The traction stress distribution has previously been mapped for fish epidermal keratocytes locomoting steadily [gliding] in a straight line (Oliver et al., 1995, Dembo et al., 1996). Such cells produced strong pinching tractions perpendicular to the direction of travel that are maximal in the vicinity of the lateral edges (wings). We now investigated how the typical pinching pattern of tractions is altered when keratocytes are ``stuck'' by their trailing edge, adopting a fibroblast-like morphology. When this occurs, the location and magnitude of both propulsive tractions driving locomotion and frictional tractions opposing locomotion are, in effect, separated. We have also determined how the normal pinching traction pattern is altered in a special phenotype of the keratocyte which undergoes continual circling motions. We find that a very simple mechanical model is able to explain how alterations of adhesion and contractility are orchestrated so as to produce this phenotype.
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