**2.1 Oocyte and semen preparation**

Oocytes were derived from bovine ovaries randomly collected at a local abattoir and prepared following the protocol of Tanghe *et al.* (2004*a*). Immature cumulus-oocyte complexes (COC) were aspirated from follicles with a diameter ranging from 2 to 8 mm. Only COCs displaying a multilayered compact cumulus and a homogeneous ooplasm were selected. Frozen-thawed bull semen from the same ejaculate was used for all inhibition experiments. Straws were thawed in a water bath (37°C) for 60 s. Subsequently, the semen was centrifuged on a discontinuous Percoll gradient (90% and 45%; Pharmacia, Uppsala, Sweden) as described by Thys *et al.* (2009*a*).

Media and chemicals were analogous to those used by Tanghe *et al.* (2004*a*). Vitronectin from bovine plasma (V9881) used in all experiments was purchased from Sigma-Aldrich (Bornem, Belgium).
