Growth Cones and Axon Pathfinding

Assembling complex adult neural circuitry begins early in development as neurons extend axons and dendrites to their intermediate and final targets. Most early guidance events do not require neuronal activity and depend critically on a range of guidance cues and their receptors. These cues belong to several protein families. Their selective expression provides unique guidance information to distinct neuronal populations, allowing for simultaneous assembly of diverse circuits. Neuronal guidance relies upon directed alteration of cytoskeletal dynamics, and extrinsic control of actin and tubulin assembly, disassembly, and translocation in neuronal growth cones determine neuronal trajectories. The same neuronal extension encounters several distinct attractants and repellents during the course of its journey, and so guidance cue signaling has evolved to allow for sequential responses to these cues and stepwise establishment of neuronal pathways. Guidance cue gradients displayed across a target fields enable topographic mapping of neuronal processes, allowing for complex circuitry organization.