HORMONAL INDUCTION OF OVULATION STIMULATES ATRESIA OF ANTRAL FOLLICLES IN GILTS

Two breeds of prepubertal gilts known to differ in their ovarian devel opment were used to compare the effect of hormonally stimulated ovulat ion on follicle numbers and proportion of follicular atresia in size c lasses other than preovulatory follicles. Hormonal treatments (hCG or eCG-hCG) were given to Meishan gilts (n = 36) at 3, 2, and 1 standard deviations (SD) before the mean age of puberty (51, 64, and 77 days of age) and to Large White gilts (n = 24) at 3 and 1 SD before the mean age of puberty (140 and 166 days of age). Ovarian follicle populations determined in the right ovaries 18 h after hCG injection alone (hCG) or 72 h after eCG injection (eCG-hCG) were compared with those in the left ovaries, which had been removed prior to injections. Follicles wi th dispersed cumulus cells around the oocytes (expected ovulations) ob served after hormonal treatments (right ovaries) were considered for f ollicular measures and categorized into specific size classes accordin g to their diameter. Human CG alone did not affect any follicular para meters measured at any age in either of the two breeds studied. The ov erall population of non atretic follicles was significantly reduced by the eCG-hCG treatment in the Large White breed when imposed at either 3 or 1 SD before the mean age of puberty (262.6 vs. 158.8; p < 0.01). Among the various follicle size classes studied, eCG-hCG treatment si gnificantly decreased the mean number of follicles in size classes 2 ( 1.13-2.00 mm in diameter) and 3 (2.01-3.56 mm in diameter) at either 3 or 1 SD before the mean age of puberty in the Large White gilts (p < 0.001). Conversely, the mean number of class 4 nonatretic follicles in Large White gilts was increased significantly by the eCG-hCG treatmen t at all of the experimental ages studied (p < 0.0001). In the Meishan breed, the overall population of antral follicles remained unchanged after eCG-hCG treatment regardless of time of injection. However, when considering the various size classes of follicles in these animals, o nly class 4 nonatretic follicles were increased significantly by eCG-h CG treatment at all of the experimental ages studied (p < 0.0006). For the atretic follicles, class 3 mean number of follicles was decreased after eCG-hCG treatment in the Meishan breed (p < 0.05). In addition, there was a significant interaction (p < 0.05) between hormonal treat ment and age at injection on the number of class 3 atretic and nonatre tic follicles. Proportion of follicular atresia in size classes 1, 2, and 3 was significantly increased by eCG-hCG treatment at either 3 or 1 SD before the mean age of puberty in Large White gilts (p < 0.01). C onversely, hormonal treatment tended to decrease the proportion of fol licular atresia of class 4 follicles in Large White gilts at all exper imental ages (p < 0.09). In Meishan gilts, however, eCG-hCG treatment increased the proportion of follicular atresia of class 4 Collides al all experimental ages (p < 0.03). Hormonal treatments increased the nu mber of late atretic follicles in size classes 3 and 4 (> 3.56 mm in d iameter) in Large White gilts. These results indicate that hormonal tr eatment with eCG-hCG limits follicular growth by increasing atresia of large follicles, thus controlling ovulation rates via a local mechani sm that is dependent on the follicular population present at the time of treatment.