We studied the effect of feeder cells (fibroblasts) and a mixture of the extracellular matrix components, Matrigel, on spreading and cytoskeleton organization of newborn rat keratinocytes (REK). REK formed lamellipodia on being plated together with feeder cells and on the Matrigel as a substrate whereas the same REK plated alone on a plastic surface formed filopodia. REK lamellipodia formation in co-cultures depended on the fibroblast addition time. Although conditioned medium from fibroblast cultures was not enough to induce lamellipodia, the extracellular matrix left after fibroblast removal was as effective as Matrigel. Our results indicate that lamellipodia formation seems to depend on the factor(s) secreted by fibroblasts and associated with the extracellular matrix.