Endometrial Content of Nuclear Estrogen Receptor and Receptivity for Ovoimplantation in the Rat

Abstract

The purpose of this study was to provide information concerning the amount of nuclear binding sites for estrogen in endometrial cells during the 1st wk of pregnancy in the rat. The insoluble (high-salt non-extractable) binding during the 1st 4 days of pregnancy follows a circadian rhythm, with a maximal value at midnight, and a minimal one at noon. By day 5, the amount plateaus at a minimal value. The number of estradiol binding sites of the soluble (high-salt extractable) nuclear fraction shows a similar pattern. All the necessary elements for the estrogen-dependent induction of endometrial receptivity for nidation appear, therefore, to be present already by day 3, but the turning-on of the endometrial response to estrogen on day 4 seems to depend on additional factor(s).