Uterine influences on embryogenesis and early placentation in the horse revealed by transfer of day 10 embryos to day 3 recipient mares.

The research investigates how the timing of embryo transfer in horses impacts the development of embryos and their placentas. When day 10 embryos were transferred to mares that had ovulated 7 days after the donors, embryonic development was slower, but it overcame a 7-day asynchrony with the uterus, highlighting the influence of the uterine environment on embryonic development.

Methods

• Researchers transferred eight day 10 horse embryos to recipient mares that had ovulated 7 days after the original donor mares.
• This transfer was non-surgical, and the development of the transferred embryos were tracked ultrasonographically.

Results

• All eight recipient mares showed an embryonic vesicle on an ultrasound scan.
• Three of the eight (38%) developed a properly formed embryo with a beating heart. This is a significant measure of successful embryonic development.
• The initial growth was slower than that in control mares, where normal pregnancy occurred, but the growth continued until day 22 post transfer (recipient day 15).
• The time when the embryonic vesicle attached to the uterus was observed to be related to the size of the vesicle, rather than the current stage of the uterine environment.
• The first visual evidence of the embryo (the embryo proper) was seen on day 22, which is in the expected timeline, though its growth and the growth of the allantois (an important part of the placenta) were slower.

Conclusion

• The development of endometrial cups and the secretion of equine chorionic gonadotropin was detected in two mares that remained pregnant for longer than 50 days, but it occurred approximately 7 days later than in control mares.
• This suggests that the horse conceptus, or embryo and associated membranes, can overcome a 7-day asynchrony with the uterus. This means even when the timing of the transfer makes the recipient uterus environment differently aged from that normally associated with the developing embryos, the embryos were able to adapt and continue their development.
• Overall, these results highlight the influence of the uterine environment on the development of the embryo in the horse. Despite timing differences, the embryonic development was able to adjust and continue, showing a remarkable resilience and plasticity in the face of environmental changes.