Follicle deviation and intrafollicular and systemic estradiol concentrations in mares

By definition, follicle deviation begins on the day the two largest follicl es of a wave begin to differ in growth rates. The relationships between fol licle deviation and intrafollicular and systemic estradiol concentrations w ere studied in ponies, using a two-follicle model in which all but the two largest follicles were ablated. A 20-mu l sample of follicular fluid was ob tained from each of the two follicles by transvaginal ultrasonography. In e xperiment 1, the two follicles were sampled when the larger follicle reache d 15 mm. No differences (p > 0.05) in post-sampling follicle characteristic s 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 d ay of sampling and the next day (0.7 +/- 0.7 mm per day) than in the contro ls (3.3 +/- 0.3 mm per day). The growth rates between 2 and 5 days after sa mpling were not different between groups. Follicular fluid estradiol-17 bet a 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 fo r day of deviation and diameters of larger and smaller follicles at deviati on. 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) w as greater (p < 0.004) than for the other two groups. In contrast, the diff erences in estradiol concentrations between the larger and smaller follicle s 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 b etween the day before and the day of the beginning of deviation. Detection of an increased difference in estradiol concentrations between the two foll icles 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.