Ultrastructural aspects of gonadal morphogenesis in Bufo bufo (Amphibia Anura). 1. Sex differentiation

The morphogenesis of gonads in Bufo bufo tadpoles was studied. Ultrastructural differences between sexes were identified. All specimens analysed initially developed gonads made up a peripheral fertile layer (cortex) surrounding a small primary cavity. Subsequently a central layer of somatic cells (medulla) developed. Both layers were kept separate by two uninterrupted basal laminae between which a vestige of the primary cavity persisted. During female differentiation, the peripheral layer continued to be the fertile layer. In males, the central layer blended into the peripheral layer and the basal laminae disappeared. The somatic cells of the central layer came into direct contact with the germi cells; this did not occur in females. Testicular differentiation continued with the migration of germ cells towards the center of the gonad. The somatic elements surrounding the germ cells appeared to play an active role in their transfer to the center of the gonad. The peripheral layer shrank and became sterile. Two basal laminae then reformed to separate the fertile central layer from the peripheral sterile one. Germ cells have always been thought to perform a passive role in sex differentiation in amphibians. Following the generally accepted “symmetric model”, the mechanism of gonad development is symmetrical with cortical somatic cells determining ovarian differentiation and medullary somatic cells determining testicular differentiation. In contrast, we found that sex differentiation follows an “asymmetric” pattern in which germ cells tend primarily toward a female differentiation and male differentiation depends on a secondary interaction between germ cells and medullary somatic cells.