Biology of Reproduction 65, 637b-637 (2001)
© 2001 Society for the Study of Reproduction, Inc.
Selection and Maintenance of the Dominant Follicle: An Introduction
J.E. Fortunea
a Department of Biomedical Sciences, Cornell University, Ithaca, New York 14853
In mammalian ovaries very few follicles successfully complete follicular development by ovulating. A developmental transition that is critical for follicular success is the selection of one follicle, from among a cohort of apparently similar follicles, for dominance, a status that allows continued growth and differentiation. The three minireviews that follow are based on the first authors' presentations in a minisymposium that was part of the 2000 annual SSR meeting.
Although there is evidence that selection of a subset of the follicles recruited into the cohort occurs in polytocous species, the mechanisms of follicular selection and dominance are more easily approached in species in which a single follicle is selected for further development. Monotocous species with fairly large preovulatory follicles, such as cattle, horses, and primates, are particularly useful models for studying mechanisms of follicular selection and dominance. Individuals of these species are large enough to allow 1) sequential assessments of endocrine changes in vivo; 2) ultrasonographic imaging of the ovaries to follow follicular growth and regression in vivo; and 3) endocrine, biochemical, and molecular analyses of follicular fluid and/or isolated follicular cells collected from individual follicles at defined stages of the cycle. Thus the minisymposium was focused on results obtained with monotocous animal models, specifically cattle, horses, and primates. The usefulness (and limitations) of these various approaches is evident in the three minireviews.
Initially I resisted the suggestion of a minisymposium on "Selection and Maintenance of the Dominant Follicle" because I believe that we are only beginning to understand some of the mechanisms involved. This is evident in the different viewpoints and divergent opinions about the factors most critical for selection and the maintenance of dominance expressed in the three reviews. Reading the manuscripts together reminded me of the parable about the six blind men and their divergent descriptions of an elephant. The blind men could have described the elephant much better if they had pooled their information. The minisymposium at the 2000 SSR meeting and this series of minireviews are attempts to pool some of the current information on follicular selection and dominance. Although I believe that we should consider the various models presented in these reviews as works in progress, the minisymposium presentations and these minireviews also remind us of how much has been accomplished in this area. There is agreement about the key role of the negative feedback loop between estradiol and FSH in the process of follicular selection. How the dominant follicle continues to develop as circulating concentrations of FSH decline is a critical question; and the relative importance of the acquisition of LH receptors by granulosa cells of the dominant follicle, changes in the intrafollicular IGF system, and changes in follicular vascularization remain to be determined. We hope that the reviews may inspire new ideas and approaches that will further unravel what is still one of the most unyielding mysteries of follicular development.
