The effects of an advanced uterine environment on embryonic survival in the mare.

REASONS FOR PERFORMING THE STUDY: During embryo transfer (ET) the equine embryo can tolerate a wide degree of negative asynchrony but positive asynchrony of >2 days usually results in embryonic death. There is still confusion over whether this is due to the inability of the embryo to induce luteostasis or to an inappropriate uterine environment.

OBJECTIVES

To assess embryo survival and development in an advanced uterine environment.

HYPOTHESIS: Embryo-uterine asynchrony, not the embryo's inability to induce luteostasis, is responsible for embryonic death in recipient mares with a >2 days chronologically advanced uterus.

METHODS

Experiment 1: Thirteen Day 7 embryos were transferred to the uteri of recipient mares with luteal prolongation, occasioned by manual crushing of their own conceptus, such that donor-recipient asynchrony was between +13 and +49 days. Experiment 2: Day 7 embryos were transferred to recipient mares carrying their own conceptus at Days 18 (n = 2), 15 (n = 2), 14 (n = 4), 12 (n = 4) or 11 (n = 4) of gestation. In addition, Day 8 embryos were transferred to 4 pregnant recipient mares on Day 11 of gestation.

RESULTS

No pregnancies resulted following transfer of Day 7 embryos to recipients in prolonged dioestrus with asynchronies between +13 and +49 days. However, the use of early pregnant mares as recipients resulted in 5/20 (25%) twin pregnancies, 4 of which came from the transfer of a Day 8 embryo to a Day 11 recipient. All transferred embryos showed retarded growth, with death occurring in 4/5 (80%).

CONCLUSIONS AND POTENTIAL RELEVANCE: The results emphasise the importance of an appropriate uterine environment for embryo growth and the inability of equine embryos to survive transfer to a uterus >2 days advanced even when luteostasis is achieved. It is possible that in normal, non-ET equine pregnancy, embryo-uterine asynchrony may account for some cases of embryonic death.