Activation of primordial follicles

The activation of primordial follicles appears to be regulated by a myriad of factors, and the current evidence suggests that whether an individual follicle stays in the resting pool or initiates growth may depend on the balance of stimulatory and inhibitory factors impinging on the follicle at a particular point in time. Although it is particularly difficult to study the factors involved in the initiation of follicle growth in large mammals, my laboratory has developed two experimental models for studying the regulation of follicular activation and growth. In one model, using organ cultures of small pieces of ovarian cortex, almost all primordial follicles activate. This model may thus be useful for testing putative inhibitors of activation. In contrast, the second model, using grafts of ovarian cortical pieces beneath the CAM of chick embryos, provides an environment in which no or little activation occurs, and this situation may be used in the future to test potential stimulators of follicular activation. Our results thus far indicate that there may be significant species differences in the regulation of follicle activation and the initial growth of activated follicles. Although encouraging progress has been made in identifying potential regulators of follicular activation in rodents, there is much to be learned and this first critical transition of follicular development is still very poorly understood in larger mammals.