The role of follicle-stimulating hormone in the selection of follicles in human ovaries: a survey of the literature and a proposed model.

A review of the literature resulted in a model consisting of three follicle-stimulating hormone (FSH)-related mechanisms of follicle selection in the human ovarian cycle. The FSH-dependent selection of dominant follicles is the result of varying FSH serum levels on the one hand, and a varying follicular sensitivity to FSH on the other hand. The first FSH-related mechanism of follicle selection is the intercycle rise of FSH which induces the ongoing development of a reasonable number of follicles during the early follicular phase. The intercycle FSH level should surpass the FSH threshold of the follicles with the highest FSH sensitivity, but the FSH level should not be too high because many other less sensitive follicles might also be stimulated to develop. The second and third mechanisms act together during the mid- to late follicular phase. During the mid- and late follicular phases, the number of dominant follicles is reduced by the synergistic actions of a decreasing FSH level and the differentiation of the follicular sensitivity to FSH. Follicles destined to become dominant gain sensitivity to FSH, whereas follicles destined to become atretic lose their sensitivity to FSH. This differentiation of follicular sensitivity to FSH is the result of several endo-, para- and autocrine factors which modulate the effect of FSH on the growing follicle. The differentiation of follicular sensitivity to FSH supports the effect of the decreasing FSH level. Only the most sensitive follicle will become dominant. The other follicles will become atretic. The presented model may be of use not only for interpretation of the results of ovarian stimulation, but also to put the growing amount of data on growth factors and other substances which modulate the effects of FSH in the dynamic context of follicle selection.