Regulation of folliculogenesis in the cycling rhesus monkey: selection of the dominant follicle.

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 one abolished the expected midcycle surges of gonadotropin secretion. In 8 monkeys of this group, surges of LH and FSH 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 fifth 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 ipsilaternal ovulation or impair progesterone secretion by the CL in 2 of 2 monkeys. These observations indicate that, by the middle of the follicular phase, the follicle destined to ovulate had been selected, and that 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: a) that the prevailing ovarian steroidal milieu at ablation had no discernible differential effect on the time-course of resumed ovarian activity, and b) that midcycle surges of estrogen or gonadotropin secretion were not required either to initiate or synchronize subsequent follicle growth.