Comparative follicular development in Meishan and Large White gilts during prepubertal periods and its relation to hormonal stimulation

Changes in follicular development in the left ovary and ovarian response to hormonal injections (hCG or eCG and hCG) in the right ovary were studied in 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 in Large White gilts (n = 24) at 3 and 1 SD (140 and 166 days of age). Left ovaries were removed at the time of hormonal treatments and the remaining ovaries were removed 18 h after hCG was injected alone (hCG) or 72 h after eCG was injected (eCG-hCG). Both nonatretic and atretic (> 4 pyknotic bodies) antral follicles > 0.6 mm in diameter were identified in the left ovaries by histological techniques. LH-sensitive follicles were identified in the right ovaries by counting follicles with dispersed cumulus cells around the oocytes. All Large White gilts showed the presence of antral follicles at both 3 and 1 SD before puberty while 6, 7, and 12 Meishan gilts had antral follicles > 0.6 mm at 3, 2, and 1 SD before puberty, respectively. In Large White gilts, the mean number of nonatretic antral follicles > 0.6 mm remained constant at 3 and 1 SD before puberty, while in Meishan gilts, the mean number increased as gilts got older. Mean percentage of class 1 nonatretic follicles (0.6-1.12 mm) of the total number of nonatretic follicles increased significantly with age in Meishan gilts (p < 0.02), while those of class 3 (2.01-3.56 mm) decreased significantly (p < 0.02). In Large White gilts, the percentage distribution of nonatretic antral follicles of size classes 1 and 3 remained unchanged with age. Mean percentage of largest size class of nonatretic follicles (> 3.57 mm) was greater (p < 0.01) at 3 and 1 SD in Meishan gilts (5.7 vs. 4.8%) but not at 2 SD (0.5%) where it was similar to those observed in Large White gilts at 3 and 1 SD before puberty, the percentage of class 1 atretic follicles increased significantly in Meishan gilts (18.3 vs. 647.8%, p < 0.0001) while those of classes 2 and 3 decreased significantly (40.0 vs. 20.3 for class 2 and 31.7 vs. 11.5% for class 3, p < 0.03). Mean percentage of class 4 atretic follicles was greater in the Meishan than in the Large White breed (5.6 vs. 0.4%, p < 0.01). The proportion of gilts with LH-sensitive follicles after hCG treatment was only 50% of those treated in the Large White group at both age periods, while in Meishan gilts it increased from 50% at 3 SD to 100% at 1 SD. Among gilts responding to hCG, the number of LH-sensitive follicles (expected ovulation rate, EOR) was greater in Meishan than in Large White gilts at 3 SD (7.0 vs. 2.3 EOR) but not at 1 SD (9.2 vs. 10.0 EOR). Injection of eCG 72 h before hCG induced positive response in 100% and 83% of Large White gilts at 3 SD and 1 SD, respectively, while the proportion of Meishan gilts with LH-sensitive follicles was 33, 66, and 100% at 3, 2, and 1 SD before puberty, respectively. Among gilts responding to eCG-hCG, the number of LH-sensitive follicles was greater in Large White than in Meishan gilts at 3 SD (14.0 vs. 5.5 EOR) and at 1 SD before puberty (14.4 vs. 12.3 EOR). LH-sensitive follicles originated from > 3.57-mm follicles for the most part in Large White gilts at 1 SD (3-SD gilts were not considered because of the low response in the few gilts with LH-sensitive follicles) and from 1.13-2.00- and 2.01-3.56-mm follicles in Meishan gilts at 3 and 1 SD before puberty, respectively. After eCG-hCG treatment, LH-sensitive follicles originated for the most part from > 3.57-mm follicles in both breeds. These results indicate that follicular development is completed at 3 SD before puberty in Large White gilts, but the sensitivity of large follicles to hCG is achieved earlier in Meishan gilts and is independent of the total population of ovarian follicles. Moreover, it appears that Large White gilts need FSH support for complete maturation of ovarian follicles for luteinization to occur.