Patterns of Ovarian Cell Proliferation in Rats during the Embryonic Period and the First Three Weeks Postpartum1

The purpose of this study was to characterize the patterns of ovarian cell proliferation during the earliest stages of folliculogenesis, which occur in the embryonic period and the first weeks postpartum in rats. Rats were given continuous infusions of [3H]thymidine (3H-TdR) or bromodeoxyuridine (BrdU), and cells that were synthesizing DNA were visualized by autoradiography or immunohistochemistry. There were dramatic changes in the patterns of cell proliferation during the period studied. Mesenchymal cells proliferated extensively in the embryonic and neonatal ovary, but their growth fraction declined precipitously as follicles formed. Epithelial cells in the medulla of the ovary left the cell cycle at about embryonic Day 12, then resumed proliferation as soon as they were incorporated into follicles just after birth. Epithelial cells towards the cortex of the organ continued to proliferate until late in the embryonic period; they apparently became quiescent around the time of birth, and incorporation into follicles did not release them from their quiescent state. After the follicles had formed, patterns of cell proliferation continued to change. At 5 days postpartum, approximately 36% of the smallest follicles (1–8 granulosa cells in cross section) had at least 1 granulosa cell that was labeled following a 24-h infusion of 3H-TdR; by Day 20 only 14% of these follicles were labeled, and by Day 30 only 4.4% were labeled.