OVARIAN FOLLICULAR POPULATIONS AND IN VITRO STEROIDOGENESIS ON THREE DIFFERENT DAYS OF THE BOVINE ESTROUS CYCLE 1,2

This study was undertaken to determine changes in follicular populations on ovaries of dairy cows during three stages of the estrous cycle and their steroidogenic capacity in vitro. Numbers of small (2.0 to 5.0 mm), intermediate (5.1 to 10 mm) and large (>10 mm) antral follicles on ovaries of multiparous cows and heifers (n = 31) in the early luteal (d 4), mid-luteal (d 12) and follicular phase (d 19) of the estrous cycle were determined (d 0 = estrus), and steroidogenic capacity of intermediate and large follicles was measured in vitro. Total number of follicles and number of small follicles were greatest (1)<.05) on d 19 compared with d 12, with numbers on d 4 not differ- ent from either d 12 or 19. Intermediate follicles were fewer (P<.05) on d 19 compared with d 4 or 12. Numbers of large follicles did not change. The proportion of estrogen active (EA) follicles was greater (1)<.05) on d 19 compared with d 4 or 12. Accumulation of estradiol-17~ (E) into culture medium by intermediate follicles decreased (P<.05) with increasing days of the estrous cycle, while accumulation of progesterone (1)) was greater on d 19. In large follicles, accumulation of E into culture medium was greatest (P<.05) on d 19 and the lowest on d 12 (P<.05). In sum- mary, the proportion of EA follicles increases during the preovulatory period, and E production increases in large EA follicles. In contrast, E production in intermediate follicles decreases and