Selection of the dominant follicle in cattle: establishment of follicle deviation in less than 8 hours through depression of FSH concentrations

摘要

In experiment 1, the role of the first follicle to reach 8.5 mm was studied by follicle ablation (0 h). The combined mean FSH concentrations for the control (n = 8) and ablation group before ablation (n = 7) progressively decreased (P<0.02) over two8-h intervals before the largest follicle reached >=8.5 mm (-16 h, 1.77±0.11 ng/ml; 0 h, 1.49 ±0.08 ng/ml). In controls, the concentrations continued to decrease (P<0.02) until 10 h (1.21±0.09 ng/ml). Ablation of the largest follicle at >=8.5 mm increased(P<0.02) FSH concentrations between 5 (1.34±0.04 ng/ml) and 8 h (1.61±0.09 ng/ml). Growth rate of the second largest follicle between 0 and 8 h was greater (P<0.05) in the ablation group than in the controls, and the second largest follicle became dominant in all of 7 heifers following ablation of the largest follicle. In experiment 2, a minimal injection of a depressant of FSH (4.4 ml steroid-reduced follicular fluid) was given when the largest follicle was 8.4 mm (0 h; controls, n = 4; treated, n = 4). An interaction of group and hour (P<0.005) for FSH concentrations was attributable to a decrease (P<0.002) in FSH by 6 h and an increase (P<0.002) between 9 and 12 h in the treated group. The growth rate of the largest follicle between 0 and 12 h waslower (P<0.05) in the treated (0.2±0.2 mm/12 h) than in the control group (1.2±0.4 mm/12 h). The reduced diameter was recorded within 6 h after suppression of FSH concentrations. It is concluded that when the largest follicle reaches a critical diameterof approximately >=8.5 mm, FSH concentrations continue to decrease and become lower than required by the smaller follicles but not the largest follicle. A close temporal coupling between a change in FSH concentrations and the follicular response could establish the deviation mechanism in <8 h or before the second largest follicle reaches a similar critical diameter.