Figure 1.

Mathematical decay curve depicting the rate of oocyte loss from ovaries of young adult rhesus monkeys. Past work from Vermande-Van Eck published in 1956 reported that female rhesus monkeys at or just after sexual maturation (~4 years of age) possess ~58,000 oocyte-containing follicles per ovary, with atresia occurring at an incidence of 4.5% and clearance of atretic follicles completed within 14 days.⁵⁶ Plotting these three variables in a mathematical decay curve, in the absence of any new oocyte input, predicts that >90% of the oocytes present at puberty will be eliminated by the time these females reach 6 years of age and <500 oocytes will remain at 8 years of age. Such an outcome is highly discordant with the fact that female rhesus monkeys exhibit ovarian function and natural fertility at ≤20 years of age. The most logical explanation for this observation is that the loss of oocytes through follicular atresia is partly offset by de novo oogenesis and folliculogenesis during adulthood, to allow maintenance of ovarian function through 20 to 25 years of age.