Electron microscopy of cytodifferentiation and its subcellular steroidogenic sites in the granulosa cells of the human ovary.

The cytodifferentiation and subcellular steroidogenic sites in the granulosa cell of the developing follicle and in vitro effect of estradiol-17 ~ (E2) on the granulosa cell of the preovulatory follicle in the human ovary were investigated using the electron microscopic cytochemistry. The follicular cell in the primordial follicle showed an elongated nucleus, rough endoplasmic reticulum, Golgi apparatus, rod-shaped mitochondria with lamellar cristae, free ribosomes and a few lipid droplets. In the secondary follicle, the granulosa cell derived from the follicular cell had a round nucleus, rough endoplasmic reticulum, lipid droplets, Golgi apparatus, microfilament, mitochondria with lamellar or tubular cristae and a small amount of smooth endoplasmic reticulum. Especially, the gramilosa cell in the preovulatory follicle considered to be a transitional form to the steroidsecreting cell was characteristic of rough endoplasmic reticulum, lipid droplets, mitochondria with lamellar or tubular cristae and moderately well developed smooth endoplasmic reticulum. On the other hand, the granulosa cell in the postovulatory follicle showing a typical steroid-secreting activity had abundant lipid droplets, round mitochondrial with tubular or vesicular cristae, well developed smooth endoplasmic reticulum and lysosomes. The hallmarks of the cytodifferentiation of the granulosa cell were i) an appearance of lipid droplets, ii) a structural change of mitochondrial cristae from lamellar to tubular configuration and iii) an appearance and development of smooth endoplasmic reticulum. Reaction products for 3~-hydroxysteroid dehydrogenase (3~-HSD) activity were localized on tubular or lamellar cristae and inner membrane of the mitochondria, and on the membrane of smooth endoplasmic reticulum in the granulosa cell of the preovulatory as well as of the postovulatory follicle. The granulosa cell of the preovulatory follicle incubated in the medium containing E2 showed a structural change of mitochondrial cristae from lamellar to tubular configuration, and an appearance and development of smooth endoplasmic reticulum. From these facts, it is suggested that the granulosa cell in the preovulatory follicle has already a steroid-secreting activity and luteinizes abruptly, and E 2 contained in the culture medium could stimulate the functional differentiation of the granulosa cell in the prevulatory follicle affected with gonadotrophin. 247 INTRODUCTION It is well known that granulosa cells and theca cells become corpus lutein cells after the