Abstract
The fertilizability of mouse oocytes matured in serum-free medium was extremely lower than those matured with fetal calf serum (FCS). The addition of FCS into maturation medium, however, complicates the analytical studies on the oocyte maturation. In the present study, a chemically defined medium supplemented with fetuin, a major protein in FCS to prevent zona hardening, and one of pregnant mare's serum gonadotrophin (PMSG), follicle stimulating hormone (FSH) or epidermal growth factor (EGF) was used to examine the effect on oocyte maturation and fertilizability of the oocytes. The fertilization rates of the oocytes matured with bovine serum albumin (BSA) alone or fetuin alone were significantly lower than that matured with FCS (35.3%, 44.3% and 95.5%, respectively). On the other hand, the addition of both fetuin and one of PMSG (10 iu/ml), FSH (0.1 armour unit/ml) and EGF (10 ng/ml) induced clear cumulus-expansion and improved the fertilization rates (84.1%, 88.4%, and 78.3%, respectively) to the comparable level as FCS supplementation. The removal of cumulus cells just before insemination abolished the effects of PMSG completely. These results suggest that the effects of FCS maintaining high fertilizability of in vitro matured oocytes work on both zona pellucida and cumulus-cells, and are fully replaced by fetuin and one of PMSG, FSH or EGF. This serum-free maturation system would be useful for the analysis of the factors and mechanisms regulating oocyte maturation.
