DEVELOPMENTAL POTENTIAL OF IN-VITRO PRODUCED BOVINE EMBRYOS FOLLOWING CRYOPRESERVATION AND SINGLE-EMBRYO TRANSFER

Information on the developmental potential of in vitro-produced, cryopreserved bovine embryos following single-embryo transfer to synchronized recipients is limited. The aim of this study was to compare the pregnancy rates of vitrified, conventionally cryopreserved, and control (unfrozen) in vitro embryos. The embryos were cryopreserved either by Vitrification in Solution 3a (VS3a) or by conventional slow freezing in 10% glycerol. Forty-two percent (n=121) of the recipients established pregnancy when in vitro produced morulae to expanded blastocyst stage embryos were transferred immediately following culture. Significantly more morulae on Day 7 and 8 of culture established pregnancy (11/14) than blastocysts on Days 7 to 9 of culture (40/107; P<0.05). Significantly fewer biastocysts on Day 9 of culture established pregnancy (12/35) than those on Days 7 and 8 of culture (39/86; P<0.05). Cryopreserved embryos yielded significantly fewer pregnancy rates than freshly transferred embryos (P<0.05). Twenty-three percent (n=85) of vitrified embryos and 14% (n=35) of conventially frozen embryos established pregnancies. When cryopreservation methods were compared in terms of embryo stage, vitrification did not yield significantly better results than the conventional freezing method, but this might be due to the low number of conventionally frozen embryos transferred. These data indicate that in vitro produced embryos yield high pregnancy rates when transferred immediately following in vitro culture for 7 or 8 d to the morula and blastocyst stages. Cryopreservation of in vitro produced embryos either by vitrification or conventional freezing reduced their ability to establish pregnancy. Vitrification was found to be the more practicable method; therefore, when transfer of fresh embryos is not possible, vitrification should be preferred to conventional freezing methods.