Development during single IVP of bovine oocytes from dissected follicles: Interactive effects of estrous cycle stage, follicle size and atresia

Previous work suggests that a number of factors such as follicle size, day of estrous cycle, and level of atresia influence the developmental potentia l of bovine oocytes in vitro. To understand better the interactions of thes e factors, 1299 follicles greater than or equal to 3 mm in diameter were di ssected from ovaries of synchronized dairy cows on four days (d2, d7, d10, or d15) during the estrous cycle. The oocyte from each follicle was collect ed and matured, fertilized, and cultured singly to d8 (d0 of culture = IVF) . Control follicles (302) were similarly dissected and processed from an ov ary pair randomly collected from the abattoir on each slaughter day. Result s showed that development to blastocyst was greater in oocytes collected du ring phases of follicular growth (d2 and d10) than those collected during p hases of follicular dominance (d7 and d15; 44.8% vs. 36.0%, respectively: P < 0.001) over all follicle size categories (3-5 mm, 6-8 mm, 9-12 mm and gr eater than or equal to 13 mm). Oocyte competence tended to increase with in creasing follicle size (P < 0.1). Follicular cells from follicles containin g an oocyte that developed to morula or greater by d8 (484 samples) were an alyzed by flow cytometry to measure the level of apoptosis. Results showed an increase in mean percent apoptotic cells in subordinate follicles (18.65 +/- 0.86 over all size categories), particularly those of medium size(25.5 5 +/- 2.2 for 6-8 mm size follicles; P < 0.001), during the dominance phase compared to growth phase (9.25 +/- 0.95 over all sizes; P < 0.05). These r esults show a significant affect of the stage of estrous cycle on both oocy te competence and levels of follicular atresia. (C) 1999 Wiley-Liss, Inc.