Figure 1. Migration is mediated by attractive and repulsive cues. Schematic representation of a cell moving on a substrate (a–c) and within a cell layer or tissue (d and e). (a) Cells can migrate either up (green arrow) or down (red arrow) the chemotactic gradient. (b) Specific components present in the ECM may provide permissive or nonpermissive cues for the migratory cells. (c) During the process of haptotaxis, cells move along an adhesive gradient towards the area of strongest adhesion. (d and e) Cells moving between other cells can use guidance cues presented by the surrounding cells via direct intercellular contact.

Figure 2. Multiple ways of pathfinding during migration. (a) Cells can migrate to their final destination via intermediate steps, using consecutive gradients of attractive factors (green circles). While finding their way, cells can stop at intermediate points to reorient themselves and then move along the next gradient. (b) Migrating cells can also be correctly positioned by the combined activity of repulsive gradients (red circles). (c) Cells can navigate through a combination of attractive and repulsive gradients with different diffusion radii. (d) Dynamic expression or morphogenetic movements may cause a repositioning of the chemotactic field, causing the cells to respond to the changing location of the chemotactic factor. (e) Correct migration may be achieved by the combination of a chemotactic factor (blue gradient) and a permissive (green) or repulsive (red) substrate. Cells move towards the source (S) of the chemotactic gradient but their migration path and final position are governed by the substrate. See text for examples.

Figure 3. Border cell migration during Drosophila oogenesis. Border cells (green) arise from the follicular epithelium of the egg chamber and migrate together with the two anterior polar cells (purple) towards the oocyte. Upon arrival at the oocyte, they turn dorsally to eventually reside at the oocyte nucleus. Migratory guidance cues are provided by ligands for either the PV receptor (blue arrow) or the EGF receptor (red arrow).

Figure 4. Migration of the posterior lateral line primordium and neuromast deposition in zebrafish. (a) The posterior lateral line primordium arises near the otic vesicle and migrates towards the tail on a path expressing the chemotactic factor SDF-1 (blue dotted line). (b) The direction of migration is probably controlled by an intrinsic self-propagating polarization mechanism in the primordium that concentrates the expression and/or activity of the SDF-1 receptor CXCR4 to the most posterior cells in the primordium (yellow). Deposition of neuromasts along the track of the primordium involves a reduction in speed of the trailing edge at predefined places.