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This study was carried out in order to investigate effects of cryoprotectant concentration and equilibration time on survival of ultrarapidly frozen 2-cell mouse embryos. Mouse 2-cell embryos, following dehydration by exposure to DMSO and sucrose, were directly immersed into liquid nitrogen and thawed in 37^{circ}C water. Viability was defined by development rate to the blastocyst stage after in vitro culture for 72 hours. The results are summarized as follows ; 1. When 0.25M of sucrose was added into the freezing medium at various concentrations of DMSO and dilution medium, higher development rate of embryo was obtained in 3.0M DMSO concentrations (82.6%). However, when sucrose concentraitons of 0.25 and 0.5 M were added to the freezing medium with 3.0 M DMSO and dilution medium, development rate of embryos were 81.7% and 24.1%, respectively. 2. In the equilibration time at room temperature, higher development rate was attained after short period of time (2.5min) in 3.0 M DMSO＋0.25 M sucrose (85.9%). 3. The development rate of embryos at in vitro 2-cell, in vitro 2-cell, solution control and untreated control was 84.6%, 90.9%, 89.9%,, and 89.7%, respectively.
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