Effects of pregnant mare's serum gonadotropin treatment on follicular populations and ovulation rates in prepuberal gilts with two morphologically different ovarian types

Two experiments were conducted to determine the effects of pregnant mare's serum gonadotropin (PMSG) on follicular populations and ovulation rates in prepuberal gilts with grape-type (GT) and honeycomb-type (HT) ovaries. The follicular populations were determined at 170 d of age (d 0) and 19 d after PMSG (d 19). In Exp. 1, the mean number of macroscopic follicles of Classes 2 and 3 was greater (P < .05) in GT (n = 11) than in HT (n = 32) ovaries at d 0, whereas the mean number of those of Class 1 was greater (P < .05) in HT ovaries. At d 19, no difference was observed between the two ovarian types for any class of follicles. The PMSG-induced ovulation rates were comparable between the two ovarian types (8.3 vs 7.9, GT vs HT, respectively; P > .10). In Exp. 2, the microscopic follicular populations were determined on right and left ovaries removed, respectively, on d 0 and 19. At d 0, GT ovaries (n = 5) contained a greater number of Class 5 nonatretic (P < .01) and atretic (P < .02) follicles than did HT ovaries (n = 5), whereas at d 19 the mean number was not statistically different between the two ovarian types (P > .10). In contrast, gilts with HT ovaries contained a greater (P < .01) number of Class 4 atretic follicles than gilts with GT ovaries at d 0, whereas at d 19 the mean number was not statistically different between the two ovarian types (P > .10). Furthermore, the rate of atresia of follicles of Classes 4 and 5 was greater (P < .08) for gilts with HT ovaries at d 0, whereas 19 d after unilateral ovariectomy and PMSG injection it was lower (P < .09) in HT ovaries for Class 5 follicles and similar (P > .10) between GT and HT for Class 4 follicles. Ovulation rates were higher (P < .05) in GT than in HT ovaries (13.4 vs 8.8). These data suggest that the higher number of Class 5 nonatretic follicles observed in GT than in HT ovaries at d 0 seems to be associated with a lower rate of atresia in the two largest size classes of follicles and that the higher ovulation rates observed in such gilts indicate the existence of some interaction between ovarian type and ovarian compensation.