Histological Observation of the Development of Follicles and Follicular Atresia in Immature Rat Ovaries

To clarify the development of follicular growth and atresia in the immature ovary ratsovaries and blood were removed at fi xed points during the period from 0 to 35 days after birth (Day 0 to Day 35. The ovaries were immunohistochemically examined and blood concentrations of serum follicle‑stimu‑ lating hormone (FSH and estrogen (E were measured. We investigated how time‑course changes in follicular cell proliferation estrogen receptor ｹ (ERｹ apoptosis and FSH and E concentrations are connected with follicular growth and atresia. Apoptosis was found in the ova from Day 0 to Day 3. On Day 15 apoptosis occurred in some granulosa cell nuclei in some follicles but BrdU uptake and the presence of cyclin D2 and ERｹ could be observed in other granulosa cells. From Day 17 apoptosis increased in the follicular granulosa cells and BrdU uptake and the presence of cyclin D2 and ERｹ were decreased. Follicular atresia continued reaching a peak on Day 30. Serum FSH and E concen‑ trations increased until Day 15 then markedly decreased after Day 17. The mechanism of apoptosis in the ova from Day 0 to 3 has not been clarifi ed. However the onset of follicular atresia was caused by apoptotic degeneration from Days 15 to 17. These results showed that the oocytes were selected by apoptosis at 2 points in the time‑course of the maturation of the ovary.