Ultrastructure of the germinal epithelium during oogenesis in the rabbit

Electron microscopic examination of newborn New Zealand white rabbit ovaries revealed numerous germ cells within the germinal epithelium during the early stages of oogenesis. The germ cells were found at all levels of the germinal epithelium, as well as in bridges connecting with the underlying sex cords. Germ cell nuclei were large and round with smooth, regular double membranes, reticular nucleoli and evenly distributed chromatin. Mitochondria were characteristically spherical with slightly eccentric cristae. The number of germ cells within the germinal epithelium gradually diminished until they were completely absent by the time oogenesis was complete. The germinal epithelial cells were characterized by irregular, elongated nuclei with eccentric nucleoli and peripherally marginated chromatin. Cytoplasm was rich in ribosomes, rough‐surfaced endoplasmic reticulum and elongated mitochondria. Junctional complexes, cytoplasmic projections and occasional cilia were present on the cell surfaces. At the junctions of germ cells and germinal epithelial cells, cell membranes were closely apposed and small interdigitating cytoplasmic projections were present. During the early stages of oogenesis, the germinal epithelium was multilayered and connected to the cortical sex cords in many places. Bridges joining the germinal epithelium to the sex cords contained both germ cells and germinal epithelial cells. Occasionally, germinal epithelial cells could be seen extending partially into the sex cords. With increase in the thickness and extent of the tunica albuginea, contact between the germinal epithelium and the sex cords was gradually diminished. As oogenesis reached completion, the germinal epithelium was reduced to a single layer of epithelial cells separated from the developing follicles below by a dense band of fibroblasts, collagen and capillaries.