30 Vitrification of Equine Embryos: Application in a Commercial Cloning Program

During a commercial horse cloning program, a critical point is the availability and maintenance of suitable recipient mares for a large quantity of embryo transfers. Usually, pregnancy rates and viable births off the breeding season decrease significantly, whereas the rate of in vitro embryo production remains constant. For this reason, an efficient vitrification system allows continuous embryo production throughout the year with the advantage of doing the transfers only during the breeding season. The vitrification technique evaluated in this study was the one described by Kuwayama et al. (2007 Theriogenology 67, 73-80). By using this method, we compared post-warming recovery efficiency, pregnancy rates, and viable foaling rates in 2 experimental groups: cloned blastocysts vitrified off-season and transferred in breeding season (VC, n = 337), and non-vitrified cloned blastocysts also transferred in breading season (no-VC, n = 516). To achieve this, equine oocytes were collected from slaughterhouse ovaries, matured, enucleated, and fused to a donor cell according to Olivera et al. (2016 PLoS One 11, e0164049, 10.1371/journal.pone.0164049). The reconstructed embryos (RE) were cultured in a well-of-the-well system by adding 3 RE per well for 7 to 8 days to reach the blastocyst stage, at which they were vitrified as mentioned above. During the breeding season, blastocysts were warmed and transferred in couples in a single cycling receptive mare. Pregnancies were confirmed by transrectal ultrasonography 15 days post-transfer. All variables were analysed by Fisher test (P u003c 0.05). The warming recovery rate was 91% (308/337) for cloned blastocysts. In addition, pregnancy and viable birth rates were similar for the VC and no-VC groups: 15.6% (24/154) v. 16.7% (43/258) for pregnancy rates, respectively, and 37.5% (9/24) v. 37.2% (16/43) for foaling rates, respectively. In summary, 9 viable cloned foals were obtained with off-season embryos warmed and transferred during the breeding season, showing that vitrification did not affect embryo quality. Hence, the proposed strategy provides the ability to maximize production efficiency of equine clones by generating a large number of pregnancies without stopping in vitro embryo production at any time of the year.