Unstable membrane outgrowth induced by filament tip spreading

We analyse the dynamics of a network of regularly spaced parallel filaments, growing perpendicularly against an elongating membrane which limits the diffusion of monomers in the half-space where filaments grow. When the membrane bulges, some filaments spread, so that less monomers are absorbed at the bulge periphery. It results in both a local increase of the free monomer concentration and an enhancement of the filament polymerisation rate, leading to an original growth instability. This one, stabilised here at short wavelengths by filament tip diffusion along the membrane and the drawback force exerted by a subjacent network of crosslinking molecules, may play a noticeable part in the initiation of neuritic and lamellipodial structures.