Sampling follicular fluid without altering follicular status in cattle: oestradiol concentrations early in a follicular wave.

A transvaginal, ultrasound-guided in situ technique was developed for sampling follicular fluid from 6.0-12.5 mm follicles in cattle, with minimal interference with the subsequent development of the sampled follicle. Each follicle was sampled only once and the status of the follicle (future dominant or subordinate) was determined retrospectively. A sample (20 microliters) was successfully obtained from 77% of 132 targeted follicles. Reasons for considering an attempt unsuccessful were as follows: (1) ovaries difficult to manipulate in two heifers (eight follicles), (2) samples contaminated with blood (11 samples), and (3) follicles sampled but the antrum subsequently developed an apparent blood clot (11 follicles). Only the 102 successful collections were used in the statistical analyses for evaluating oestradiol concentrations in the follicular fluid and the follicle growth profiles before and after sampling. The follicles were sampled once on days 1, 2, 3, or 4 after the emerging dominant follicle was 4 mm in diameter. On day 1, there were no differences in the oestradiol concentrations among follicles that later became the dominant follicle, largest subordinate follicle, or smaller subordinate follicles (means for diameters, 6.7-6.9 mm; means for oestradiol, 30-42 ng ml-1). The mean diameter of the dominant follicle increased linearly over the 4 days. Concentrations of oestradiol in the dominant follicle increased curvilinearly, resulting from a slower increase between days 1 (mean, 42 ng ml-1) and 2 (110 ng ml-1) than between days 2 and 3 (313 ng ml-1) and 3 and 4 (554 ng ml-1). Neither mean diameter nor mean oestradiol concentration of the largest subordinate follicle increased after day 2. Data were available from 19 follicular waves in which both the dominant and largest subordinate follicles were sampled on the same day. Oestradiol concentrations were not higher in the dominant follicle than in the largest subordinate follicle until the day after the two follicles began to deviate in growth rates (mean day of deviation, 2.5 +/- 0.2 days after emergence). These observations indicate that the future dominant follicle cannot be identified reliably by either its diameter or oestradiol production before the deviation in growth rates between the two largest follicles.