Effects of heat stress on follicular development in PMSG-treated immature rats
2000
Shimizu, T. (Tsukuba Univ., Ibaraki (Japan). Inst. of Agricultural and Forest Engineering) | Ohshima, I. | Kanai, Y.
Effects of heat stress on follicular development was studied in three-week-old immature female rats kept under 25 degrees C and 50% relative humidity (RH). In experiment 1, animals were divided into two groups (Control: C and Heat stress : HS, n = 5 each). They received subcutaneous injections of 20 IU of PMSG and 20 IU of hCG 48 h apart and the number of oocytes in the oviducts was counted 24 h after hCG injection. HS consisted of exposure to a 35 degrees C and 70% RH condition from 48 h before PMSG until time of hCG injection. In experiment 2, animals were subjected to the same temperature and PMSG treatments as experiment 1, and their ovaries were removed for histological examinations immediately before (n=5 each) or 48 h after (n=4 each) PMSG injection. The number of ovulated oocytes was significantly lower in HS than in C (10.0 +- 1.7 vs 17.8 +- 1.8, P0.001). Histological observations revealed that while the number of follicles dwindled as the follicles grew from preantral to antral stages, the percentage of atretic follicles drastically increased. PMSG apparently reduced the rate of atresia of antral follicles in C, but not in HS. Instead, heat stress hastened this stage-dependent follicular atresia. These results indicate that heat stress diminishes PMSG-induced ovulatory responses and enhances atresia of growing follicles in immature female rats
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