Doubts about the 'first phase of testis descent' in the rat as a valid concept.

It has become customary to distinguish between two stages in descent of the testis: first transabdominal migration of fetal testes from the posterior body wall to the inguinal region, and second, true descent into the developing cremaster sacs. The present study of rats examines the validity of the concept of transabdominal testis migration by histological analysis, between days 15 and 22 of fetal life, of the position of testes and ovaries relative to the site where, postnatally, the male cremaster sacs are to develop. The analysis revealed that rat testes do not migrate from the posterior body wall to the inguinal region during the last days of fetal life. It appeared that, during that period of fetal life, ovaries ascend in a cranio-lateral direction, together with the caudo-lateral poles of the kidneys, and are closely connected to them via the ovarian cranial suspensory ligaments. A similar ascent of testes seemed to be prevented by failure of cranial suspensory ligament growth. This failure may have occurred through the exposure of male fetuses to androgen, since cranial ligaments developed in male fetuses exposed to anti-androgen. The above results allow for the following conclusions. There is no evidence for active testis migration from the posterior abdomen towards the inguinal region during the latter part of rat fetal life. There is clear evidence of cranial migration of the ovaries during the latter part of rat fetal life together with the ascending kidneys.(ABSTRACT TRUNCATED AT 250 WORDS)