Relationship between the estrus duration in cyclic mares and the endometrial gene expression on day 7 post-ovulation.

This study investigated the effect of estrus duration on endometrial gene expression in cyclic mares on Day 7 post-ovulation, focusing on transcripts relevant to early pregnancy: uterocalin (P19), insulin-like growth factor 1 (IGF1), fibroblast growth factor 2 (FGF2), progesterone receptor (PGR), and estrogen receptor alpha (ESR1). Twelve mares were classified according to the estrus duration in long estrus (LE, ≥ 3 days of endometrial edema; n = 7) or short estrus (SE, < 3 days, n = 5). Transcervical endometrial biopsies were collected and gene expression quantified via qPCR. The only significant difference between groups was found in P19 expression, which was elevated in LE mares (P = 0.048). No differences were observed for IGF1, PGR or FGF2 (P > 0.1). ESR1 expression was undetectable in all samples. These findings suggest that prolonged estrus enhances P19 expression, potentially improving uterine support for the embryo; however, other genes involved in uterine receptivity appear unaffected by estrus duration at this time point. These findings highlight P19 as a potential indicator of uterine priming and suggest that minor differences in estrous expression may be resolved by the mid‑luteal phase.