Early sequential development in avian gonads. An ultrastructural study using selective glycogen labeling in the germ cells.

The early development of chick embryo gonads was studied by light and transmission electron microscopy and by cytochemical detection of glycogen in primordial germ cells (PGC's). The sequential events are presented from day 2 to day 5 (stages 14 to 27), together with the ultrastructural characteristics of the various cell populations. This study attempts to contribute data on the controversial origin of the avian gonadal medulla. It is shown that the main part of the primitive medulla arises from the association of the proliferating germinal epithelium (GE) and epithelioid cords (formed early from the condensed splanchnopleural mesenchyme); later deep cords, probably developed from the intermediate mesoderm mesenchyme, join the medulla. The process of mesenchymal condensation is described (cells aggregated by dense plaques and membrane anchorages, deposits of fibrillar extracellular material around the cords) together with ultrastructural changes in the GE basal lamina, which is progressively pushed back then disorganized. The study of the morphological relationships between the PGC's and the somatic cells has been facilitated by glycogen labeling in the former. Whereas the PGC's settled in the germinal epithelia do not display any junctional complex with the epithelial cells up to stage 26, those which are migrating through the dorsal mesentery and in the deep gonadal mesenchyme are associated to fibroblast-like somatic cells by dense plaques. The cytological characteristics of migrating PGC's (polymorphism, pseudopodia and filopodia, paucity of microfilaments, high alpha-glycogen particle content) have been described. In addition, the cell pattern in mesodermal tissues is thought well-suited to PGC invasion owing to a wide extracellular compartment, distributed in the mesenchyme and the early epithelium, and to discontinuities in the basal lamina.