Regulation of Folliculogenesis in the Cycling Rhesus Monkey: Selection of the Dominant Follicle

Abstract

To identify factors regulating the initiation of follicle growth in adult primates, the ovarian cycle of sexually mature rhesus monkeys was interrupted by surgical ablation of the preovulatory follicle or functioning corpus luteum (CL). In 10 of 10 animals, cautery of the largest visible follicle on day 8-12 of the cycle blocked ovulation, and in all but 1 abolished the expected midcycle surges of gonadotropin secretion. In 8 monkeys of this group, surges of LH [luteinizing hormone] and FSH [follicle-stimulating hormone] release occurred 12.4 .+-. 0.9 days (d) (mean .+-. SE) after cautery, coincident with elevations in serum estrogens, and succeeded by typical luteal phase patterns of circulating progesterone (P). No gonadotropin or estrogen surges were observed during the next 32 days of sampling in the remaining pair, despite visible new vesicular follicles. Removal of the CL in 5 of 5 monkeys 4-6 days after the midcycle LH surge was followed by a reduction in serum P to less than 0.25 ng/ml within 24 h and by the onset of menses within 3-4 days. After luteectomy in 4 of the 5 animals, preoperative levels of LH and FSH were maintained until 12.8 .+-. 0.9 days, when typical surges of gonadotropin secretion occurred, followed by a normal luteal phase pattern of P. The 5th luteectomized monkey menstruated again 25 days after ablation without intervening surges of estrogen or gonadotropin release and did not ovulate. Sham follicle cautery did not block ipsilateral ovulation or impair progesterone secretion by the CL in 2 of 2 monkeys. By the middle of the follicular phase, the follicle destined to ovulate had probably been selected, and no other follicles were soon competent to mature. That the interval from ablation, at either phase of the cycle, until the next ovulation was the same indicates: that the prevailing ovarian steroidal milieu at ablation had no discernible differential effect on the time-course of resumed ovarian activity, and that midcycle surges of estrogen or gonadotropin secretion were not required either to initiate or synchronize subsequent follicle growth.