Human antral folliculogenesis: what we have learned from the bovine and equine models

The study of ovarian folliculogenesis has been of great interest to scientists and clinicians in the human and veterinary health fields for more than 20 centuries. Initial studies of the ovarian follicle were based on anatomical descriptions post-mortem, followed by histologic and endocrinologic evaluation of ovarian status. The introduction of high resolution ultrasonography in the 1980s provided a long-awaited tool to image the reproductive tissues in situ in both animal and human species. The bovine and equine species have been established as models for the study of human ovarian folliculogenesis. Profound similarities in the dynamics of follicle development exist between the menstrual cycle in humans and the estrous cycle in cattle and horses. Disparities between species appear specific rather than general. Research performed in women thus far has led to the concepts that: 1) follicle development occurs in a wave-like manner during the menstrual cycle, 2) the number of waves per cycle correlates positively with the length of the cycle, 3) the emergence of follicle waves in women are preceded by a rise in circulating FSH, 4) selection of a dominant follicle may occur in each wave of the cycle, and 5) a decline in circulating FSH and increase in follicular estradiol, inhibin A, and IGF-II act collectively to enable the dominant follicle to continue to grow in an endocrine environment of decreasing FSH and increasing LH, while subordinate follicles undergo regression. The goal of continued research using animal models for studying human ovarian function is to provide the hypothetical basis for further studies in women, which will ultimately lead to the development of safer and more efficacious infertility and contraceptive therapies.