Effect of duration of estradiol exposure on embryo survival and endometrial gene expression in anestrous embryo recipient mares.

Previous studies indicate a positive correlation between the duration of estrus prior to ovulation and likelihood of pregnancy in embryo recipient mares. However, the mechanisms by which the duration of estrus before may affect fertility remains unclear. This study aimed to determine the effect of different durations of estradiol exposure, prior to progesterone administration, on embryo viability in anestrous recipient mares, and endometrial expression of genes thought to influence embryo survival. Three groups of anestrous recipient mares treated with different duration of estradiol were used: long (LE), short (SE) and no treatment (NE). Day 8 embryos were transferred into recipient mares four days after long-acting progesterone administration and recovered 48h later to examine embryo growth and viability. The endometrial gene expression profile of selected genes was also investigated. The likelihood of recovering an embryo 48h after transfer was 46.1% (6/13), 62.5% (5/8) and 85.7% (6/7) for recipient mares from the NE, SE and LE groups, respectively (P = .09). Embryos recovered from the different groups of recipients did not, however, differ in size, morphology or the proportion of nuclei undergoing mitosis (P > .05). Abundance of mRNA for uterocalin (P19) and insulin-like growth factor 1 (IGF1) were increased in the LE compared to the NE group, while fibroblast growth factor 2 (FGF2), progesterone receptor (PGR) and insulin-like growth factor 1 receptor (IGF1R) transcript abundances were increased (P < 0.05) in the NE group compared to both SE and LE groups. In conclusion, a longer exposure of the endometrium to estradiol before progesterone tended to improve embryo survival within 48h of transfer. However, the grade, growth rate, and proportion of mitotic cells in surviving embryos did not differ among groups. If embryos are destined to fail in a suboptimal endometrial environment, they die and disappear quickly. Moreover, a more adequately estradiol-primed uterus, before the progesterone rise, seems to create a uterine environment, in terms of P19, IGF1, FGF2 and PGR gene expression, more conducive to embryo survival and further development.