Electron Microscopy of the Cytodifferentiation of the Theca Cell in the Mouse Ovary

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Abstract
There are only fibroblast-like cells around the primordial follicle. Three different cell types are recognized around the secondary and Graafian follicles; these are fibroblast-like cells, theca gland cells (steroid-secreting cells) and transitional cells (partially or incompletely differentiated theca cells). The fibroblast-like cell has well-developed rough endoplasmic reticulum, moderately developed Golgi apparatus, rod-shaped mitochondria with lamellar cristae, a few lipid droplets and free ribosomes in their cytoplasm. A large, ellipsoidal nucleus, abundant lipid droplets, mitochondria with tubular cristae and smooth endoplasmic reticulum are seen in the theca gland cell. The transitional cell, which is considered to be an intermediate form between the fibroblast and the theca gland cell, has a round or oval nucleus, many lipid droplets, mitochondria with tubular or lamellar cristae and a small amount of smooth endoplasmic reticulum. As to the cytodifferentiation of the theca cell, an appearance of lipid droplets, a structural change of the mitochondrial cristae and an appearance of the elements of smooth endoplasmic reticulum are important signs. Even after the differentiation of the theca gland cell and the administration of PMS (pregnant mare serum gonadotropin) or HCG (human chorionic gonadotropin), many fibroblasts exist in the theca folliculi of the Graafian follicle, which might be without any ability to differentiate into theca gland cells.