Quantitative aspects of follicular development in the untreated and PMS‐treated cyclic hamster

Abstract Hamsters injected at 0900 on day 1 of the cycle (metestrus) with either 0, 5 or 15 IU pregnant mare's serum (PMS) were killed at 1500 of days 1 to 4 of the cycle and the ovaries prepared for light microscopy and for a quantitative evaluation of follicular development. In the untreated cyclic hamster, the maximal number of preantral follicles with eight or more layers of granulosa cells occurred between the afternoon of day 4 (proestrus) and day 1, coinciding with the highest blood levels of FSH and LH. It is concluded that the elevated preovulatory levels of gonadotropins not only induce the ovulation of the mature antral follicles but at the same time recruit the next set of follicles for development during the new cycle. By the afternoon of day 1, treatment with either 5 or 15 IU PMS recruited more follicles into large preantral and incipient antral stages than in the untreated hamsters. However, by day 2 the pattern of follicular distribution was similar between the 5 IU PMS and untreated group whereas considerably more antral follicles had differentiated in the animals given 15 IU PMS. The ability of 15 IU PMS to elicit superovulation therefore depends on the levels being initially high enough to mature more follicles at critical stages of their development; the prolonged biological half life of PMS then sustains these follicles throughout the cycle.