In vitro-produced horse embryos exhibit a very narrow window of acceptable recipient mare uterine synchrony compared with in vivo-derived embryos.

The study investigates the success rates of pregnancy in horses that receive in vitro and in vivo derived embryos. It finds that the timing of the recipient mare’s oestrous cycle greatly affects the likelihood of successful pregnancy, with in vitro embryos having a narrower window for successful transfer compared to in vivo embryos.

Research Objective and Methodology

• The researchers aimed to explore how the recipient mare’s oestrous cycle affects the likelihood of a successful pregnancy after transferring in vitro-produced (IVP) and in vivo-derived embryos.
• They conducted a retrospective study of 526 equine embryos (264 IVP and 262 in vivo-derived). They examined the effects of the timing and length of recipient mares’ oestrus on the outcomes of the transferred embryos.

Key Findings

• IVP embryos tolerated only a narrow period of recipient mare’s “synchrony.” Transfer on Day 4 after ovulation led to a significantly higher likelihood of an ongoing pregnancy (69%) than transfers on Days 3, 5, or 6, which resulted in reduced success rates (53.2%, 41.3%, and 23.1% respectively).
• However, in vivo-derived embryos exhibited more flexibility, showing no significant difference in pregnancy or pregnancy loss for recipients that ovulated between Day 4 and Day 9 prior to the transfer.
• The researchers found that transferring in vivo-derived embryos to mares that had a longer oestrus preceding the transfer resulted in higher Day 12 and ongoing pregnancy rates. Surprisingly, IVP embryos did not show this trend, suggesting that the length of oestrus may not affect their success rates.

Conclusion

• The researchers concluded that Day 6-8 IVP embryos survive better after transfer to mares on Day 4 of their cycle.
• In contrast, Day 8 in vivo-derived embryos survive equally well in recipients on Day 4-9 of their oestrus cycle, but perform best in mares with a longer preceding oestrus. The findings highlight notable differences in the flexibility of successful embryo transfer between IVP and in vivo-derived embryos, suggesting the need for better synchronization protocols for IVP embryos transfer.