Comparison of the effects of controlled-rate cryopreservation and vitrification on 2-cell mouse embryos and their subsequent development.

Effects of two cryopreservation procedures (conventional slow controlled-rate freezing using a programmable freezer and vitrification by direct plunging into liquid nitrogen) were compared on 2-cell embryos and their subsequent development to blastocysts, fresh or cryopreserved 2-cell mouse embryos were developed into blastocysts in vitro. The percentage of vitrified embryos which developed into blastocysts was significantly lower than that of fresh and slow controlled-rate frozen embryos. Although blastocysts from each cryopreservation procedure appeared morphologically normal and neither number of cells in the blastocysts nor in-vitro trophoblast spreading differed significantly, there were significant differences in their functional viability. First, the glucose incorporation activity in terms of [(3)H]2-deoxyglucose (2-DG) uptake in vitrified and thawed 2-cell embryos significantly decreased compared with fresh or slow controlled-rate frozen and thawed 2-cell embryos. Second, 2-DG uptake by blastocysts developed in vitro from fresh 2-cell embryos and from slow controlled-rate frozen or vitrified 2-cell embryos was 105 +/- 75, 43.0 +/- 28.3 and 22.0 +/- 11.4 fmol/embryo/h respectively. Third, the implantation rate of blastocysts developed in vitro from vitrified 2-cell embryos (10.2%) was significantly lower than that from fresh 2-cell embryos (30.8%) or slow controlled-rate frozen 2-cell embryos (22.1%). Since these data suggest that cryopreservation may have ulterior consequences on the functional development of embryos and that vitrification may exert a more harmful effect than slow controlled-rate freezing, more attention should be paid to its safety before vitrification is used routinely in a clinical programme.