Ultrastructural events during early gonadal development in Rana pipiens and Xenopus laevis.

The establishment of the undifferentiated gonad was studied in Xenopus laevis and Rana pipiens using high resolution techniques. It was found that the cells of the so-called "mesonephric blastema" had no structural resemblance to the cells of the gonadal medulla in both species. Furthermore, there was no morphological evidence that would suggest a migration of the former cells towards the incipient gonad at the time of its appearance. However, the basal lamina of the coelomic epithelium was interrupted in the region of the genital crest, and there was a definite ultrastructural similarity between the cells of this epithelium and those that first form the medulla. These observations suggest that, in amphibians, the cells of the gonadal medulla come from a cellular line arising from the coelomic epithelium and not from the "mesonephric blastema," as has been proposed.