migration

ati such a scenario, the processing unit that computes the direction in which tomigrate would not be in the individual ing structure has an inherent polarity, a free ’front ’ and an attached ‘back’. Such directionally migrating groups can Opinion TRENDS in Cell Biology Vol.17 No.12cell but the multicellular group. Why do cells migrate? Some free-living cells, such as Dictyostelium amoeba, move to find food or to find each other in times of crisis. Many immune cells move to locate and incapacitate intruders within a multicellular organ-ism. In these cases, both speed and directionality are important, as is rapid response to change. For hardwired developmental migrations, cells are born in one place and migrate to act in another place or whole tissues are also be created artificially by enabling epithelial-type tis-sue-culture cells to organize on a collagen-gel substrate [17]. Finally, Drosophila border cells [18,19] are a group or cluster of cells performing a directional movement during oogenesis. These migrating cells are associated tightly but the cluster is free, without an inherent ‘back ’ (Figure 1f). Tight cell–cell association is expected to impose physical constraints on movement. This will depend on the cell–cell adhesive forces within the group relative to the cell–sub-strate forces exerted during movement. Maintaining cell– cell cohesion during migration and remodeling is probably an advantage when making a continuous tissue. Cell–cel