The development of a dominant ovarian follicle through the production of increasing amount of estradiol toward midcycle normally elicits a surge of gonadotropins both LH and FSH-of very exact specifications. The ultimate role of the midcycle gonadotropin surge is to prepare the follicle and its cells for oocyte extrusion (ovulation), to induce resumption of meiosis in the arrested oocyte, and, finally, to induce the changes in the follicular cells to form an active corpus luteum.
Estradiol plays a central role in the secretion of LH by the pituitary gland. It seems that an initial priming effect of estradiol is needed to achieve the subsequent LH rise, as well as a second stage rapid estradiol increase that is required for an acute and sustained LH surge. Another mechanism of the midcycle gonadotropin surge control has been that the estradiol controls the number of synapses on GnRH cells and between the presynaptic elements connecting with GnRH releasing cells. It may be that GnRH-producing elements are under constant inhibition by higher neural centers, and the midcycle peak of activity is a result of estradiol-induced removal (depression) of this inhibition.
Progesterone is also a key signal in the complicated mid cycle dynamics. An increase in progesterone can be detected in the venous effluent of the ovary bearing the preovulatory follicle as early as day 10 of the cycle, but significant rises in progesterone can be measured some 12 hours before the LH surge onset. Progesterone synthesis in the preovulatory follicle is probably the result of LH activation of FSH-induced LH receptors on granulosa cells. This small increment in progesterone, appearing after ≥ 48 to 60 hours of estrogen priming, will facilitate both a timely and adequate LH surge, as well as an adequate FSH surge.
Incremental (rising) estradiol sustained for approximately 60 hours constituted the ovarian signal for initiating the mid cycle surge, that a small rise in progesterone was required to establish the normal dimension of the LH and FSH surge, and this will advance the surge onset by approximately 15 hours.
