Figure 10

Cartoon illustrating the gradient of CXCR4 signalling that drives the primordium to migrate towards the right even if the concentration of SDF1 along the trail is constant. In the leading region (right part of the primordium) CXCR4 is present at high levels and is activated by binding to its ligand SDF1, resulting in active migration and repression of cxcr7. In the trailing region (left part) the high level of CXCR7 masks or sequesters the ligand, making it unavailable for binding and thereby preventing activation of CXCR4. Not mentioned in this cartoon are the possibility that SDF1 internalization will progressively reduce the concentration of ligand as the primordium moves along, and the possibility that the expression of cxcr4b depends at least in part on CXCR4 signalling, in which case cxcr4b would be down-regulated whenever CXCR7 is present at high levels.