HIGH INCIDENCE OF POLYSPERMIC FERTILIZATION IN BOVINE OOCYTES MATURED IN VITRO AFTER CRYOTOP VITRIFICATION

BACKGROUND: Vitrification with the Cryotop device is the most promising technique for oocyte cryopreservation, but the high post-wauning morphological survival of bovine oocytes does not guarantee high developmental competence after in vitro fertilization (IVF). OBJECTIVE: This study was designed to examine achievement of normal fertilization in bovine oocytes vitrified-warmed with the Cryotop device. MATERIALS AND METHODS: Oocytes were matured in vitro and vitrified-warmed after complete removal of the cumulus layers. Distribution of cortical granules (CGs) was assessed by Lens culinaris agglutinin (LCA) lectin staining. Ten hours after IVF, presumptive zygotes were analyzed for pronuclear formation. Day-8 blastocysts were harvested and stained with Hoechst-33342 for total cell counting. RESULTS: Both yield and mean cell number of the blastocysts were impaired by Cryotop vitrification. Incidence of polyspennic fertilization was three-times higher in vitrified oocytes compared to fresh oocytes. No difference in CG distribution was found between vitrified and fresh oocytes. CONCLUSION: Polyspermic fertilization induced in vitrified warmed bovine oocytes may be one of the possible causes responsible for their low developmental potential.