The effect of temperature at which slow cooling is terminated and of thawing rate on the survival of one-cell mouse embryos frozen in dimethyl sulfoxide or 1,2-propanediol solutions.

We investigated the slow freezing of one-cell mouse embryos with either dimethyl sulfoxide (Me2SO) or 1,2-propanediol (PROH) as the cryoprotectant. One-cell embryos, collected from superovulated C57BL/6J x CBA/Ca females were exposed to 1.5 M solutions of either Me2SO or PROH. The embryos were cooled at 0.3 degrees C/min to temperatures between -10 degrees and -80 degrees C before being plunged into LN2 and then warmed at either 20 degrees C/min or 450 degrees C/min. Survival was expressed as the percentage of hatching or hatched blastocysts per frozen-thawed embryo. When the slow cooling was in 1.5 M PROH, the temperature at which survival rates after slow thawing began to increase was -35 degrees C (52.6 +/- 5.2% survival). For slow cooling in 1.5 M Me2SO this temperature was -50 degrees C (45.0 +/- 2.9% survival). The addition of sucrose to the 1.5 M PROH solution raised the temperature at which survival rates after slow thawing began to increase to -30 degrees C (54.8 +/- 3.7% survival). If slow cooling was stopped at high subzero temperatures, embryos survived better after rapid thawing than slow thawing. If slow cooling was stopped at low subzero temperatures, the survival rate was not dependent on the thawing rate if freezing was done in 1.5 M PROH. When freezing was in Me2SO solutions and to subzero temperatures of -60 degrees and -80 degrees C, slow thawing gave better survival than rapid thawing. The addition of sucrose to the Me2SO freezing solution restored the survival rates at -60 degrees and -80 degrees C. These results indicate that high rates of survival may be obtained from one-cell mouse embryos by a rapid or a slow thawing procedure, as has been found for other developmental stages. The results also indicate that PROH provides superior protection compared to Me2SO against freezing-thawing damage and that the addition of sucrose to the freezing solutions prior to freezing improves the overall survival rates. Embryos that survived freezing and developed in culture implanted and formed normal fetuses at rates similar to those of nonfrozen control embryos (60% vs 68% and 53% vs 58%, respectively.