Follicle Deviation and Intrafollicular and Systemic Estradiol Concentrations in Mares1

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Abstract
By definition, follicle deviation begins on the day the two largest follicles of a wave begin to differ in growth rates. The relationships between follicle deviation and intrafollicular and systemic estradiol concentrations were studied in ponies, using a two-follicle model in which all but the two largest follicles were ablated. A 20-μl sample of follicular fluid was obtained from each of the two follicles by transvaginal ultrasonography. In experiment 1, the two follicles were sampled when the larger follicle reached 15 mm. No differences (p > 0.05) in post-sampling follicle characteristics were found between control (n = 6) and sampled (n = 8) groups except that the growth rate was slower (p < 0.01) in the larger follicle between the day of sampling and the next day (0.7 ± 0.7 mm per day) than in the controls (3.3 ± 0.3 mm per day). The growth rates between 2 and 5 days after sampling were not different between groups. Follicular fluid estradiol-17β concentrations were higher (p < 0.007) in the larger follicle (460 ± 67 ng/ml; diameter, 16.4 ± 0.4 mm) than in the smaller follicle (322 ± 50 ng/ml; diameter, 14.6 ± 0.6 mm). In experiment 2, the pair of follicles was sampled when the larger follicle reached 15 mm, 20 mm, or 25 mm (n = 5 per group). There were no significant differences among the three groups for day of deviation and diameters of larger and smaller follicles at deviation. The difference in diameter between the larger and smaller follicles was similar for the 15-mm (2.2 ± 0.9 mm) and 20-mm (3.1 ± 1.0 mm) groups, but the difference between follicles for the 25-mm group (7.9 ± 1.2 mm) was greater (p < 0.004) than for the other two groups. In contrast, the differences in estradiol concentrations between the larger and smaller follicles increased (p < 0.0001) progressively for the 15-mm (13.0 ± 86.8 ng/ml), 20-mm (722.0 ± 173.8 ng/ml), and 25-mm (1873.5 ± 310.3 ng/ml) groups. The first significant (p < 0.007) increase in systemic estradiol occurred between the day before and the day of the beginning of deviation. Detection of an increased difference in estradiol concentrations between the two follicles before the detection of a change in differences in diameter suggests, on a temporal basis, that estradiol is a candidate for involvement in the mechanism that leads to follicle-diameter deviation in mares.