It was previously shown that proliferation of CG-5 cells, an estrogen supersensitive variant of the MCF-7 cell line, is inhibited by tamoxifen starting three days after addition to culture medium. In this paper we report here on the effect of tamoxifen on CG-5 cell membrane morphology using scanning electron microscope. The addition of the antiestrogen to CG-5 cells causes marked alterations of cell surface, consisting of reduction in the length and the number of microvilli. Moreover, cells appear flattened and of reduced thickness. These modifications are more pronounced in cells plated at low density, but they are observed also in nearly confluent monolayers. In this case alterations are more evident in the center of cell islands, while at the periphery there is a certain mobility as reflected by the persistence of few microvilli and philopodia. These early effects of tamoxifen on cell membrane raise the possibility that parallel modifications in membrane function may occur which are responsible for alterations impairing transport mechanisms.
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