Fig. 9. A model for the migration of fast-moving irregularly shaped cells. (A) The migrating cell comprises a frontal towing zone (a) and a rear trailing and contractile zone (p). These two zones are connected by an elastic transition zone (e) that includes the cytoskeleton. Polymerization of actin (red-filled circles) produces a force (F) for pseudopod elongation in the frontal towing zone and a rearward force (rf) at stress points (s) in the anterior or middle of the cell. Part of F' is transmitted to the rear zone through the elastic transition zone, and this passively generates a forward force (ff) at the stress points in the rear trailing and contractile zones. F' increases as the elastic transition zone extends. (B) When the passive force F' reaches a threshold level, an active strong force (Fm) is exerted by accumulated myosin II at the rear trailing and contractile zones. After contraction of the rear zone by myosin II, a new cycle of the cell migration process begins (panel A).