Development of mouse embryos following conventional and ultra rapid cryopreservation.

A conventional computer-controlled program for cryopreservation of mouse embryos at the 4 to 8 cell stage was compared with various methods of ultra rapid cooling. 104 embryos were frozen by using the conventional cooling program. 79% of the embryos developed to blastocysts after thawing. Sixty-three embryos were vitrified and 81% reached the blastocyst stage within 2 days of culture. Using the ultra rapid cooling program A (4-step equilibration of embryos in cryoprotectant) 62% of embryos survived after freezing and thawing. Only 12% survival rate could be achieved by employing the ultra rapid program B (2-step equilibration of embryos in cryoprotectant). The significantly higher survival rate of program A compared to program B was due to the longer equilibration time and therefore presumably higher intracellular concentration of cryoprotectant. In program B the short equilibration time with the cryoprotectant seemed to result in an inadequate dehydration of blastomeres leading to intracellular ice formation and cell destruction.