The locomotion of murine myoblasts over the extracellular matrix components laminin and fibronectin was analyzed using quantitative videomicroscopy, and the organization of the cytoskeleton was observed in parallel immunofluorescence studies. Cells plated on the laminin-nidogen complex locomoted twice as fast as on laminin alone. The main form of translocation on laminin was a jerky cycle of prolonged lamellipod extension followed by rapid (approximately 200- less than 500 microh h-1) movement of the cell body into the extended lamellipod. The locomotion-stimulating activity of laminin resides in the elastase digestion fragment E8, part of the laminin long arm, while the E1-4 fragment containing the three short arms is inactive. Myoblasts moved poorly over fibronectin irrespective of whether high, intermediate, or low coating concentrations were used (approximately 5,000- approximately 10 fmol cm-2). In contrast, the locomotory responses both to laminin and to E8 peaked sharply at coating concentrations approximately 20-50 fmol cm-2 and decreased at higher concentrations. This response corresponds to that expected for a haptotactic stimulant. When cells locomoted over a mixed substrate of laminin and fibronectin, the fibronectin effects appeared to predominate. The cytoskeleton has been implicated in many cellular motile processes. Within 6 h on fibronectin many cells expressed vinculin-containing focal contacts, elaborated stress fibers and had periodically organized alpha actinin, whereas on laminin, most cells showed diffuse vinculin and alpha actinin and a fine meshlike actin cytoskeleton. We conclude that the poor locomotion of cells over fibronectin is because of the cytoskeletal stabilization it induces.

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