Comparison of DNA apoptosis in mouse and human blastocysts after vitrification and slow freezing

Vitrification is a novel cryopreservation method for mammalian blastocysts. This study was designed to compare different vitrification methods and slow freezing for their effects on survival rate and DNA integrity in mouse and human blastocysts. In Experiment 1, embryo survival and DNA integrity were compared between mouse blastocysts with collapsed and non-collapsed blastoceles. In Experiment 2, embryo survival and DNA integrity were compared between vitrified and slow-frozen mouse blastocysts. In Experiment 3, embryo survival and DNA integrity were compared between vitrified and slow-frozen human blastocysts. Fresh blastocysts were used as controls in all experiments. Higher (P < 0.05) blastocyst survival rates were obtained in mouse blastocysts vitrified with collapsed versus intact blastoceles, although DNA-integrity indices in the surviving blastocysts were the same among vitrified and fresh blastocysts. More mouse blastocysts (P < 0.05) survived after vitrification (100%) as compared to slow freezing (82.5%). DNA-integrity indices examined in the surviving blastocysts were also higher (P < 0.001) in fresh (93.6%) and vitrified/warmed (93.7%) blastocysts than in slow-frozen/thawed (75.8%) ones. More human blastocysts survived with a higher DNA-integrity index after vitrification/warming than after slow freezing/thawing. These results indicate that higher survival rates can be obtained by vitrification of blastocele-collapsed blastocysts, and that vitrification causes less cell apoptosis in both mouse and human blastocysts compared to slow freezing. Vitrification of blastocysts after blastocele collapse by single laser pulse supports a higher survival rate and less DNA apoptosis, suggesting that laser blastocele collapse is a safe procedure for blastocyst vitrification.