Fig. 6. Forces involved in cell spreading. Myosin II maintains the integrity of the actin cytoskeleton (small white arrows) as well as generating the traction force on the substratum (large black arrows). In the presence of serum, myosin II is fully activated. Hence the traction force is detectable (A). Without serum, the traction force is not detectable; either with or without serum, myosin II activity lasts briefly and counterbalances the protruding force (small gray arrows) due to actin polymerization (B). Hence, the traction force is not detectable. When myosin II activity is inhibited, the cell can spread without the counterbalancing force (C). Therefore, inhibition of myosin II activity increases the rate of cell spreading.