Bovine cumulus expansion and corona-oocyte disconnection during culture in vitro

Cumulus-oocyte complexes were aspirated from small antral follicles (3-5 mm in diameter) and divided into 2 groups: complexes in which a dark rim of corona cells were visible around the zona pellucida (group 1); and those in which the corona displayed the same density as the rest of the cumulus cell mass (group 2). Cumulus complexes of both groups were evaluated by acetoorcein staining, scanning electron microscopy (SEM) and 3H-uridine uptake during culture in vitro. Germinal vesicle breakdown was initiated at 7 h of culture in group 1 while more than half of the oocytes in group 2 already displayed germinal vesicle breakdown at the time of aspiration. However, whereas more than 80% of the oocytes in group 1 had reached metaphase II at 24 h of culture, almost half of the oocytes in group 2 were arrested in metaphase I after this time interval. As revealed by SEM the complexes of group 2 showed signs of expansion such as elongation of cumulus cells and presence of extracellular matrix already at the time of aspiration. In group 1 these features were noticed at 7-9 h of culture. A high level of metabolic coupling between corona cells and oocyte was maintained up to 9 h of culture in group 1 followed by a decrease to a constant low level at 13 h. In group 2 a high degree of coupling was maintained up to 7 h followed by a gradual decrease to a constant low level at 13 h. It is concluded that cumulus-oocyte complexes with a dark rim of corona cells as judged by stereomicroscopy mature at a higher rate and maintain unexpanded characteristics and efficient corona-oocyte coupling longer than complexes with even density of the cumulus mass. Consequently, the presence of a dark rim of corona cells may be used as a criterion for selection of oocytes for in vitro embryo production.