Estrogen receptors in the nidatory sites of the rat endometrium.

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In ovariectomized rats treated with progesterone, implantation was induced by a minute dose of 17 beta-estradiol. Twenty-four hours later, the concentrations of estradiol receptor in nuclear and cytosol fractions prepared from the endometrium surrounding the blastocyst and the inter-implantation areas remained very low. This indicates that estrogen was not secreted by the blastocyst. The higher receptor content in cytosol from inter-implantation sites may reflect modifications accompanying the decidual reaction since our results show that there is no translocation of the receptor to the nuclei. The choice of the dye used to reveal the implantation sites is critical, since Trypan blue but not Evans blue binds steroids and thereby interferes with receptor measurements.