The morphological effects of estrogen on the luminal surfaces of rat endometrial cells were investigated by scanning electron microscopy. Ovariectomized rats were injected intravenously with estradiol-17 beta (E2 beta), 0.5 micrograms/0.25 ml per 100 g body wt. At various intervals thereafter, the lumen of a uterine horn was flushed with buffered 2% glutaraldehyde and then prepared for scanning electron microscopy by conventional methods. In control rats that had received an equivalent volume of placebo vehicle, the luminal cell surface was characterized by short, sparse microvilli (MV) and, in most cells, a single, central cilium. At 30 s after E2 beta injection, the number of MV was significantly increased. By 1 min, MV density was further increased and MV were frequently clustered; also, the central cilium of many cells was no longer evident. Similar results were obtained after exposure to diethylstilbestrol for 30 s to 1 min, whereas neither a subthreshold dose of E2 beta nor a dose of the relatively inactive congener E2 alpha equivalent to a saturating concentration of E2 beta gave statistically significant responses in surface changes by the present criteria. After 3-7 min of E2 beta exposure, MV had increased greatly in length and density. These effects underwent dramatic regression by 15-30 min after E2 beta treatment, with distinct diminution of microvillar lengths and numbers, reduction of clustering, and reappearance of the central cilium in many cells. This was succeeded at 1 h by a renewed surge of surface activity. These results are consistent with cumulative evidence for rapid alterations of the surface membrane of estrogen-sensitive cells in response to physiological levels of active hormone. Whether these responses in the luminal surfaces are primary, or are secondary reflections of receptor-mediated membrane alterations at the basolateral blood-front, remains to be determined.

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