In cattle, the two largest follicles of a wave (F1, F2) begin to deviate into a dominant follicle and a subordinate follicle when F1 is a mean of 8.5 mm in diameter. After the beginning of deviation, F1 and F2 are diameter-defined dominant and subordinate follicles. Changes associated with the conversion of F2 into a future dominant follicle were studied by ablating F1 at the expected beginning of deviation (F1, 8.5 mm; Hour 0) and assessing the follicular-fluid factors in F2. Follicles were designated F1C and F2C in controls and F2A in F1-ablated heifers. Follicular-fluid collections were made at Hours 0, 4, 8, or 12 (n = 7 heifers per hour; fluid from F1C, F2C, and F2A; experiment 1) or at Hours 4, 6, 8, 10, or 12 (n = 9 heifers per hour; fluid from F2A; experiment 2). Postablation concentrations of circulating FSH increased (P < 0.05) between Hours 2 and 6. Diameter of F2A increased (P < 0.05) after Hour 8 in both experiments so that the diameter of F2A at Hours 10 or 12 was not different (P > 0.1) from the diameter of F1 at Hour 0. A transient elevation (P < 0.05) in follicular-fluid activin A occurred in F2A at Hour 8 in both experiments. Concentrations of estradiol (P < 0.05) and insulin-like growth factor I (IGF-I; P < 0.1) decreased in F2C by Hour 8. In F2A, the concentrations of both factors began to increase (P < 0.05) after Hours 4 or 8 so that there was no difference (P > 0.1) between F1C and F2A at Hour 12. Concentrations of IGF-I and IGF binding protein 2 (IGFBP-2) in F2A changed in opposite directions at the same hours. No differences between follicles were found for concentrations of progesterone, androstenedione, inhibin A, and inhibin B. The order of events in the conversion of a future subordinate follicle to a future dominant follicle was an increase in systemic FSH, a transient elevation in follicular-fluid activin A, and a simultaneous increase in follicular-fluid estradiol and restoration of an apparent growth-compatible balance of free IGF-I and IGFBP-2.
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