Figure 1.

Models of collective migration. (A) Drosophila border cell cluster. Cluster of two polar cells (purple) surrounded by epithelial cells migrating between nurse cells (blue). Main adhesion molecule: Drosophila epithelial (DE)-cadherin (in order of decreasing expression: polar cells, border cells, nurse cells) (Niewiadomska et al. 1999; Cai et al. 2014). (B) Drosophila tracheal branching. Branches sprout from dorsal trunk by active migration of the leader cell and elongation and intercalation of follower cells. Follower cells are also polarized apicobasally toward the lumen (Lebreton and Casanova 2014). Main adhesion molecule: DE-cadherin (Affolter and Caussinaus 2008). (C) Xenopus mesendoderm. Migrates as a multilayered sheet over fibronectin-rich extracellular matrix (ECM). Leader and follower cells in contact with the ECM show front–rear polarization and extend lamellipodia. Main adhesion molecule: C-cadherin (Weber et al. 2012). (D) Zebrafish lateral line. Leading zone is polarized in the direction of migration. Behind the leading zone, there is a transitional zone in which cells acquire apicobasal polarity and are organized into rosette-like sensory organs. Main adhesion molecules: E-cadherin (throughout), N-cadherin (accumulates apically at rosette) (Revenu et al. 2014). (E) Wound healing assay. 2D sheet migration assay, in which a gap is created in a confluent monolayer. Leader cells extend lamellipodia and migrate into the wound area. Main adhesion molecules: cell-type dependent. (F) Vascular sprouting. Migrates as cord with lumen led by two leader cells. The trailing cells elongate parallel but polarize apicobasally perpendicularly to the direction of migration (Lee and Bautch 2011). Main adhesion molecule: vascular endothelial (VE)-cadherin. (G) Mammary branching. Migration of multilayered structure called terminal end bud (TEB) consisting of luminal epithelial cells that lose apicobasal polarity but retain E-cadherin and are partially covered by myoepithelial cells (blue). Leader cell in the TEB lack protrusive structures (Ewald et al. 2012). Main adhesion molecules: myoepithelial cells, P-cadherin; luminal epithelial cells, E-cadherin (Knudsen and Wheelock 2005). (H) Heterotypic cancer cell migration. Cancer cohorts may recruit other cell types (blue), such as fibroblasts or myeloid cells as leader cells (Gaggioli et al. 2007; Kitamura et al. 2007; Li et al. 2016). Main adhesion molecules: Heterotypic E-N-cadherin contacts (Omelchenko et al. 2001) or homotypic N-N and tight junctions (Smalley et al. 2005).