Alterations in intrafollicular regulatory factors and apoptosis during selection of follicles in the first follicular wave of the bovine estrous cycle

Abstract

Changes in follicular fluid (FF) concentrations of estradiol, inhibin forms, and insulin-like growth factor binding proteins (IGFBPs), percentage of apoptotic granulosa cells (%A), and follicular size for individual follicles in a growing cohort were determined throughout the first wave of follicular development during the bovine estrous cycle and related to FSH decline. Four groups of heifers (n = 31) were ovariectomized between Days 1.5 and 4.5 of the estrous cycle at 5 +/- 1, 33 +/- 2, 53 +/- 1, and 84 +/- 2 h after the periovulatory peak in FSH concentrations. Follicles > or = 2.5 mm were dissected, measured, and FF aspirated. The five largest follicles were ranked based on their diameter (F1 to F5). Diameters of F1 to F5 were positively correlated with interval from FSH peak (r > or = 0.6, P < 0.05). Five hours after the FSH peak, follicular diameter and FF concentrations of estradiol, inhibins, and IGFBPs were similar for F1 to F5. From 5 to 33 h, amounts of the six precursor inhibin forms (> or = 48 kDa) increased (P < 0.05) in F1 follicles. The IGFBPs in F1 follicles remained low at all time periods. At 33 h, amounts of IGFBP-4 and -5 were higher (P < 0.05) in F4 and F5 compared with F1 follicles. At 84 h, IGFBP-2, -4, and -5 were increased (P < 0.05) in F3, F4, and F5 compared with F1. At 5, 33, or 53 h, %A was not different between follicles in any size class. At 84 h %A was increased (P < 0.05) in follicles <6 mm in diameter. However, at that time, %A did not differ between the selected DF and the largest subordinate follicle. For individual heifers, the selected DF at 84 h was largest in size, highest in estradiol, and lowest in IGFBP-2 and -4. The F1 follicle had highest estradiol in 23 of 27 heifers irrespective of stage of the wave and lowest IGFBP-4 in 19 of 21 heifers from 33 h. We concluded that the earliest intrafollicular changes that differentiate a dominant-like follicle from the growing cohort are enhanced capacity to produce estradiol and maintenance of low levels of IGFBPs.