A two-dimensional monolayer culture system is described in which mammary stromal cells and colonies of normal epithelium are allowed to confront each other en bloc. Epithelial (and stromal) cell growth was inhibited by confrontation. Epithelial cell growth was reinitiated by estradiol (10−8m) but not by dexamethasone. Reinitiated growth was inhibited by tamoxifen (10−6m). Contact between stromal cells and epithelium was critical for the response to estradiol, and photomicrographic evidence was obtained that estradiol stimulated invasion of the stroma-epithelia interface. These observations are organized into a model for mitogenic action of estradiol that seems to reconcile observed disparities in the action of the hormone in vivo and in vitro.

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Much of the work developing the “confrontation” culture system was done while the author was a visiting fellow in the laboratory of Dr. J. C. Heuson at the Institute Jules Bordet in Brussels, Belgium, in 1971 and 1972. The developmental work was supported by a grant from “The Friends of the Jules Bordet Institute.” More recent work was supported by NIH Grants CA 25482 and CA 18175 to the author.

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