Polymerization, bending, tension: What happens at the leading edge of motile cells?

The forces experienced by filaments in actin based propulsion in reconstituted systems and cell motility, the mechanical properties of the lamellipodium of motile cells due to filament branching and cross-linking, the free filament contour length between branch points, the mechanisms of the force-velocity relation and velocity oscillations are all topics of ongoing debate. Here, we review results with a modelling concept considering the F-actin network as weakly cross-linked in a region with dynamic depth close to the propelled obstacle and gel-like further back. It offers quantitative explanations for steady motion and oscillation mechanisms in reconstituted systems and motile cells, and the force-velocity relation of fish keratocytes.