QUANTITATIVE STUDY OF FOLLICULAR DEVELOPMENT IN THE OVARY OF THE INTACT OR UNILATERALLY OVARIECTOMIZED HAMSTER

Abstract

Summary. This study is an analysis of the changes occurring in the population of follicles during the oestrous cycle of the hamster. Findings in the intact animal were compared with those in animals semispayed at various times. The results indicate that the number of large follicles (267µ or greater) is largely determined for each ovary by Day 1 (9 a.m. of the morning of ovulation) and is maintained until the 3rd day of the cycle. Follicular atresia between Days 3 and 4 reduces the number of large follicles per ovary to nearly the number that will eventually ovulate. In general, both ovaries of any one animal contain approximately equal numbers of follicles during the cycle. During the first 3 days of the cycle of the intact hamster, each ovary of a pair contains nearly as many large follicles as will finally ovulate from both ovaries. After the removal of one ovary on any one of the first 3 days of the cycle, numerous follicles persist in the Day-4 ovary that ordinarily would degenerate. This suggests that a decrease in follicular atresia rather than an increase in the proliferation of medium-sized follicles is responsible for compensatory ovulation after semispaying. After unilateral ovariectomy at Day 3, the remaining ovary on the 1st day of the next cycle contains significantly more follicles than the Day-1 ovary of the intact animal or of hamsters that were semispayed on Days 1 or 4. Removal of one ovary at 9 a.m. during the first 3 days of the oestrous cycle was followed by a doubling in the number of ovulations from the remaining ovary. These results agree with the findings on follicular development in indicating that the follicles destined to ovulate at the next oestrus are determined on the 3rd day of the cycle.