Estrogen and progestogen receptors in the implantation sites and interembryonic segments of rat uterus endometrium and myometrium.

Estrogens play a central role in the mechanism of blastocyst implantation. Whether the blastocyst itself contributes to this hormonal effect by locally releasing estrogens at the site of implantation remains debatable. Indirect evidence of estrogen production by the embryo could be obtained if specific estrogenic effects were found to a greater extent at the implantation sites, when compared to the interembryonic segments. Six-day pregnant rats were injected in the morning with Evans' blue, and the uterine blue stripes revealing the implantation sites were separated from the interembryonic segments. Endometrial and myometrial portions of the two sites were separately pooled and analyzed for protein, estradiol receptor (E2R) and progesterone receptor (Prog.R) contents, in cytosol and nuclear fractions. The present results show a significantly higher protein concentration in cytosol of endometrium (20 +/- 6.2 vs. 12 +/- 7.6) (means +/- SD) and, to a lesser extent, in the cytosol of myometrium (10 +/- 2.5 vs. 8.1 +/- 2.2 mg/mg DNA) at the implantation sites as compared to interembryonic segments. Protein levels were slightly higher in nuclei of endometrium only (8.3 +/- 3.4 vs. 6.4 +/- 4.5 mg/mg DNA). E2R concentrations were significantly lower in cytosol of endometrium from implantation sites (3.5 +/- 1.4 vs. 5.0 +/- 2.6 pmol/mg DNA), whereas nuclear levels were higher (0.63 +/- 0.38 vs. 0.47 +/- 0.24 pmol/mg DNA); nuclei-cytosol ratios were significantly higher in endometrium from implantation sites (16 +/- 7 vs. 9.7 +/- 5%). In myometrium no differences were observed between the two sites. Prog.R were higher both in cytosol (3.3 +/- 1.0 vs. 2.0 +/- 0.3) and in nuclei (3.0 +/- 1.2 vs. 1.4 +/- 0.7 pmol/mg DNA) of endometrium from implantation sites; nuclei-cytosol ratios were also higher (97 +/- 32 vs. 71 +/- 34%). In myometrium, differences between the sites were minimal. Our results show higher protein concentration in endometrium from implantation sites, mostly in cytosol and to a lesser extent in nuclei; lower cytosol but higher nuclear E2R concentrations, and both higher cytosol and nuclear Prog.R concentrations in endometrium from implantations sites, and strongly suggest a local estrogenic effect on tissues in close vicinity of the blastocyst. Thus they favor the hypothesis of estrogen release by the embryo itself.