Estradiol-17␤ (E 2 ) acts through the estrogen receptor (ER) to regulate uterine growth and functional differentiation. To determine whether E 2 elicits epithelial mitogenesis through epithelial ER versus indirectly via ERpositive stromal cells, uteri from adult ER-deficient ER knockout (ko) mice and neonatal ER-positive wild-type (wt) BALB͞c mice were used to produce the following tissue recombinants containing ER in epithelium (E) and͞or stroma (S), or lacking ER altogether: wt-S ؉ wt-E, wt-S ؉ ko-E, ko-S ؉ ko-E, and ko-S ؉ wt-E. Tissue recombinants were grown for 4 weeks as subrenal capsule grafts in intact female nude mice, then the hosts were treated with either E 2 or oil a week after ovariectomy. Epithelial labeling index and ER expression were determined by [ 3 H]thymidine autoradiography and immunohistochemistry, respectively. In tissue recombinants containing wt-S (wt-S ؉ wt-E, wt-S ؉ ko-E), E 2 induced a similar large increase in epithelial labeling index compared with oil-treated controls in both types of tissue recombinants despite the absence of epithelial ER in wt-S ؉ ko-E tissue recombinants. This proliferative effect was blocked by an ER antagonist, indicating it was mediated through ER. In contrast, in tissue recombinants prepared with ko-S (ko-S ؉ ko-E and ko-S ؉ wt-E), epithelial labeling index was low and not stimulated by E 2 despite epithelial ER expression in ko-S ؉ wt-E grafts. In conclusion, these data demonstrate that epithelial ER is neither necessary nor sufficient for E 2 -induced uterine epithelial proliferation. Instead, E 2 induction of epithelial proliferation appears to be a paracrine event mediated by ER-positive stroma. These data in the uterus and similar studies in the prostate suggest that epithelial mitogenesis in both estrogen and androgen target organs are stromally mediated events.

Myometrial development from the prenatal to adult period was examined in rats and mice 1) by histologic and immunocytochemical methods with anti-actin, -vimentin, and -laminin to assess cytodifferentiation of smooth muscle and fibroblastic cells; and 2) by morphometric procedures to assess quantitatively the expression of cellular orientation in the emerging inner circular myometrial layer. Uterine mesenchymal cells initially were uniformly vimentin-positive, undifferentiated, and randomly oriented during the late fetal period. By the early neonatal period, three mesenchymal layers became recognizable histologically, the middle one of which (prospective circular myometrium) developed distinct circular orientation and differentiated into a layer composed of actin-positive smooth muscle cells. The cells of the inner mesenchymal layer initially exhibited radial orientation. By 10 days postpartum, the outer longitudinal mesenchymal layer differentiated into bundles of smooth muscle cells representing the longitudinal myometrium. The inner mesenchymal layer remained vimentin-positive and differentiated into the randomly ordered endometrial stroma. The cells of the middle and outer mesenchymal layers that were destined to form myometrium initially expressed vimentin throughout and then coexpressed vimentin and actin, but with time vimentin staining disappeared in the maturing smooth muscle cells as they expressed actin.

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