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This experiment was carried out to investigate the optimal cooling rate and the plunging temperature into liquid nitrogen of the 8-cell hamster embryos. The female hamsters were induced to superovulate by intraperitoneal injection of 30 i.u. PMSG. Embryos were flushed from oviduct and uterine horn. Embryos were frozen and incubated with a modified Dulbecco’s phosphate buffered saline, and equilibrated with 1.5M-dimethyl sulfoxide by a 3-step procedure. The cooling rate of samples was 1^{circ}C/min from room temperature to -5^{circ}C and the samples were seeded at -5^{circ}C. The plunging temperatures into liquid nitrogen were -20, -25, -30, -35, -40, -45, -50 and -55^{circ}C at 0.3^{circ}C/min, 0.5^{circ}C/min and 1^{circ}C/min cooling rates, respectively. This mean numbers of ovulation points and recovered embryos were 59.4 and 48.4 appearing 81.6% recovery rate. The percentage of 8-cell embryos in recovered embryos was 68.2. The survival rates of embryos plunged at -45^{circ}C were 73.5% at 0.3^{circ}C/min, 44.8% at 0.5^{circ}C/min and 30.3% at 1^{circ}C/min cooling rates, respectively. This mean numbers of ovulation points and recovered embryos were 59.4 and 48.4 appearing 81.6% recovery rate. The percentage of 8-cell embryos in recovered embryos was 68.2. The survival rates of embryos plunged at -45^{circ}C were 73.5% at 0.3^{circ}C/min, 44.8% at 0.5^{circ}C/min and 30.3% at 1^{circ}C/min cooling rates, respectively. The survival rates at 0.3^{circ}C/min were significantly high. Under the condition of 0.3^{circ}C/min cooling rate, the survival rates of embryos according to the plunging temperature were 70.0% and 73.5% at -40 and -45^{circ}C, and those were higher than other plunging temperatures. Under the condition of 0.5^{circ}C/min and 1^{circ}C/min cooling rates, the survival rates according to the plunging temperatures were lower than the cooling rate of 0.3^{circ}C/min, showing the similar tendency at all the plunging temperatures. In conclusion, 8-cell hamster embryos showed the best survival rates at 0.3^{circ}C/min cooling rate and -45^{circ}C plunging temperature.
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