THE TESTES AND THEIR ADNEXA

The Testis

The testis combines endocrine and exocrine compo­nents within a common capsule. The endocrine compo­nent functions normally at the core temperature of the body, but in most mammals the successful production of the male gametes requires a temperature a few degrees lower than that within the abdomen.

Figure 5-35 Pelvic organs of the bitch. The lateral pelvic wall and the lateral wall of the vestibule have been removed. 1, Rectum; 2, anal sac; 3, anus; 4, uterus; 5, vagina; 6, ureter; 7, bladder; 8, urethra; 9, vestibule; 10, clitoris; 11, vulva.

ability of spermatogenesis to occur normally at the core temperature in a few mammals (described as testicond, e.g., elephants, hyraxes) in which the testes remain within the abdomen throughout life. It is consistent with the periodic changes exhibited in many small mammals (chiefly found among rodents, insectivores, and bats) in which the testes descend into the scrotum for the breed­ing season, after which they return to the abdomen. This is brought about by contraction of the cremaster muscle sac found in these species.

The testes are solid ellipsoidal organs whose bulk bears no fixed proportion to the body size. Among domestic species they are conspicuously small in cats and impressively large in sheep and goats. Their orientation also varies. They are carried with their long axes vertical in ruminants (necessitating a deep and pendulous scrotum), horizontal in horses and dogs, and tilted toward the anus in pigs and cats.

The outer surface of the testis is made smooth by the direct peritoneal investment, except at the poles and along one margin where the testis is attached to the epididymis, a structure formed by the coiled initial portion of the external duct system. The peritoneum covers a thickish capsule (tunica albuginea) mainly composed of dense connective tissue but sometimes including smooth muscle. The larger branches of the testicular artery and vein run within the capsule, where they are visible in a pattern that is species characteristic. The parenchyma is contained under moderate pressure, which accounts for its pouting through any incision of the capsule. It is probable that slight swelling of the parenchyma can be accommodated by the testis assum­ing a more globular form, but any significant expansion raises the intratesticular pressure and produces pain, which may be severe when the testis is inflamed (orchi­tis). The capsule detaches septa and trabeculae that divide the parenchyma into lobules. The septa are not always conspicuous, but in those species in which they

Figure 5-36 The perineal, intermediate, and inguinal posi­tions of the scrotum exhibited by the tomcat (A), dog (B), and bull (C). 1, Testicular artery; 2, deferent duct; 3, testis; 4, pelvic symphysis.

are well developed, they may be seen to converge on a substantial thickening (mediastinum testis); this may be axial or displaced toward the side bordering the epididymis.

The soft, yellowish or brownish parenchyma consists of intermingled seminiferous tubules and interstitial tissue (Figure 5-38). The latter consists of massed inter­

Figure 5-37 Median section of testis (bull). 1, Mediastinum testis; 2, testicular parenchyma.

stitial (Leydig) cells supported by a delicate connective tissue framework in which run small blood and lym­phatic vessels (Figure 5-39). The interstitial cells are the principal producers of the steroid androgenic hor­mones. The greater part (60% in boars and stallions, 90% in rams and bulls) of the parenchyma is formed by the tubules in which the process of spermatogenesis is conducted.

Each seminiferous tubule (Figure 5-38) is much con­torted and also looped so that both ends open into the rete testis (Figure 5-38/5), a plexus of spaces within the mediastinum. Within the seminiferous tubules two cell types can be discerned: the Sertoli cells, which support and nourish the germ cells by the production of hor­mones and growth factors, and the seminiferous epithe­lium (Figure 5-39). The rete drains by a dozen or so efferent ductules (Figure 5-38/d) that pierce the capsule to join the head of the epididymis.

The endocrine functions of the testis are performed by the interstitial (Leydig) cells, responsible for androgen production, and the sustentacular (Sertoli) cells, responsible for inhibin production. Both types are normally under the pulsatile but more or less tonic control of gonadotropins (luteinizing hormone [LH] and follicle-stimulating hormone [FSH], respectively) produced in the pituitary (p. 217). Among other func­tions, the sustentacular cells produce activin and inhibin, whose names clearly indicate their effects upon the syn­thesis and release of FSH through mechanisms that may be direct or mediated via the hypothalamus. Andro­gens clearly have distinct local function but are also responsible for secondary sex characteristics such as the maturation of the accessory sex glands, male skeleto- muscular development, skin characteristics, and even the prenatal differentiation of certain brain and spinal cord nuclei.

Figure 5-38 Longitudinal section of a testis and epididymis, schematic. 1, Tunica albuginea; 2, mediastinum; 3, seminifer­ous tubules; 4, straight tubules; 5, rete testis; 6, efferent ductules; 6', epididymal duct; 7, deferent duct; 8, head of epididymis; 9, body of epididymis; 10, tail of epididymis; 11, pampiniform plexus.

behavior typical of the male. They also exert a negative feedback on pituitary gonadotropin secretion; part of this feedback is effected at the level of the hypothala­mus. In the fetal period, active production of androgens may take place without pituitary control. The intersti­tial cells in this period are also responsible for the pro­duction of the insulin-like factor 3 that is associated with gubernacular outgrowth and thus with testicular descent. In the fetal period the sustentacular cells produce the AMH that exerts an inhibitory effect on the paramesonephric ducts (p. 171), causing the disappear­ance of most of the female duct system.