Identification of potential intrafollicular factors involved in selection of dominant follicles in heifers.

A surgical procedure to aspirate follicular fluid concurrently from individual follicles from the same heifer was validated and used to determine if intrafollicular amounts of estradiol, progesterone, inhibins, activin-A, follistatins, and insulin-like growth factor binding proteins (IGFBP) differed for the future dominant compared with subordinate follicles during selection of the first wave dominant follicle. Heifers were subjected to surgery and aspiration of follicular fluid from the two or three largest follicles on Day 3 of the estrous cycle (approximately 1.5 days after emergence). Ultrasound was used to determine the fate of each aspirated follicle after surgery. At aspiration, diameter of the future dominant and largest subordinate follicle was similar in heifers. However, estradiol was higher, whereas IGFBP-4 was lower in the future dominant compared with the largest or next largest subordinate follicles. Also, the future dominant follicle in most cohorts had the highest estradiol and lowest IGFBP-4 compared with future subordinate follicles. We concluded that: IGFBP-4 and estradiol may have key roles in determining the physiological fate of follicles during selection of the first wave dominant follicle in heifers, and that both are reliable markers to predict which follicle in a growing cohort of 5- to 8.5-mm follicles becomes dominant.