Effects of asynchrony between embryo development and uterine environment on embryo survival and development in rabbits.

The aim of this study was to estimate the effects of asynchrony between embryo development and uterine environment on embryo survival and subsequent foetal development in rabbits. Embryos at 48 and 72 hours post-coitum (p.c.) were transferred into 22 females at 48 h of pseudopregnancy and into 20 females at 72 h of pseudopregnancy. Similar embryonic survival was obtained in synchronous recipients with embryos transferred at 48 h p.c. (0.40) and asynchronous recipients with embryos transferred at 72 h p.c. (0.47). However, embryonic survival was 26% higher in synchronous recipients with embryos transferred at 72 h p.c. compared to asynchronous recipients with embryos transferred at 48 h p.c. (0.56 vs. 0.30). These findings suggest that embryos have the ability to wait for the favourable uterine environment for implantation and that a less advanced embryo development into a more advanced uterine environment increases implantation failures. Similar foetal survival rates were found in recipients at 72 h p.c. regardless of the embryo development. Nevertheless, foetal survival was 23% higher in synchronous recipients with embryos transferred at 48 h p.c. than in asynchronous recipients with embryos transferred at 72 h p.c. (0.91 vs. 0.68). Most foetal losses in asynchronous recipients with embryos transferred at 72 h p.c. occurred close to implantation, in comparison to synchronous recipients with embryos transferred at 48 h p.c. (38.1 vs. 12.4%). Blood supply seems to be key in implantation losses and foetal development of asynchronous recipient females with embryos transferred at 72 h p.c. The percentage of dead foetuses with placentas receiving fewer than three blood vessels was higher than those with placentas receiving more than three blood vessels in embryos transferred at 72 h p.c. into asynchronous recipient females than in embryos transferred at 48 h p.c. in synchronous recipient females (69% of dead foetus with <3 vessels vs. 17% with >3 vessels in embryos transferred at 72 h p.c. in asynchronous recipients in comparison to 14% with <3 vessels vs 12% with >3 vessels in embryos transferred at 48 h p.c. into synchronous recipients). In conclusion, a less advanced embryonic development than that of the pseudopregnant uterine horn increases pre- and peri-implantation losses, while a more advanced embryonic development than that of the pseudopregnant uterine horn increases foetal losses as a consequence of lower blood supply in each implanted site. [ABSTRACT FROM AUTHOR]