Quantitative Determination of Follicle Size Distribution in the Guinea Pig Ovary after Hemicastration and PMSG Treatment

In order to investigate the action point of intraphysiological or supraphysiological elevation of FSH during the preovulatory period on follicular development, adult guinea pigs underwent unilateral ovariectomy on days 10, 12 and 14 of the estrous cycle (N=6 each group). Thereafter, guinea pigs were injected twice daily with either vehicle or pregnant mare's serum gonadotropin (PMS). After 2 days, the remaining ovaries were removed. The resected ovaries were fixed, embedded in paraffin, serially sectioned (7μm) and stained with Azan. All follicles >70μm were classified by size and atretic stage. The follicular size distribution was not affected by hemicastration at day 10, although the ratio of atretic follicles (>400μm) decreased from 51% to 32%(P400μm) decreased from 35% to 23%. The peak size distribution of follicles was shifted from 70-99μm to 200-299μm by PMS, and follicles 600-899μm in size contained an increased percentage of atresia, which resulted in the bimodal distribution of viable follicles >400μm. These data suggest that 2 day hemicastration promotes an influx of primordial follicles into growing follicles and suppresses the atretic process by a different mechanism depending on the date of hemicastration in the estrous cycle. Conversely, hemicastration+PMS accelerated viable follicle growth to increase the percentage of atresia.