Freshly plated 3T3 cells send out radial projections or filopodia. We observed cells which happended to settle on glass near the borderline of a gold-plated area. When some of the filopodia contacted the gold-plated area and others the glass substratum and remained attached for a few minutes, lamellipodia then extended preferentially toward the gold-plated area. 1-2 h later, most of the cells were found in the gold-plated area. When the filopodia of a spreading 3T3 cell contacted another already spread 3T3 cell and also the glass substratum, the first lamellipodia extended preferentially towards the glass. These observations suggest a directionally differentiated extension of lamellipodia after the filopodia of a spreading 3T3 cell have contacted different substrates in their environment. Before filopodia contact a substrate, they perform a rapid "scanning" motion. Therefore, we suggest that the filopodia of a spreading 3T3 cell serve as organs which explore the nonfluid environment and react to a certain quality of the substrate that is presently unknown. Subsequently, they mediate the extension of lamellipodia into the direction in which this quality is found. The described phenomena are reversibly inhibited by Cytochalasin B at concentrations above 5 mug/ml although filopodia are produced.

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