Initiation of the primate ovarian cycle with emphasis on perimenarchial and postpartum events

In collating the issues discussed in this review, we have summarized the initiation of the primate ovarian/menstrual cycle into two conceptual illustrations. Figure 27 depicts the progression of folliculogenesis in perimenarchial monkeys; Figure 28 compares the restoration of recruitment, selection, and maturation of the dominant follicle in postpartum monkeys, with and without an infant at breast. The following conclusions, some tentative or partial, seem warranted by the data. In perimenarchial monkeys: 1. The late pubertal cascade included progression toward greater ovarian asymmetry of estradiol secretion. 2. This asymmetry of estrogen synthesis may be indicative of recruitment and selection of the dominant follicle, even though ovulation is not yet achieved. 3. Quantitative and qualitative changes in pulsatile pituitary gonadotropin secretion accompany the onset of cyclic ovarian function. 4. The enhanced biological activity of LH, especially during the preovulatory surge induced by the instigation of estrogen positive feedback in the advanced postmenarchial interval, may be a critical development step toward normal ovulatory menstrual cycles. In postpartum monkeys: 1. Reestablishment of ovulatory cycles is a gradual process in non-nursing monkeys, with initial follicular growth supporting endometrial proliferation but failing to culminate in ovulation. Immediately thereafter, ovulatory menstrual cycles ensue. 2. After nursing and weaning, resumption of ovulation is prompt, with the first menses coming after the first luteal phase. 3. Gestation-induced refractoriness of the hypothalamic-pituitary unit to estrogen positive feedback or GnRH challenges is highly transient. 4. While nursing continues, inhibition of the H-P-O axis is absolute; but after weaning the reacquisition of its functional integrity is rapid. 5. Nursing sustains the block to estrogen positive feedback, probably emanating from a mechanism(s) distinctly unlike that imposed by the pregnancy milieu. 6. Persistence of postpartum anovulation surely illustrates inadequate tonic gonadotropin secretion, as opposed to constraints arising from an inoperative surge mode of FSH/LH release.