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

Overview

• This study investigated how different lengths of estradiol hormone exposure before progesterone treatment affect embryo survival and gene expression in the uterine lining of anestrous (non-cycling) mares receiving embryo transfers.
• The research aimed to clarify the biological mechanisms by which estradiol exposure duration may influence fertility and embryo viability in these recipient mares.

Background and Purpose

• Previous observations suggest that the length of estrus (the period when mares are receptive to breeding) prior to ovulation correlates positively with greater pregnancy rates in mares receiving embryos.
• However, the specific physiological and molecular mechanisms underlying this correlation were not well understood.
• This study focused on anestrous recipient mares, which naturally lack regular estrous cycles, to isolate the effect of controlled estradiol exposure.
• The main goal was to assess how different durations of estradiol exposure before administering progesterone influence:
• Embryo survival and growth after transfer
• Endometrial gene expression relevant to embryo support

Experimental Design

• Anestrous recipient mares were divided into three groups based on the length of estradiol treatment before progesterone administration:
• Long Exposure (LE)
• Short Exposure (SE)
• No Estradiol Treatment (NE)
• Day 8 embryos (i.e., embryos 8 days post-fertilization) were transferred into these mares four days after administering long-acting progesterone.
• Embryos were recovered 48 hours after transfer to assess survival, size, morphology, and cell division activity.
• The endometrial expression levels of selected genes were measured to evaluate changes related to embryo support.

Key Findings: Embryo Survival and Development

• Embryo recovery rates 48 hours post-transfer showed a trend toward improvement with longer estradiol exposure:
• NE (no estradiol): 46.1% recovered embryos
• SE (short estradiol): 62.5%
• LE (long estradiol): 85.7%
• This difference approached statistical significance (P = 0.09), indicating a tendency rather than definitive proof.
• Among embryos that survived, no significant differences were found between groups in:
• Embryo size or overall morphology
• Percentage of cells undergoing mitosis (cell division)
• This suggests that estradiol exposure duration affected survival likelihood but did not influence the growth or quality of embryos that survived.
• The authors propose that embryos failing in a suboptimal uterine environment die and disappear rapidly, so only embryos capable of surviving appear similar across groups.

Key Findings: Endometrial Gene Expression

• Gene expression analysis focused on genes implicated in uterine function and embryo support, including:
• Uterocalin (P19)
• Insulin-like growth factor 1 (IGF1) and its receptor (IGF1R)
• Fibroblast growth factor 2 (FGF2)
• Progesterone receptor (PGR)
• The main findings were:
• Higher abundance of P19 and IGF1 mRNA in the LE group compared to NE
• Higher levels of FGF2, PGR, and IGF1R transcripts in NE compared to both SE and LE groups
• This gene expression pattern suggests that longer estradiol exposure primes the uterus in a way that enhances conditions for embryo survival, potentially by modulating growth factors and receptors involved in uterine receptivity.

Conclusions and Implications

• A longer period of estradiol exposure before progesterone treatment tended to improve embryo survival 48 hours after transfer in anestrous mares.
• Embryos that survived did not differ in developmental metrics across treatment groups, indicating that the estradiol effect primarily influences whether embryos survive initial uterine transfer conditions.
• The improved survival is likely due to changes in the endometrial environment evidenced by altered gene expression of factors critical to uterine receptivity and embryo support.
• This research enhances understanding of hormonal preparation of embryo recipient mares and suggests that optimizing estradiol exposure could increase pregnancy rates in equine embryo transfer programs.