**2.5 Effect of Vn on sperm penetration of the cumulus oophorus**

*In vitro* matured COCs were randomly assigned to 4 groups (3 replicates). Half of the oocytes (2 groups) were denuded by vortexing (cumulus denuded or CD) and the other half (2 groups) were kept cumulus-enclosed (CE). Both CD and CE oocytes were fertilized under control conditions (in standard fertilization medium) and in the presence of 500 nM Vn (Tanghe *et al.* 2004*b*). The oocytes were co-incubated with sperm at a concentration of 106 sp mL-1 for 20 h (39°C; 5% CO2). Each group contained 76 to 100 oocytes. Prior to fixation the presumed zygotes were vortexed to remove excess spermatozoa. After staining, the oocytes were evaluated for fertilization and polyspermy.
