Intrafollicular changes in the largest follicle (F1) and second-largest (F2) follicle were examined in relation to follicle diameter deviation. Deviation is characterized by continued growth of the largest follicle and the cessation of growth of the smaller follicles. Granulosa cells and follicular fluid were obtained from slaughterhouse ovaries (n = 95 pairs, experiment 1), and follicular fluid was collected in vivo (n = 28 heifers, experiment 2). Several ranges in the diameter of F1 were used to represent the progressive growth of the follicle. The diameter range with the first significant increase in the difference between F1 and F2 was determined for each end point and was used as an indicator of the sequence of events associated with diameter deviation. An increased difference for diameter and for estradiol concentration occurred (P < 0.05) simultaneously at the 8.5- to 8.9-mm range in both experiments. In experiment 1, the increased difference between F1 and F2 in LH receptor (LHr) mRNA expression occurred (P < 0.05) at the 8.0- and 8.4-mm range. In F2 of experiment 2, there was a progressive decrease (P < 0.05) in free insulin-like growth factor (IGF)-1 and a progressive increase (P < 0.05) in IGF binding protein (BP)-2 across the follicle-diameter ranges (7.5-11.2 mm). No differences were detected between F1 and F2 for 3β-hydroxysteroid dehydrogenase mRNA expression in experiment 1 and testosterone, total inhibin, and dimeric inhibin-A concentrations in experiment 2. The results indicated that the acquisition of granulosa cell LHrs by F1, as indicated by increased LHr mRNA expression, occurred one diameter range before an increased difference between F1 and F2 for diameter or estradiol concentrations. On a temporal basis, it is concluded that LHr acquisition plays a role in the establishment of diameter deviation. In addition, the reduced growth of F2 may have involved the reduced bioavailability of IGF-1 in association with elevated IGFBPs.
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