Cryopreservation of rabbit zygotes.

Rabbit embryos at the pronuclear stage (n = 480) were frozen using a mixture of propanediol and sucrose. The survival rate was 71% according to their morphological features shortly after thawing. The viability rate after 5 days of culture was 10% according to the number of embryos that reached the blastocyst stage, while the rate of formation of blastocysts from freshly collected and cultured pronuclear embryos was higher (59%). The implantation rate was higher (24%) if the frozen/thawed zygotes were directly transferred to foster mothers after 2-4 hours of culture, whereas with zygotes cultured for 24 hours (4 to 8-cell stages) the implantation rate was poor (4%). Both rates were low compared with the implantation rate (52%) achieved with freshly collected and directly transferred pronucleate embryos.