Comparative studies of normal and neoplastic ovarian germ cells: 1. Ultrastructure of oogonia and intercellular bridges in the fetal ovary.

Electron microscopic studies were performed on human fetal ovaries to gain insight into the process of mitotic proliferation in developing germ cells. Three stages of germ cell differentiation are present during the early gestational period: primitive germ cells, oogonia, and oocytes. Oogonia, representing the mitotic stage of differentiation, are the predominant cell type present between 9 and 12 weeks' gestation and then progressively decrease in number as a result of transformation into oocytes in meiosis and degeneration. Mitotic division of oogonia, which is extensive during the late first and early second trimesters, is characterized by incomplete separation and persistence of intercellular bridges between germ cells. Intercellular bridges were found in large numbers from 10 weeks until the time of follicle formation at midgestation. The bridges contained microtubule arrays consistent with remnants of the spindle apparatus. The findings support the role of germ cell bridges in maintaining coordination of proliferative activity during the early developmental period.