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

The study tested whether how long a mare shows estrus changes the uterine lining’s gene activity one week after ovulation. Mares with longer estrus showed higher expression of uterocalin (P19), a nutrient-transport protein important for early embryos, while IGF1, FGF2, and PGR were unchanged and ESR1 was not detected.

What the researchers asked and why it matters

• Question: Does the duration of estrus (a proxy for the length of estrogen exposure) influence endometrial gene expression relevant to early pregnancy by Day 7 post-ovulation?
• Importance: Around Day 7, the equine embryo is in the uterus and depends on endometrial secretions (histotroph). Gene programs that support nutrient delivery and uterine receptivity at this time can affect embryo survival and development.

Study design and grouping

• Animals: 12 clinically cyclic mares.
• Grouping by estrus duration using endometrial edema as the clinical indicator:
  • Long estrus (LE): ≥ 3 days of endometrial edema (n = 7).
  • Short estrus (SE): < 3 days of endometrial edema (n = 5).
• Sampling time: Day 7 post-ovulation endometrial tissue.
• Transcripts quantified: uterocalin (P19), insulin-like growth factor 1 (IGF1), fibroblast growth factor 2 (FGF2), progesterone receptor (PGR), and estrogen receptor alpha (ESR1).
• Analysis: Comparison of transcript abundance between LE and SE groups; ESR1 also assessed for detectability.

Genes examined and why they matter

• P19 (uterocalin): A major uterine secretory lipocalin in mares that binds and transports lipids and other small molecules into the histotroph, aiding embryo nutrition and development.
• IGF1: A trophic factor linked to glandular function and conceptus growth signaling, potentially influencing endometrial-embryo communication.
• FGF2: Involved in angiogenesis and tissue remodeling, supporting uterine changes and early conceptus interactions.
• PGR (progesterone receptor): Mediates progesterone-driven differentiation and secretion in the endometrium during diestrus.
• ESR1 (estrogen receptor alpha): Mediates estrogen effects; typically high during estrus and downregulated during the luteal phase.

Key findings

• P19 expression was significantly higher in mares with long estrus compared with short estrus.
• IGF1, FGF2, and PGR showed no significant differences between groups (reported as p > 0.1).
• ESR1 transcripts were undetectable in all samples at Day 7.

Interpretation and biological context

• Prolonged estrus likely reflects longer or stronger estrogen priming before ovulation, which is associated here with increased P19 transcription by Day 7. This points to enhanced capacity for lipid/nutrient delivery to the embryo via histotroph.
• The lack of differences in IGF1, FGF2, and PGR suggests that, by the mid-luteal phase, many aspects of uterine receptivity converge regardless of preceding estrus length.
• Undetectable ESR1 is consistent with physiological downregulation of ERα during the progesterone-dominant luteal phase, when direct estrogen signaling in the endometrium is minimal.
• Together, the data highlight P19 as a sensitive readout of prior estrogen-driven uterine priming that persists into early diestrus, whereas other pathways appear buffered or time-normalized by Day 7.

Implications for reproductive management

• Estrus duration, easily observed via endometrial edema over days, could serve as a practical indicator of subsequent P19 upregulation and potentially richer histotroph around Day 7.
• For embryo transfer programs, recipient mares with longer estrus might provide a uterine milieu with higher uterocalin, possibly benefiting embryo support; however, this remains a hypothesis until linked to actual pregnancy outcomes.
• Because IGF1, FGF2, and PGR were unaffected, selecting mares solely on estrus duration is unlikely to influence these broader receptivity pathways at Day 7.

Limitations and caveats

• Small sample size (n = 12; LE = 7, SE = 5) limits power and could mask modest effects on other genes.
• Estrus duration was inferred from endometrial edema, which may vary with examiner scoring and does not directly quantify estradiol exposure.
• Single time point (Day 7) cannot capture dynamic pre- and post-ovulatory gene regulation; earlier and later windows may differ.
• mRNA abundance does not guarantee corresponding protein levels or secretion; uterine fluid composition was not measured.
• ESR1 non-detection could reflect true biological suppression or technical sensitivity limits.
• Mares were cyclic rather than confirmed pregnant; embryo–maternal signaling could modify these patterns in pregnancy.

Future directions

• Pair endometrial transcript data with serum estradiol and progesterone profiles and direct quantification of P19 protein in uterine fluid.
• Expand panels to include additional secretory proteins, transporters, and immune mediators central to histotroph quality.
• Test causality by experimentally extending estrus or modulating estrogen exposure and assessing downstream P19 responses.
• Link molecular findings to outcomes: embryo mobility, growth, recovery rates at flushing, and pregnancy establishment in natural breeding and embryo transfer.
• Stratify analyses by age, parity, breed, uterine health, and cycle management to assess generalizability.

Bottom line

• Longer estrus in mares is associated with higher endometrial P19 expression on Day 7 post-ovulation, pointing to enhanced uterine nutrient support, while other key receptivity genes appear unaffected at this time. P19 may be a practical molecular indicator of uterine priming driven by preceding estrogen exposure.