Electron microscopy of cytodifferentiation and its subcellular steroidogenic sites in the theca cell of the human ovary.

The cytodifferentiation and subcellular steroidogenic sites in the theca cell of the human ovary during the follicular phase were investigated using the electron microscopic cytochemistry. Only fibroblast-like cells were seen around or near the primordial follicle. In the theca interna of the secondary and Graafian follicle however there were three different cell types: fibroblast-like cells, theca gland cells (steroid-secreting cells) and transitional cells (partially or incompletely differentiated theca cells). On the other hand the theca externa of these follicles consisted mainly of fibroblast-like cells. The hallmarks of the cytodifferentiation of the theca cell were: 1) the appearance of lipid droplets, 2) a structural change of the mitochondrial cristae from lamellar to tubular form and 3) the appearance and development of smooth endoplasmic reticulum. Reaction products of 3 beta-hydroxysteroid ferricyanide reductase, indicating 3 beta-hydroxysteroid dehydrogenase activity, were localized on tubular or lamellar cristae and inner membrane of the mitochondria, and on the membranes of smooth endoplasmic reticulum in the transitional cell as well as in the theca gland cell of the secondary and Graafian follicle. From these data, it is suggested that the transitional cell has a steroid-secreting activity and also plays an important role in follicular development in human reproduction.