Figure 2.

Topological terminology of collective migration. (A) Leader cells comprise one or several cells at the front of the migrating unit, which respond to guidance signals, form protrusions or a leading edge, and determine direction of migration. Leader cells face an asymmetric environment of lateral and rear cell–cell adhesions and extracellular matrix (ECM) at the front, and show cellular front–rear polarity. (B) Inner follower cells are behind leader cells inside of the cluster and lack asymmetric cell–cell adhesions but front–rear polarization may be induced. Migrating cohorts of simple epithelia or endothelia lack inner follower cells. (C) Peripheral follower cells locate at the outside of the cluster and face the ECM on one side, while adhering to other cells on other sides (C1). Peripheral follower cells form actomyosin cables at the outer surface of the cluster. When inner follower cells are absent, peripheral followers (C2) show apicobasal polarization, with an apical domain toward an internal lumen (L).