Figure 3 - Schematic representations of models of assembly for lamellipodia, lamella, peripheral ruffles and filopodia.

(a) Top view of lamellipodia and lamella. Lamellipodial actin filaments (blue) are dendritically branched and Arp2/3-complex-dependent. These filaments assemble at the leading edge plasma membrane (due to assembly factors bound there) and disassemble abruptly at a band corresponding to the first sites of stable adhesion (green), generally 1–3 m from the leading edge. Lamellar actin filaments (red) are in a less defined network, and are not Arp2/3 complex-dependent. They can initiate throughout the lamellipodium and lamellum, but their frequency increases as one moves back from the leading edge. Lamellar filaments persist many micrometres behind the lamellipodium, and disassembly can occur throughout this region. (b) Side views of lamellipodium and lamellum. Lamellipodial filaments may lie above lamellar filaments. At the leading edge, lamellipodial filaments predominate, but lamellar filaments increase in abundance further back. A lamellipodium maintaining weak attachment to the substratum is shown in the upper panel. Myosin II activity at the stable adhesion site causes retrograde flow of the lamellipodial actin network. If the lamellipodial attachment with the substrate is broken, myosin II activity causes this membrane sheet to move rearward as a peripheral ruffle, which disassembles at the stable adhesion site (lower panel). (c) Filopodial assembly from the lamellipodium or lamellum. Filopodial assembly from the lamellipodial network through the convergent-elongation mechanism is shown on the left. Possible filopodial assembly from the lamellar network, through an as yet undefined mechanism, is shown on the right.