Objective: To find out the optimal freezing time in terms of developmental stage of mouse embryo and the effectiveness of vitrification method for freezing them.
Methods: Superovulation induction was performed using PMSG and hCG. Total 1,437 mouse embryos (vitrification group: 743, slow-freezing group: 694) were obtained and cultured with the T6 containing 0.4% BSA medium. Each developmental stage of embryos (1-, 2-, 4-, 8-cell, morula and blastocyst) were cryopreserved by vitrification and also by slow freezing method for comparison of the results. After thawing, the recovery rate, the survival rate and the blastocyst developmental rate were analysed and compared in two different settings. Student’s t-test was used for statistical analysis.
Results: The survival and developmental rates at all subgroups of vitrification method are significantly higher than those of slow-freezing groups, but not in the recovery rates. In vitrification group, the survival rate and the blastocyst developmental rate are highest when frozen at morula stage, 98.4% and 86.4%, respectively. In slow-freezing group, the survival rate is also highest when frozen at morula stage, 87.2%, and the blastocyst developmental rate is highest when frozen at 8-cell stage, 78.1%.
Conclusion: The vitrification method is more efficient for mouse embryo freezing compared with slow freezing one. Among various developmental stages of mouse embryos, morula stage seems to be the optimal stage for cryopreservation, whatever the freezing method applied. Therefore, we recommend embryo freezing at morula stage by vitrification method.
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