Cryopreservation and Transfer of Sheep Embryos Recovered at Different Stages of Development and Cryopreserved Using Different Techniques

Simple Summary Due to gene pool insufficiency, it is clear to use the best breeds of sheep for meat. Multiple Ovulation and Embryotransfer Technology (MOET) is applied to improve the genetic and production performance of high-value ewes. This article presents cryoresistance data for Charollais sheep embryos, considering the stage of embryo development and the freezing method. No significant differences in survival rate and morphological structure between cryopreserved and fresh embryos were found. But, cryoresistance largely depended on embryonic development stages. Thus, cryopreservation of embryos at the 6 days after insemination was more effective than freezing 2-3-day embryos after insemination. The data obtained is believed to make assisted reproductive technologies (ART) more effective and cheaper. This article will be interesting for farm producers, breeders and researchers. This article presents data from experiments to determine the cryoresistance of Charollais sheep embryos, depending on the stage of embryo development and the method of freezing, as well as the results of embryo transfer. The study design consisted of a study on the cryopreservation of ewe embryos at different developmental stages (early, 2-8 blastomeric and late, at the morula/blastocyst stage), two cryopreservation protocols (slow freezing and ultra-fast vitrification), and embryo transfer of cryo- and fresh embryos. Embryos from Charollais sheep donors (n = 12) were recovered after induction of superovulation. The embryos were recovered surgically (laparotomy) on days 2 and 6 after insemination. Before there was transfer to recipients, part of embryos was cryopreserved using standard slow freezing and ultra-fast vitrification methods. The average ovarian response was 7.54 ovulations per donor, and 5.83 embryos per donor were collected. No effect of the cryopreservation method and embryo development stage on the preservation of the morphological structure of embryos was found. There were no significant differences in the survival rate of cryoembryos at different development stages, frozen using different techniques, and after transfer to recipients. Differences in cryoresistance between embryonic developmental stages in favor of the morula/blastocyst stage were found (survival after thawing 86.4% vs. 75.0% in early embryos). At different stages of development, the survival rate of fresh embryos (45.8%) compared to cryopreserved ones (30.2%) was significantly higher (p < 0.05), while among fresh ones, the best survival rate (50.0%) was observed after the transfer of morules and blastocysts.