In vivo development of vitrified rabbit embryos: effects of vitrification device, recipient genotype, and asynchrony.

This study was designed to evaluate the effects of vitrification device, recipient genotype, and recipient asynchrony on implantation rate, offspring rate at birth, and fetal losses of rabbit embryos. Morphologically normal embryos (N = 787) recovered at 72 hours of gestation were kept at room temperature until transfer or vitrification. Vitrified embryos in Cryotop and ministraw devices were transferred into females induced to ovulate 60 hours (asynchrony) or 72 hours (synchrony) before transfer. In addition, recipient genotypes were analyzed (maternal and paternal genotype). The number of implanted embryos was estimated by laparoscopy as number of implantation sites at day 14 of gestation. At birth, total kits born were recorded. Fetal losses were calculated as the difference between total born at birth and implanted embryos. Our data show that a combination of Cryotop device and recipient asynchrony at -12 hours provides the most successful rate of offspring at birth, although a similar implantation rate was obtained with both devices. Thus, low fetal loss rates were observed for embryos vitrified in Cryotop independently of recipient synchrony, and embryos vitrified in straws revealed a two-fold higher rate of fetal losses. Moreover, when an asynchrony between vitrified embryo and recipients was applied, higher rates of embryos developed to term were obtained regardless of the device used. Finally, we found a highly significant association of the recipient genotype with implantation rate, offspring rate at birth, and fetal losses. In conclusion, the current study findings show that Cryotop enhances offspring rate because it is associated with a lower rate of fetal loss. This study thus provides additional evidence that recipient genotype and recipient asynchrony affect offspring rate at birth and indicates that the genotype of the recipient and the recipient asynchrony have a significant effect on implantation rate and fetal losses after vitrification.