A crawling cell (like a fibroblast or immune cell) moves by remodeling its internal scaffolding — the cytoskeleton. At the front (leading edge), branched actin filaments polymerize against the plasma membrane, physically pushing it forward into a thin sheet called a lamellipodium. At the rear, myosin II motor proteins slide actin filaments past each other to contract the cell body and haul the trailing edge forward. A radial network of microtubules, nucleated from the centrosome, provides internal tracks and polarity cues that orient the whole process.
A single actin filament can grow by adding subunits at up to several microns per minute, but it's the coordinated polymerization of thousands of filaments in a branched Arp2/3-nucleated network that generates enough force to push a cell's leading edge forward — a process sometimes called the "actin treadmill."
A crawling cell rebuilds its internal scaffolding on the fly: branched actin filaments polymerize at the leading edge to push the membrane forward while myosin motor proteins contract the rear, and a microtubule network radiating from the centrosome keeps the whole process organized.
Actin polymerization at the front generates the pushing force behind lamellipodial protrusion, while myosin-driven contraction at the rear retracts the trailing edge — together producing net forward crawling.
Raise actin polymerization to bulge the leading edge further and faster, raise myosin activity to strengthen rear contraction, and adjust microtubule dynamics to see the internal network grow and shrink. Watch crawl speed and distance update live.
Because actin monomers are added at the front and removed at the back at similar rates, the filament network appears almost stationary relative to the substrate even as it drives the whole cell forward — a phenomenon called "treadmilling."