Cell-cell interactions in primordial follicle assembly and development.

Two critical processes in ovarian biology are the assembly of the primordial follicles early in development and then the subsequent development and transition of the primordial follicle to the primary follicle. These processes directly effect the number of oocytes available to a female throughout her reproductive life. Once the pool of primordial follicles is depleted a series of physiological changes known as menopause begins in humans. The inappropriate coordination of these processes contributes to ovarian pathologies such as premature ovarian failure. Studies demonstrate primordial follicle assembly and development are coordinated by locally produced paracrine and autocrine factors. Factors have been identified that influence follicular assembly such as neurotropins. Several local factors that promote the primordial to primary follicle transition have also recently been identified. These include growth factors such as kit-ligand, leukemia inhibitory factor and basic fibroblast growth factor. Interestingly, recent studies demonstrate Müllerian inhibitory substance appears to inhibit the primordial to primary follicle transition. Therefore, observations suggest a mechanism for both positive and negative control of the primordial to primary follicle transition. The studies reviewed regarding the control of primordial follicle assembly and the primordial to primary follicle transition help elucidate these poorly understood aspects of ovarian biology.