Comparison of vitrification and slow freezing efficiencies of human zygotes.

The use of cryopreservation of embryos in assisted reproduction technologies can not only reduce the occurrence rate of multiple pregnancies, but also increases the cumulative pregnancy rate, due to embryo transfer in any other cycle with no stimulation of superovulation. Due to this the cryopreservation procedures objective is to obtain a high rate of survival and viability of embryos after thawing, which largely depends on the choice of freezing method. Since one of the key parameters of cryopreservation protocols is the rate of cooling, the aim of this study was to compare the efficacy of slow freezing and vitrification of human embryos at the zygote stage. There was shown that the most effective zygote cryopreservation was at 18 hrs. after insemination, when 2 pronuclei were clearly visualized. At this time, the early zygotes were less sensitive to the damaging effects of freeze-thawing process than the zygotes were in G2 phase of the cell cycle, the stage which was prior to mitosis (26 hrs. after insemination). The further development of these zygotes was blocked. Our data are helpful in explaining the conflicting results about successful or failed cryopreservation of human embryos at the zygote stage.