COMPARATIVE FOLLICULAR DEVELOPMENT IN MEISHAN AND LARGE WHITE GILTS DURING PREPUBERTAL PERIODS AND ITS RELATION TO HORMONAL-STIMULATION

Changes in follcular development in the left ovary and ovarian respons e 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). L eft ovaries were removed at the time of hormonal treatments and the re maining ovaries were removed 18 h after hCG was injected alone (hCG) o r 72 h after eCG was injected (eCG-hCG). Both nonatretic and atretic ( > 4 pyknotic bodies) antral follicles > o.6 mm in diameter were identi fied in the left ovaries by histological techniques. LH-sensitive foll icles were identified in the right ovaries by counting follicles with dispersed cumulus cells around the oocytes. All Large White gilts show ed 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 mea n number of nonatretic antral follicles > 0.6 mm remained constant at 3 and 1 SD before puberty, while in Meishan gilts, the mean number inc reased as gilts got older. Mean percentage of class 1 nonatretic folli cles (0.6-1.12 mm) of the total number of nonatretic follicles increas ed 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 Wh ite gilts, the percentage distribution of nonatretic antral follicles of size classes 1 and 3 remained unchanged with age. Mean percentage o f 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 a t 3 and 1 SD before puberty (0.6 and 1.5%). Rate of atresia increased with age in both breeds (p < 0.01). Mean percentage of class 1 atretic follicles of the total number of atretic follicles was greater in Lar ge White than in Meishan gilts (p < 0.002), while that of class 3 was greater in Meishan than in Large White gilts (p < 0.01). Between 3 and 1 SD before puberty the percentage of class 1 atretic follicles incre ased significantly in Meishan gilts (18.3 vs. 64.8%, p < 0.0001) while those of class 2 and 3 decreased significantly (40.0 vs. 20.3 for cla ss 2 and 31.7 vs. 11.5% for class 3, p < 0.03). Mean percentage of cla ss 4 atretic follicles was greater in the Meishan than in the Large Wh ite breed (5.6 vs. 0.4%, p < 0.01). The proportion of gilts with LH-se nsitive 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, EO R) was greater in Meishan than in age 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 2 SD, respectively, while the proportion of Meishan gilts wi th LH-sensitive follicles was 33, 66, and 100% at 3, 2, and 1 SD befor e 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 f or the most part in Large White gilts at 1 SD (3-SD gilts were not con sidered 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 treatm ent, LH-sensitive follicles originated the most part from >3.57-mm fol licles in both breeds. These results indicate that follicular developm ent is completed at 3 SD before puberty in Large White gilts, but the sensitivity of large follicles to hCG is achieved earlier in Meishan g ilts and is independent of the total population of ovarian follicles. Moreover, it appears that Large White gilts need FSH support for compl ete maturation of ovarian follicles for luteinization to occur.