In the majority of mammalian species the testes migrate from their developmental position within the abdomen near the kidneys to a location outside of the body wall, usually in a scrotum (). For these species, including the dog, if neither testis descends (bilateral cryptorchid) spermatogenesis is eliminated and the animal is infertile. If only one testis descends (monorchid or unilateral cryptorchid) fertility is lessened. (Many rodents have testes that descend periodically coincident with breeding. In such species the testes can be gently squeezed back into the body cavity at any time because of the large inguinal canal.)
Much important research on testicular descent in domestic animals has been conducted at the Institute of Veterinary Anatomy of the State University at Utrecht, the Netherlands, by Wensing and co-workers: Wensing (1968- 1980); Baumans et al. (1981- 1983); Baumans (1982); Wensing and Colenbrander (1986).
Whereas in most mammals testicular descent occurs in fetal life, in the dog it occurs at approximately the time of birth and this makes the dog a good subject for the study of the mechanics and hormonal control of descent. A descriptive and illustrated experimental study of normal development and the factors responsible for the descent of the testes in the dog was published as a thesis in the Netherlands by Baumans (1982). (Included in the thesis are six papers with co-workers, each with a bibliography.) Baumans found that the major factors essential for the descent were first an outgrowth and then a regression of the gubernaculum testis. The gubernaculum is a mesenchymal mass enclosed in a fold of peritoneum that extends from the testis across the mesonephros (kidney ridge) to the inguinal area. It is attached to the caudal pole of the testis and epididymal part of the mesonephric duct. Its distal end continues through the abdominal wall where the inguinal canal forms around it. Here, the gubernaculum is invaded by an outgrowth of parietal peritoneum, the vaginal process. During fetal growth the testis moves caudally to the level of the inguinal canal as the trunk elongates. There are two phases of the migration of the testis through the inguinal canal and into the scrotum. In the first phase the extra-abdominal part of the gubernaculum increases enormously in length and volume expanding beyond the inguinal canal and dilating it. When this expansion exceeds the size of the testis and passive resistance is reduced, the testis passes through the canal adjacent to the vaginal process. Here it rests in a mass of swollen gubernaculum, which is still covered by vaginal process peritoneum. An important feature of this testicular migration is that the testis brings with it the visceral peritoneum that formed around it at its site of development in the dorsal abdominal wall. Completion of this first phase of testicular descent initiates the second phase in which the gubernaculum is transformed by tissue degeneration from an expanded mucoid mass into a small fibrous structure in the scrotal sac, thus making room for the testis. This converts the gubernaculum into the proper ligament of the testis, which attaches the tail of the epididymis to the testis, and the ligament of the tail of the epididymis, which attaches the tail to the distal portion of the vaginal tunic and spermatic fascia. The latter is derived from the remnants of the gubernaculum that surrounds the vaginal tunic. The degeneration of the gubernaculum results in the testis being located at the distal extent of the vaginal tunic.
Baumans found that on the day of birth the testis was located halfway between the kidney and the deep inguinal ring. The gubernaculum had reached its maximum development and was beginning to show signs of regression histologically and histochemically. By day 3 or 4 after birth the testis passes through the inguinal canal and the gubernaculum regresses. By 35 to 40 days after birth the testis reaches its definitive position in the scrotum.
To test the factors responsible for gubernacular growth and regression, Baumans removed the testes in fetal dogs and found that gubernacular outgrowth ceased and there was no epididymal descent. Removal of the testis at birth resulted in retarded gubernacular regression and a delayed epididymal descent. After further investigation it was concluded that the testis induces gubernacular outgrowth and regression and thereby regulates its own descent. It was found that testicular hormones, particularly testosterone, are synthesized in the testis. In the first phase of descent an unidentified testicular factor stimulated gubernacular proliferation and swelling. Susten-tacular cells are thought to be the source of this factor. In the second phase of descent testosterone induced gubernacular regression. In regard to the mechanical role of the gubernaculum, it was found that a connection between the testis and the gubernaculum () was essential for normal testicular descent. By the end of the second phase of descent the testis plays a mechanical role in distending the scrotum.
Wensing and Colenbrander (1986), in discussing normal and abnormal descent of the testis, make a distinction between the morphologic characteristics of testicular descent in mammals with a striplike cremaster muscle (ungulates, dog, and humans) and those with a saclike cremaster muscle (rodent, lagomorph). They believe that the bilaminar saclike cremaster muscle of rodents and lagomorphs continues to grow and increase in size even after the regression of the gubernaculum and thus plays a role in descent of the testis. Regardless of the morphologic characteristics of the cremaster muscle, they stress the importance of a normal gubernacular outgrowth and regression for affecting testicular descent. Their experimental results showed that the first phase in the descent process (gubernacular outgrowth) does not depend on the presence of active interstitial cells, testosterone, testosterone receptors, or gonadotropins, although the presence of the testis is essential.