Comparison of slow freezing and vitrification on ovine immature oocytes.

BACKGROUND: Immature oocytes are more sensitive to cold injury than mature oocytes. OBJECTIVE: The study was to evaluate the post thaw normal oocytes, cleavage and blastocyst rates of ovine cumulus oocyte complexes (COC's) using different cryoprotectants by slow freezing and Open pulled straw (OPS) vitrification. METHODS: In five replicates, abattoir derived COC's were collected and distributed into three groups. In Experiment 1, COC's were cryopreserved by a slow freezing protocol using 10% concentration of ethylene glycol (EG), 10% dimethyl sulphoxide (DMSO) or 5% EG and 5% DMSO mixture. In Experiment 2 and 3 embryos were cryopreserved by OPS vitrification using either 33% or 40% (EG, DMSO or an equal mixture of EG and DMSO mixture. Normal oocytes post thaw were in vitro matured and parthenogenetically activated. RESULTS: Although, there was no difference in the number of post thaw normal oocytes between the groups, cleavage and blastocyst rates were higher in 10% slow freezing group than any of the vitrified groups. CONCLUSION: The study demonstrates better cryopreservation of ovine COC's by controlled slow freezing than OPS vitrification.