Cell migration requires sustained forward movement of the plasma membrane at the cell's front or "leading edge." To date, researchers have uncovered four distinct ways of extending the membrane at the leading edge. In lamellipodia and filopodia, actin polymerization directly pushes the plasma membrane forward, whereas in invadopodia, actin polymerization couples with the extracellular delivery of matrix-degrading metalloproteases to clear a path for cells through the extracellular matrix. Membrane blebs drive the plasma membrane forward using a combination of actomyosin-based contractility and reversible detachment of the membrane from the cortical actin cytoskeleton. Each protrusion type requires the coordination of a wide spectrum of signaling molecules and regulators of cytoskeletal dynamics. In addition, these different protrusion methods likely act in concert to move cells through complex environments in vivo.
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