Open pulled straw vitrification of in vitro matured sheep oocytes using different cryoprotectants

The aim of this study was to compare the survival and in vitro development of sheep oocytes after open pulled straw vitrification and different final concentrations of permeable cryoprotectants. In 5 identical replicates of two experiments, in vitro matured (IVM) oocytes were vitrified by the Open Pulled Straw (OPS) method, then warmed, and the surviving ones were subjected to parthenogenetic activation. In Experiment 1, survival rate of oocytes after vitrification in 33% ethylene glycol was higher than in 33% DMSO or a mixture of 17.5% ethylene glycol and 17.5% DMSO (87.64 vs. 77.43 vs. 69.39%, respectively). The cleavage and blastocyst rates were higher after vitrification in mixture group than in ethylene glycol and DMSO (46.81 and 15.5 vs. 37.55 and 9.12 vs. 29.51 and 6.40%, for cleavage and blastocyst rates in different groups, respectively). In Experiment 2, elevated concentrations of vitrification solutions were used. The survival rate was higher after vitrification in 40% ethylene glycol and in the mixture of 20% ethylene glycol and 20% DMSO than in 40% DMSO (90.22 vs. 87.56 vs. 75.34%, respectively). Cleavage and blastocyst rates were also higher in the ethylene glycol and ethylene glycol – DMSO mixture group than in DMSO alone group (50.67 and17.60 vs. 49.13 and 14.45 vs. 33.86 and 9.81% for cleavage and blastocyst rates in different groups, respectively). The survival rates between the two experimental groups was higher in 40% ethylene glycol group, 40% mixture group and 33% ethylene glycol group than in 40% DMSO group, 33% mixture group and 33% DMSO group. Cleavage and blastocyst rates were higher in 40% ethylene glycol group, 40% mixture group and 33% mixture group than in 40% DMSO group, 33% ethylene glycol group and 33% DMSO group. All cleavage and blastocyst rates in both the experiments were lower than those of the non-vitrified control group (87.00 and 45.00, respectively). In conclusion, although ethylene glycol group and ethylene glycol – DMSO mixture group gave better survival and cleavage – blastocyst rates than DMSO group, the survival rates were lower than the control group and hence the technique could be further improved to get better results after OPS vitrification of IVM sheep oocytes.