The testicular descent in human. Origin, development and fate of the gubernaculum Hunteri, processus vaginalis peritonei, and gonadal ligaments.

Testicular descent has to be divided into the turn-out of the testis and epididymis from the abdomen proper and an inner abdominal descent of genital organs. Both events are closely related to and depend on the development and reorganisation of ligaments, mainly the gubernaculum Hunteri. These seemingly unambiguous events are controversially described since the first description of the gubernaculum, and results and specifics of other species were intermingled with data from humans, thus giving more confusion than lucidity in this important step of gonadal development. Here, we concentrate on human embryos, chronologically investigated by serial sections, scanning electron microscopy, three-dimensional reconstructions, microdissection and immunohistochemistry. The first question to be answered was whether a real inner descent of gonads occurs. We demonstrated this inner descent by showing the relations of the gonads, mesonephros, cranial mesonephric ligament and the anlage of the diaphragm with the vertebral segments. No explosion-like increase in certain vertebral segments was observed which might simulate a gonadal descent. The inner descent is coupled with the growth of the gonad (ovary or testis), the involution of mesonephros, the descending septum transversum or the anlage of the diaphragm, and the intercalated cranial mesonephric ligament. This ligament always inserts medially at the border between the gonad and mesonephros in close relationship to the abdominal ostium of the müllerian duct, a region where hydatides often occur. In contrast to the testes, the ovaries arrive very early--20-25 mm CRL--at their definitive position of S2/3 (level of linea terminalis pelvis), yet, are transversally oriented. The cranial gonadal ligament does not exhibit notable increase in length during inner descent. It does not contain blood vessels. While regressing in both sexes, it will be replaced by the plica formed by the ovarian vessels, that is the suspensorium ligament of the ovary as known in adults. The second point to be investigated was the origin, development, structure and fate of the gubernaculum Hunteri as well as the processus vaginalis peritonei. Their arrangement and composition is crucial for testicular descent. We discriminated five phases of their development and differentiation. Phase I characterises the early development of the gubernaculum of stage 14 CC (5-7 mm CRL) embryos. It arises as conus inguinalis and connects the abdominal wall lateral to the umbilical artery with the caudal part of the mesonephric fold. It is in this early stage that the localisation of the inner inguinal ring is defined. In phase II, stage 20-23 CC (21-30 mm CRL), three parts of the gubernaculum--abdominal, interstitial and subcutaneous--can be distinguished. The processus vaginalis peritonei appears with its dorsal layer firmly adhering to the ventral side of the gubernaculum. The gubernaculum inserts cranially into the mesenchyme of the genital ducts at their crossing-over. Opposite to it, but at some distance, a ligament connects the caudal pole of the testis with the dorsal mesenchyme of the genital ducts. In female embryos, the analge of the ovarian ligament appears as a U-shaped, double peritoneal fold. Phase II is subdivided in phase IIa (32-55 mm CRL), characterised by an enormous increase in length and volume of the gubernaculum and also an enlargement of the processus vaginalis peritonei. In phase III, sex-specific differences in gonadal position and gubernacular structure can be observed for the first time. Testes increase in volume and come close to the mesenchyme of the genital ducts. The caudal pole of the testis overlaps both ducts. We also subdivide this phase into phase IIIa (about 100 mm CRL) where two very important events occur in male foetuses: 1, the swelling of the gubernaculum, and 2, the gliding of the testis across the genital ducts. This gliding is permitted by both, the regression of the müllerian duct and t