| Gap junctions, comprised of transmembrane proteins (connexins), permit intercellular communication through sharing of small hydrophilic molecules. Electron microscopy, immunohistochemistry and in situ hybridization were employed to characterize the spatio-temporal expression pattern of connexins, during the estrus cycle and early pregnancy in equine and porcine endometrium. For mares, additional experiments determined the influence of ovarian steroids and pregnancy on endometrial Cx43 (Cx43) expression. The expression pattern of estrogen and progesterone receptors (ER and PR) were also determined to examine the relationship between steroid receptor expression and endometrial Cx43 expression.;Cx43 was the predominant connexin expressed in the endometrial stroma of mares and sows. While no gap junctions were found to be expressed in the luminal epithelium for either species, in mares, Cx43 was colocalized with tight junctions in the endometrial glands during diestrus and prior to Day 30 of pregnancy. Hence, glandular Cx43 expression may serve as a physiologically relevant marker of receptivity to the preimplantation embryo. Analysis of mRNA expression revealed that glandular Cx43 was posttranscriptionally downregulated during estrus and transcriptionally downregulated by Day 30 of pregnancy. Exogenous estrogen or progesterone stimulated glandular Cx43 mRNA expression but concurrent protein expression was not stimulated by estrogen alone suggesting that posttranscriptional regulation was mediated by estrogen. Transcriptional downregulation was also evident in pseudopregnant mares suggesting that Cx43 modulation by Day 30 of gestation was not pregnancy specific.;Endometrial ER and PR mRNA and protein were strongly expressed in all endometrial compartments during estrus and through Day 6 postovulation. By Day 12 postovulation in cyclic and pregnant mares, luminal and glandular ER and PR were reduced while stromal PR expression remained elevated. The relationship between established serum estrogen levels, and the spatio-temporal expression pattern of ER expression was consistent with the hypothesis that estromedin action may have a key role in modulation of glandular Cx43 expression in mares. In addition, short-term treatment with progesterone alone caused increased ER and PR expression in the endometrial stroma and glands suggesting that both may have important roles in establishing uterine receptivity by the time the embryo enters the uterus. |
