Schematic diagram of the "FSH threshold" model for the selection of the preovulatory follicle in primates. During the luteal phase of the menstrual cycle circulating FSH concentrations are held below the FSH threshold by secretions of the corpus luteum. As a result, growing follicles do not advance beyond the preantral stage and undergo atresia. Upon the regression of the corpus luteum at the end of the nonfertile menstrual cycle, the negative feedback suppression of FSH secretion is released and FSH concentrations rise above threshold levels. One (or occasionally more) of the maturing preantral follicles is stimulated in response to the elevation of FSH and develops both the aromatase enzyme and LH receptors. The acquisition of aromatase results in a rise in systemic levels of estradiol which result in the suppression of FSH secretion, which, in turn, prevents the maturation of less mature follicles. The FSH-stimulated induction of LH receptors and the acquisition of LH responsiveness of granulosa cells of the stimulated follicle permit it to mature in the presence of FSH concentrations which are insufficient to stimulate the maturation of other less mature follicles.