The massive cellular migration that occurs in our bodies during early development works on the same principle as the healing of wounds in adulthood: cells move in large monolayer sheets, responding to chemical cues that guide migration. A paper published online this week in Nature Physics suggests that there is an additional feedback mechanism at work in cell migration, beyond chemical-gradient sensing, involving physical forces.

Cells, as they move, apply traction forces to their surroundings, and transmit forces to one another via intercellular junctions. In doing so, they build up gradients of tension across the group, which then migrates in the direction of maximum stress. Xavier Trepat, Jeffrey Fredberg and colleagues now report that these stress gradients are established via a slow mechanical wave that propagates through the cell population from the leading edge of migration.

Their observation challenges the long-held assumption that viscous stress dominates elastic stress on the timescales of migration. The reinforced patterns of stress and strain that emerge in the study provide direct evidence of a physical process enabling cell migration, which may play a role in wound healing, morphogenesis and the early stages of metastasis.