Fig. 1. Characterization of S2R+ cells. (A) Morphology of cell population (by phase contrast microscopy). Many cells display broad circumferential lamellipodia. (B) Protrusive activity (time-lapse sequence). Lamellipodium protrudes with regular and smooth outline. Dotted line shows the initial position of the cell edge. Time in min:sec. (C) Actin cytoskeleton (phalloidin staining). Circumferential lamellipodium is rich in actin; filopodia are not seen. (D) Structural organization of lamellipodia (EM). Actin filaments form dense, highly branched dendritic network. Circled region is enlarged in the inset to show actin branches. Bars, 10 µm (A), 5 µm (B,C) and 0.5 µm (D).