Evidence for a Discontinuous Requirement for Estrogen in Stimulation of Deoxyribonucleic Acid Synthesis in the Immature Rat Uterus*

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Abstract
Estriol is rapidly lost from uterine target cell nuclei. It was used to study the relative time of exposure to estrogen that is necessary to elicit the late growth responses of increased DNA polymerase activity or increased rate of DNA synthesis observed after 17.beta.-estradiol treatment. Estriol administered during a 2nd critical phase from 9-15 h after the initial injection elicits maximal DNA synthesis. The results suggest that there is a discontinuous requirement for estrogen in the sequence of events which result in the uterine growth response to estrogenic hormones. If estriol is present during critical phases of this sequence of events, it is equipotent to estradiol in eliciting the full uterotropic response.