The Process of Testicular Descent
The descent of the testis into a scrotal position is necessary in most mammals to obtain normal fertility. The process depends on the existence of a mesenchymal condensation, the gubernaculum testis, within a detachment from the genital fold that leads from the testis toward and through the inguinal canal (Fig.
5.18). At a certain critical period of development (which varies in timing among different species) the distal part of the gubernaculum, which extends through the inguinal canal to the groin, enlarges very rapidly and considerably (Fig. 5.19A and B). The gubernaculum is invaded by an extension of the peritoneal lining of the abdomen. In this way the vaginal process, which provides the space into which the testis will be drawn, is formed (Fig. 5.18/3). The invasion by the vaginal process divides the gubernaculum into three parts: the proximal part (pars propria) is enclosed by the inner (future visceral) peritoneal lining of the process; the second part (pars vaginalis) surrounds the outer (future parietal) peritoneal lining of the process; and the third part (pars infravaginalis) lies distal to the invagination and is thus continuous with the other parts. The swelling of the gubernaculum commences distally, causing it to exert pressure on the body wall about the superficial ring of the inguinal canal. The swelling displaces the testis distally, toward the abdominal entrance of the canal. The swelling then gradually extends proximally, and at its peak the part adjacent to the testis (and within the inguinal canal) is as thick as the testis itself (Fig. 5.19A and B). At this stage any slight increase in intraabdominal pressure may be sufficient to expel the testis from the abdomen into the inguinal canal, although for a time its return to the abdomen is still possible. The descent is complete and irreversible once the core of the gubernaculum has regressed (Fig. 5.20). A well-timed gubernacular regression is therefore as indispensable to normal descent as is the earlier swelling. Because the timing is critical and the process is subject to various disturbances, it is not surprising that abdominal retention and abnormal descent are both relatively frequent. Failure of the testis to appear in the groin is known as cryptorchidism (hidden testis). It takes various forms: it may be unilateral or bilateral, and the testis may be held within the abdomen or trapped within the inguinal canal. As a result of the higher temperature to which an undescended testis is exposed, spermatogenesis is not initiated at puberty. The condition is clearly undesirable, and although unilaterally cryptorchid animals may be fertile, they should be excluded from breeding because the condition is often hereditary.
FIG. 5.15 The fusion of the combined paramesonephric ducts with a bud from the urogenital sinus forms the vagina. 1, Rectum; 2, caudal part of urogenital sinus (vestibule); 3, cranial part of urogenital sinus (bladder, urethra); 4, bud from urogenital sinus; 5, fused paramesonephric ducts; 6, vagina; 7, cervix uteri; 8, uterine horn.
FIG. 5.16 Different degrees of fusion of the paramesonephric ducts. (A) Uterus duplex (rabbit). (B) Uterus bicornis (small body: sow, cow). (C) Uterus bicornis (large body: mare). (D) Uterus simplex (woman). 1, Infundibulum; 2, uterine tube; 3, uterine horn; 4, fusion site of the two ducts; 5, cervix; 6, vagina; 7, vestibule.
FIG. 5.17 Development of the female external genitalia. 1, Genital tubercle; 2, cloacal fold; 3, urogenital fold; 4, lateral swelling; 5, anus; 6, labia of vulva; 7, clitoris.
Similar structures are formed in the female sex but do not develop significantly, except in the bitch among domestic mammals, in which the existence of the vaginal process is occasionally troublesome (p.
448).In several species when a twin pregnancy occurs, the circulation of the two fetuses can become interconnected, resulting in the exchange not only of cells but also of hormones (see Fig. 29.18). The hormonal influence of the male fetus can interfere with the development of the female co-twin. In cattle this interference can result in a "freemartin," in which the ovary and the female duct system are severely underdeveloped or absent. It can also result in the outgrowth of the gubernacula in the female twin (see Fig. 35.8A and B). Very seldom, this problem can also occur in a pig fetus that is interconnected with a male fetus in utero.
FIG. 5.18 Schematic representation of the testis and gubernaculum within the peritoneal fold in which descent takes place. 1, Testis; 2, gubernaculum; 2', pars propria; 2", pars infravaginalis; 2'", pars vaginalis; 3, vaginal process; 4, testicular artery.