[Guidance Mechanisms in Neuronal and Axonal Migration]

During development, neurons migrate from their birthplace to their appropriate positions and extend their axons toward their proper targets. These events are crucial for the establishment of CNS cytoarchitectures and neural cuircuits essential for brain functions. Recent genetic and biochemical studies have led to the notion that neurons and axons appear to be guided by similar guidance mechanisms and molecules. They appear to be guided by at least four distinct mechanisms: contact attraction, chemoattraction, contact repulsion, and chemorepulsion. These mechanisms are mediated by evolutionarily conserved ligand-receptor system. Extracellualr guidance cues, including Netrins, Slits, Semaphorins and Ephrins, act as ligands, while membrane proteins which are expressed in migrating neurons or axons serve as receptors. A certain guidance cue can bind to several distinct receptors and vice versa, leading to elicit diverse responses in migrating neurons or axons with a relatively small number of guidance cues.