Survival of Mouse Embryos Frozen to -196° and -269°C

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Abstract
Mouse embryos survived freezing to -196°C. Survival required slow cooling (0.3° to 2°C per minute) and slow warming (4° to 25°C per minute). Depending on the specific rates used, 50 to 70 percent of more than 2500 frozen and thawed early embryos developed into blastocysts in culture after storage at -196°C for up to 8 days. When approximately 1000 of the survivors, including some frozen to -269°C (4°K), were transferred into foster mothers, 65 percent of the recipients became pregnant. More than 40 percent of the embryos in these pregnant mice gave rise to normal, living full-term fetuses or newborn mice.