cell crawling and lamellipodia
/ lah-MEL-ih-POH-dee-uh /
Watch a sheet of plastic wrap creep forward by pushing out a thin, flat edge, gripping the ground there, and then pulling the rest of itself along. That, roughly, is how many animal cells crawl. Cell crawling is the way cells move across a surface, and the flat, fan-shaped front edge that does the pushing is called a lamellipodium (plural lamellipodia).
The driving force is actin. At the leading edge, the cell rapidly assembles a dense, branching mesh of actin filaments. Because the filaments grow against the membrane, their growth physically pushes the membrane forward, extending the lamellipodium like the prow of a ship. The cell then forms grip points where it sticks to the surface (using proteins such as integrins), and myosin motors pulling on actin haul the bulk of the cell body forward. Finally the rear releases its grip and is reeled in. Repeating this push-stick-pull-release cycle lets the cell travel.
Cell crawling is not an exotic trick; it underlies some of biology's most important events. Immune cells crawl through tissue to reach infections; skin cells crawl to close wounds; and during embryo development, cells crawl long distances to build organs. The same machinery has a dark side: cancer cells use crawling to invade neighboring tissue and to spread, or metastasize, which is why the actin-based motility machinery is studied intensely in cancer research.
A neutrophil, a kind of white blood cell, can crawl out of a blood vessel and chase a bacterium by repeatedly pushing out lamellipodia in the direction of the chemical trail the bacterium leaves.
Crawling toward a chemical cue lets immune cells home in on infections.
Cell crawling is fundamentally different from swimming with cilia or flagella: crawling needs a surface to grip and is powered by actin, while ciliary swimming works in open fluid and is powered by microtubule-based beating.