The bimodular development of ovarian follicles in cyclic guinea pigs and differences between antral follicles developed at different phases of the cycle

Abstract

The results of a histological, [3H]thymidine uptake, and autoradiographic study of the cyclic guinea pig ovary suggest that there are two consecutive waves of antral follicle development within each estrous cycle, with the first wave extending from days 2 to 10 and the second wave from days 7 to 16 of the 16-day cycle. The bimodular development is apparently made possible by the continuous daily recruitment of growing follicles from the pool of dormant primordial follicles and by the occurrence of two occasions (days 1–3 and days 6–9) when gonadotropin secretory activity allows some of these follicles to progress into the antral phase of development. The antral follicles of the first wave attain their maximum size (0.5 mm3) on day 9. However, they have disappeared by day 12 and in their place, large atretic follicles and the smaller healthy antral follicles of the second wave have become the most prominent follicular features. On days 14 and 15, the antral follicles of the second wave have grown to 0.5 mm3 themselves, and within the next 24 h their volume increases rapidly by 150% to 1.23 mm3. Extrusion of first polar bodies, vaginal estrus, and ovulation then follow in quick succession. The results indicate that the follicles of the first wave become atretic before the final rapid growth phase and resumption of meiotic division. The absence of the rapid growth phase probably indicates that they have never reached final maturity, nor have they achieved the competence to ovulate in response to gonadotropin stimulation.