Multicellular organisms harbor armies of cells on the move. Most are on goodwill missions—immune cells chase bacteria, and wound-healing fibroblasts rush in to fill gaps after injuries. Others, such as metastatic cancer cells, travel with deadly intent. The biochemical signals that set cells on a journey are as diverse as the tissues they move through, but the engine is driven by constant remodeling of a protein network built from a box of cellular Legos.

The cytoskeletal network of a cell is somewhat similar to an animal skeleton: it provides a scaffolding and a means for stepping forward. But unlike a bony skeleton, the cytoskeleton works only when it is unstable. Most locomoting cells move not by discrete steps but through continuous scaffold extension on the front end and destruction at the rear—a process sometimes likened to a treadmill.

Illustration: Jamie Cullen