Part of “CHAPTER 95 – THE NORMAL MENSTRUAL CYCLE AND THE CONTROL OF OVULATION“

The basic activity of the pituitary gonadotropes is determined by the direct input of GnRH but is modulated by the feedback effects of E₂. Data now suggest the existence of two functionally separate pools of gonadotropin: one that is acutely releasable (and has been termed sensitivity) and a second that is released only with sustained stimulation (termed reserve).86 Together, these pools define pituitary capacity. During the early follicular
phase, when E₂ levels are low, both gonadotropin sensitivity and reserve are at a minimum. As E₂ levels increase during the midfollicular phase, a preferential increase in reserve occurs first. As E₂ increases further toward midcycle, both sensitivity and reserve become maximal until the sensitivity becomes such that the midcycle release of LH occurs. The surge also is partly a result of what is an estrogen-dependent self-priming effect of GnRH: A second pulse of GnRH elicits greater release of gonadotropin than does the first pulse in the estrogen-primed state87 (Fig. 95-12).