Effect of a low dose of pregnant mare's serum gonadotropin on follicular recruitment and atresia in cycling rats.

Atresia appears to play a central role in selecting the correct number of follicles for ovulation in the rat. A wave of atresia, apparent by noon on metestrus, reduces the number of large healthy follicles to the appropriate quota for ovulation. The purpose of this study was to test the hypothesis that falling levels of gonadotropin on the morning of estrus precipitate the wave of atresia of large follicles seen on metestrus. Endogenous concentrations of follicle-stimulating hormone (FSH) were augmented by a single injection of pregnant mare's serum gonadotropin (PMSG; 0.025 IU/gram body weight) administered at different times during the periovulatory period. Animals given PMSG at 0800 h on estrus (when the endogenous FSH surge was waning) had a supernormal number of large healthy follicles in their ovaries at 1200 h on metestrus. This increase in large healthy follicles was accompanied by a decrease in large atretic follicles in the ovaries. The same dose of PMSG, when administered at other times during the periovulatory period, did not affect any of the parameters measured. These observations suggest that the wave of atresia normally seen in large follicles on metestrus is triggered by the decline in the concentration of FSH during the morning of estrus and can be prevented by prolonging the surge of FSH with administration of PMSG.