system.

Functionally, the testis is considered to have three compart­ments. The interstitial tissue compartment, containing the Leydig cells, surrounds the seminiferous tubules and bathes them with testosterone-rich fluid. The other two compartments reside within the seminiferous tubules. The basal compartment con­tains spermatogonia, which divide through mitosis, whereas the adluminal compartment represents a special environment where spermatocytes undergo meiosis and continue their meiotic divisions to differentiate into spermatids and finally into spermatozoa. Within the seminiferous tubules, the Sertoli cells, which provide support and nourishment to the developing germ cells, extend from the basal compartment into the adlum- inal compartment. Tight-junctional complexes between the Ser­toli cells separate the basal and adluminal compartments and form the major component of the blood-testis barrier, which functionally prevents many compounds found in the blood and interstitial fluid from entering the adluminal compartment.

In domestic mammals, normal testicular function, espe­cially normal spermatogenesis, is temperature dependent and requires an environment that is lower than core body tem­perature. Therefore, in normal domestic males, the testes are located outside the abdominal cavity, in the scrotum. Failure of one or both of the testes to descend into the scrotum is known as cryptorchidism. Although the cryptorchid testis is still capable of producing androgens, it is incapable of pro­ducing normal spermatozoa. Consequently, a bilaterally crypt­orchid male would be sterile. The cryptorchid testis is more prone to torsion of the spermatic cord and 10 times more

Table 40-1

Male Reproductive Parameters

likely to be neoplastic. Cryptorchidism appears to be genetic, although the exact mechanism is not completely understood and may vary among species.

■ Horse: 9 to 11 months of gestation

■ Cattle: 3.5 to 4 months of gestation

■ Sheep: 80 days of gestation

■ Pig: 90 days of gestation

■ Dog: 5 days after birth

■ Cat: 2 to 5 days after birth

■ Llama: usually present at birth

For the majority of domestic species, passage of the testes through the internal rings by 2 weeks after birth is necessary for a final scrotal position to occur. Many animals may have testes in the inguinal region at birth, and the testes may remain there for weeks or months before descending into the scrotum. In the dog, testicular descent is uncommon after 14 weeks of age and does not occur after 6 months of age. In the stallion, although it is considered abnormal, descent of inguinally retained testes has been known to occur as late as 2 to 3 years of age.

The seminiferous tubules empty their contents into the rete testis, which subsequently transports the spermatozoa and seminiferous tubular fluid into the epididymis. The epi­didymis is a single tortuous duct of considerable length (from 2 m in the cat to 80 m in the stallion) that is anatomically divided into three segments: head, or caput; body, or corpus; and tail, or cauda. The epididymis not only is a conduit for spermatozoa but also provides a special environment in which spermatozoa are concentrated, undergo maturation, and acquire fertilizing capacity. Spermatozoa that enter the caput from the rete testis are immotile and incapable of fertilization. Only after they undergo migration and maturation through the caput and corpus are both motility and the capacity for fertilization achieved. The cauda epididymis and the deferent duct, into which the cauda empties, serve as a storage depot for mature spermatozoa; together, these are known as the extragonadal sperm reserves.

The deferent ducts, or vasa deferentia, pass through the inguinal rings into the abdomen and connect the cauda epi­didymis with the pelvic urethra. In most species the terminal portion of the deferent ducts enlarges to form prominent ampullae such as those found in the bull and the stallion. In other species the ampullae either are absent or are anatomic­ally indistinct from the vasa deferentia. The ampullae serve as an additional storage depot for spermatozoa, and in some species, such as the bull, stallion, and dog, ampullary glands add to the ejaculate. Along with spermatozoa, ejaculated semen is composed primarily of accessory gland secretions that add volume, nutrients, buffers, and a number of other sub­stances whose exact functions are unknown. The contribution to the ejaculate by each of the accessory glands varies with the species and is responsible for the variation in concentration, volume, and character between ejaculates. The seminal vesicles lie lateral to the ampullae near the neck of the bladder. In the bull, ram, and buck, these organs are firm and IobuIated with a narrow lumen, whereas in the stallion and boar, they are more saclike. The dog and tom lack seminal vesicles but have relatively prominent prostate glands, especially the dog. The prostate gland is present in all domestic males and is intimately associated with the pelvic urethra, but it varies in size and appearance among species. The bulbourethral glands of the tom are almost as large as the prostate, but these glands are absent in the dog.

The Copuiatory organ of the male is the penis. It is more or less cylindrical in all species and extends from the ischial arch to near the umbilicus on the ventral abdominal wall, except in the tom and the llama, in which the penis points posteriorly in the relaxed state. The body of the penis is surrounded by a thick fibrous capsule (the tunica albuginea) that encloses numerous cavernous spaces (the corpus cavernosum penis) as well as the corpus spongiosum penis, which immediately sur­rounds the urethra. Erection is a psychosomatic event that involves mutually occurring actions of the vascular, neuro­logical, and endocrine systems. Contraction of the ischio­cavernosus muscle during erection results in occlusion of venous outflow. At the same time, the parasympathetically mediated relaxation of corpus cavernosum and corpus spongio­sum results in these cavernous spaces becoming engorged with blood, and the penis becomes elongated and turgid.