Ovarian follicle size or growth rate can both be determinants of ovulatory follicle selection in mice

The endocrinology regulating ovulation of the desired number of oocytes in the ovarian cycle is well described, particularly in mono-ovulatory species. Less is known about the characteristics that make one follicle suitable for ovulation while most other follicles die by atresia. Bromodeoxyuridine (BrdU) injection was used to characterize granulosa cell proliferation rates in developing ovarian follicles in the estrous cycle of mice. This methodology allowed identification of follicle diameters of secondary (80-130 mu m), follicle-stimulating hormone (FSH)-sensitive (130-170 mu m), FSH-dependent (170-350 mu m), and preovulatory (>350 mu m) follicles. Few preovulatory-sized follicles were present in the ovaries of mice at estrus, the beginning of the cycle. Progressive increases were seen at metestrus and diestrus, when full accumulation of the preovulatory cohort (similar to 10 follicles) occurred. BrdU pulse-chase studies determined granulosa cell proliferation rates in the 24-48 h before the follicle reached the preovulatory stage. This showed that slow-growing follicles were not able to survive to the preovulatory stage. Mathematical modeling of follicle growth rates determined that the largest follicles at the beginning of the cycle had the greatest chance of becoming preovulatory. However, smaller follicles could enter the preovulatory follicle pool if low numbers of large antral follicles were present at the beginning of the cycle. In this instance, rapidly growing follicles had a clear selection advantage. The developing follicle pool displays heterogeneity in granulosa cell proliferation rates, even among follicles at the same stage of development. This parameter appears to influence whether a follicle can ovulate or become atretic.