Rate and course of atresia during follicular development in the adult cyclic rat

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Abstract
Ovarian follicles (.gtoreq. 100 x 105 .mu.m3 or a mean diameter of .gtoreq. 275 .mu.m) in adult rats were classified as non-atretic and atretic during the estrus cycle and recorded in 5 volume classes. The atretic follicles were also categorized in several stages according to the progress of atresia. The degeneration of the entire granulosa wall until the induced changes in the oocyte took at least 24 h. Another 24 h elapsed before the oocyte became denuded. Therefore the percent of atretic follicles, i.e., follicles in all stages of atresia, could not be used as indicator for the rate of atresia. The atretic portion in the follicle population .gtoreq. 100 x 105 .mu.m3 increased from early diestrus 1 to early diestrus 3, reached a plateau during diestrus 3 and pro-estrus, and decline at late estrus to the level of early diestrus 1. The sudden decrease in number of atretic follicles after late pro-estrus was caused by the discard of many atretic follicles in the advanced stages due to various deformities as revealed by histological observation. By using the percent of atretic follicles in the earliest stage as indicator of atretic rate, 2 waves of atresia were found affecting the population of antral follicles during their growth, the first at diestrus 1 amounting to 15-20% and then at diestrus 3, affecting 35% of the population. This study also shows the extension of atresia in the various volume classes of follicles during the estrus cycle. A pool of .apprx. 7 follicles in the smallest volume class was maintained after ovulation, grew further in the next cycle with a new cohort of 20 follicles and seemed to provide the required number of follicles destined to ovulate. The follicles that ovulate were already present at an antral stage in the preceding cycle and needed 2 cycles for their growth to ovulation.