Molecules making waves in axon guidance

The exquisite complexity of neural connectivity depends upon the precise navigation of axons to their targets in the developing nervous system. How is this achieved? Signals from the environment are assumed to impinge on growth cone receptors and thus steer axons in the right direction, but the molecular details of this process have been largely unknown. Recently, impressive progress has been made in identifying families of molecules that may underlie this process. Candidate diffusible guidance molecules include the netrins and semaphorins, whereas membrane-associated Eph receptors and their ligands are proposed to influence guidance by cellcell contact. Characterization of receptors for some of these molecules now promises the dissection of the signalling pathways that dictate axonal responses to pathfinding cues.