Effect of ovarian phase and follicle quality on morphology and developmental capacity of the bovine cumulus-oocyte complex.

In order to study the effect of the follicular environment on the quality and developmental competence of cumulus-oocyte complexes (COC), COC were collected from nonatretic (NA), light atretic (LA), atretic (A), and heavily atretic (HA) follicles. Cumulus-oocyte complexes were also collected from early-luteal phase ovaries (EL), from late-luteal phase ovaries (LL), from follicular phase ovaries (F), and from ovaries from non-cyclic animals (NC). Each COC was assigned to one of three quality groups based on the appearance of the cumulus investment: 1) COC-A: compact and bright, 2) COC-B: less compact and dark, and 3) COC-C: strongly expanded cumulus with dark spots. There was a high correlation between follicle quality and the distribution of the COC over the three COC qualities. The COC-A were mainly but not exclusively derived from NA follicles, COC-B were mainly derived from all classes of atretic follicles, and COC-C nearly exclusively originated from HA follicles. The developmental capacity of COC-A and COC-B, which was measured by in vitro embryo production, was consistent over the follicle qualities, except for COC-B obtained from HA follicles. They showed lower development (10.6%, P < .05) compared with COC-B from the other follicle qualities (18.9, 18.7, and 19.8%, respectively, for COC-B from NA, A, and LA follicles). The COC-B from atretic follicles produced more blastocysts (19.8%) than COC-A (12.7%, P < .05). The overall percentage of produced embryos per follicle class seemed to increase with increasing signs of atresia, except if the COC were derived from HA follicles. This increased percentage of embryos was, however, not due to a better quality of COC, but to a higher percentage of COC-B coming from these follicles. The stage of the cycle had no effect on the distribution of the COC over the three COC qualities or on the developmental capacity of COC-A or COC-B, except for COC-A from EL ovaries, which produced more (P < .05) blastocysts than COC-A from the other luteal phases (12.5% vs approximately 8%). A follow-up study was performed trying to elucidate why COC-B possess a higher developmental potency than COC-A. The answer was sought in the oocyte maturation. At several time points during maturation, oocytes were evaluated for their nuclear stage. At all time points COC-B seemed to be a few hours ahead of COC-A, and after 24 h of maturation more COC-B had reached the metaphase-2 stage. This might mean that COC-A need a longer maturation period than COC-B.