The ultrastructure of the interstitial cells in human fetal ovaries.

Steroid-active interstitial cells (stromal lutein cells) in human fetal ovaries of 12, 15 and 22 weeks of gestation were studied by means of electron microscope. The large cells, which are predominantly situated in the vicinity of capillaries, show high structural similarity to Leydig cells (type A) of the fetal testis. Both are characterized by extensively development of smooth endoplasmic reticulum, accumulation of lipids, and numerous mitochondria of the tubular type containing osmiophilic inclusions. Whether the interstitial cell system of the fetal ovary has any specific function in developmental physiology of the genital system cannot be decided on the basis of the present results. Its differentiation may only indicate adequate response of competent mesenchymal cells to gonadotropic stimulation.