An integrative toy model of cell flattening, spreading, and ruffling

The processes of cell spreading and crawling are frequently associated with mysterious waves and ruffling cycles of the leading edge.

To develop a physical model that can account for these phenomena based on a few simple and plausible rules governing adhesion, contractility, polymerization of cytoskeleton, and membrane tension.

Extension of a continuum mechanical model of phagocytosis [J Cell Sci. (2006);119(Pt 9):1903-13] adding a simple coupling between membrane curvature and cytoskeletal polymerization.

We show that our generalized model has just the right nonlinearity needed for triggering of stochastic/chaotic cycles of ruffling similar to those that are observed in real cells.

The cycles are caused by a branching instability at the leading edge that leads to bifurcations of protrusion into forward moving lamellipodium and upward and rearward folding ruffles. The amplitude of the instability is modulated by the surface tension, with higher tension stabilizing against ruffling (but inhibiting protrusion) and lower tension promoting ruffling and protrusion.