Figure 1. Collective migration depends on internal and external factors.

(a) Collective migration depends on interactions within the migrating collective (blue arrows) although external factors may also influence the movement, such as physical obstacles (brown arrows) or external gradients (orange arrows). (b) Collective versus coordinated migration: coordinated movement is simply the sum of the parts while collective movement depends on interactions within the group. (c-e) Examples of epithelial collective migration: (c) Border cells (light blue) during collective migration in the Drosophila ovary acquire outwards polarity due to interactions with “polar cells” (dark blue) within the cluster, while the whole cluster polarizes in the direction of an external chemoattractant gradient (indicated by red/green outlines). (d) Internal structure and interactions of the posterior lateral line primordium during collective migration in zebrafish development (lateral view of the cluster). The trailing population (light blue) sequesters the underlying chemoattractant (black arrow) generating a gradient that stimulates forward chemotaxis of the leader population (dark blue). In return the leaders secrete FGF that attracts the trailing cells. (e) Cell-cell interactions within epithelial sheets: during plithotaxis, cells move (blue arrows) within an epithelial sheet to minimize intercellular shear (red arrows). (f) Schematic representation of neural crest (NC) migration within the embryo. The NC differentiates and undergoes EMT at the borders of the neural plate and then invades the surrounding tissues, including placodes. During its migration the NC interacts with the ECM and external chemoattractants and maintains interactions within the migrating cluster. Migration occurs in the head first, where streams are wider and larger than at more posterior locations.