Survival of mouse embryos frozen-thawed slowly or rapidly in the presence of various cryoprotectants.

Eight-cell mouse embryos were frozen by using glycerol, ethylene glycol, or erythritol as the cryoprotectant. The samples were slowly cooled (0.5 degrees C/min) to temperatures between -10 and -79 degrees C before direct transfer to liquid nitrogen (-196 degrees C) and thawed slowly (approximately 20 degrees C/min) or rapidly (approximately 500 degrees C/min). The highest survival rates of embryos in 1 and 2 M glycerol were obtained after transfer to -196 degrees C from -40 to -79 degrees C, regardless of the thawing rate (72 to 90%), except for those of embryos thawed slowly in 2 M glycerol, in which case highest survival rates were obtained after transfer from -70 and -79 degrees C (78 to 82%). With slow thawing, survival of embryos in 1.2 M ethylene glycol reached maximal levels at transfer temperatures between -50 and -79 degrees C (82 to 89%) and with rapid thawing at transfer temperatures between -40 and -79 degrees C (84 to 90%). The highest survival rates of slowly thawed embryos in 0.6 M erythritol were obtained after transfer to -196 degrees C from -35 and -40 degrees C (53 to 55%) and of rapidly thawed embryos after transfer from -30 degrees C (46%). These results show that the transfer temperatures to liquid nitrogen, at which the highest survival rates of mouse embryos are obtained, vary with the cryoprotectant and the thawing rate. It appears that with glycerol and ethylene glycol mouse embryos are less sensitive to the thawing rate when slowly cooled to lower subzero temperatures before rapid cooling.