Final answer:
A lamellipodium provides temporary anchor sites for the cell to pull itself forward by pushing against the cell membrane and creating focal adhesions with the ECM to anchor and transmit contractile forces.
Step-by-step explanation:
A lamellipodium is an actin-driven extension at the leading edge of a cell, contributing to cell motility. It acts like a dynamic, semi-rigid network, increasing in volume and area through the rapid polymerization and branching of actin filaments. As actin monomers are added to the strands at the leading edge, they push against the cell membrane, creating outward pressure. This expanding actin network forms the lamellipodium, which encounters resistance from both the extracellular matrix (ECM) through adhesion sites and internally from cytoplasmic components.
These adhesion sites, known as focal adhesions, temporarily connect the actin cytoskeleton to the ECM through integrin proteins. The interplay between lamellipodial expansion against the membrane and focal adhesions connecting to the ECM allows the cell to effectively 'grip' the substrate, pulling itself forward. Actin polymerization provides the protrusive force at the front, while myosin II generates contractile forces within the cell body, both contributing to traction force generation that propels the cell.
Focal adhesions are initially small and increase in size as they are transported backward by retrograde flow of actin, resulting from the counterforces exerted by the membrane. If the adhesions mature, they can anchor more strongly, exerting larger forces. Conversely, if they do not mature and the flow is fast, they may exert smaller forces, possibly contributing to faster cell movement.