Hormonal Control of Gene Expression in the Ovary

The maturation of ovarian follicles involves several sequential stages: initiation, growth, selection, ovulation, and luteinization (Fig. 1). Although the factor(s) that initiates the growth of “resting” primordial follicles remain elusive, the key endocrine events controlling follicular growth in mammals have been known for many years. For example, once follicles have begun to grow, basal concentrations of the gonadotropins, FSH and LH, maintain growth up until the small antral stage. Small antral follicles are selected to continue growth by subtle increases in basal concentrations of gonadotropins, such as those that occur at menses in primates (1) or at the end of pregnancy in rodents (2). Once selected, the growing dominant follicle(s) acquires specific functional characteristics that permit it to differentiate to the preovulatory stage and synthesize estradiol. Increased serum estradiol triggers the surge of gonadotropins that, in turn, stimulates the preovulatory follicle(s) to ovulate and luteinize (form the corpus luteum).