The electron microscopic autoradiographic studies described here revealed the presence of specific silver grains over nuclei, lysosomal vesicles, rough endoplasmic reticulum and Golgi apparatus after incubation of placental tissue for 2 h at 38 degrees C with 1 nM-[125I]EGF. Three-step mask analysis, which corrects for radiation spread, showed that the relative grain density was the highest in nuclei, followed by lysosomal vesicles, then Golgi and rough endoplasmic reticulum, equally. The nuclear grain density, however, was lower than that in microvillus plasma membranes. There were very few grains in basolateral plasma membranes, none in the basement membrane area and a considerable number in capillary endothelial cells. The present results demonstrating the association of internalized [125I]EGF with a variety of intracellular organelles raise the possibility of EGF acting on the intracellular sites in addition to cell surface sites.

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