Optimization of protocols for oocyte vitrification and thawing of cattle embryos

Cryopreservation of oocytes and embryos has become an integral part of assisted reproduction in both humans and animals. However, the methods used for cryopreservation of bovine oocytes still have significant drawbacks. We used modified protocols of vitrification of fertilized bovine eggs and thawing of bovine blastocysts after vitrification. The aim of our study was to determine the oocyte survival rate, their competence for further development to the blastocyst stage, and to evaluate the viability of the resulting embryos and their ability to implant after a combination of modified ultra-rapid methods: vitrification at the fertilized oocyte stage and repeated vitrification/ rapid thawing of the blastocyst during embryo transfer. Results: Overall bovine oocyte survival rates were comparable with the ultra-rapid and standard vitrification protocols: 90,6–86,8%, respectively. MII oocytes obtained from immature cells post mortem were capable of activated development and began cleavage to produce blastocysts. No statistically significant differences were found between the groups vitrified using the standard and optimized protocols. The resulting embryos at the blastocyst stage were vitrified. The protocol of modified thawing (rehydration) of vitrified blastocysts was first used to transfer bovine embryos into the uterus of a recipient cow. One pregnancy was obtained when three vitrified blastocysts were transferred and defrosted using the modified rapid protocol. The methods of ultra-rapid vitrification of oocytes and rapid thawing of vitrified embryos ensure high survival and reduce the time of vitrification and thawing of embryos, and, consequently, the time the embryos spend outside the incubator at suboptimal temperature and osmolarity.