Collective cell migration in development.

Collective cell migration is a key process during the development of most organisms. It can involve either the migration of closely packed mesenchymal cells that make dynamic contacts with frequently changing neighbour cells, or the migration of epithelial sheets that typically display more stable cell-cell interactions and less frequent changes in neighbours. These collective movements can be controlled by short- or long-range dynamic gradients of extracellular signalling molecules, depending on the number of cells involved and their distance of migration. These gradients are sensed by some or all of the migrating cells and translated into directed migration, which in many settings is further modulated by cell-contact-mediated attractive or repulsive interactions that result in contact-following or contact-inhibition of locomotion, respectively. Studies of collective migration of groups of epithelial cells during development indicate that, in some cases, only leader cells sense and migrate up an external signal gradient, and that adjacent cells follow through strong cell-cell contacts. In this Commentary, I review studies of collective cell migration of differently sized cell populations during the development of several model organisms, and discuss our current understanding of the molecular mechanisms that coordinate this migration.