Histological analysis of the 'critical point' in follicular development in mice

Aim: To histologically examine ovarian follicular development in cycling mice. Methods: Mice were observed at 13:00 h at each stage of the estrous cycle. The ovaries were cut into complete serial sections. All sections were observed, and the size of each follicle was measured by using a micrometer. Follicles with advanced atresia were excluded and non-atretic healthy follicles were differentiated from atretic follicles. Results: The measurement of the number of follicles in each stage of the estrous cycle, with reference to their size, showed that in each mouse, the number of small healthy follicles (100-249 μm in average diameter) was approximately 100 for all stages and the number of medium-sized healthy follicles (250-349 μm) was close to 20 in only the metestrus and diestrus stages. In contrast, large healthy follicles (≧350 μm) showed marked changes throughout the estrous stage. Many healthy large follicles were observed in the proestrus stage, but they disappeared in the estrus stage, which suggests that they have ovulated. This was supported by observations of oocytes resuming meiosis in large healthy follicles. Conclusion: As follicular atresia was frequently observed in follicles of 250-399 μm diameter, this size range may be a 'critical point' for atresia. The results suggest that there is a 'critical point' in follicular development, and that only follicles that pass this point will ovulate, while those that do not will become atretic follicles