Molecular mechanisms mediating axon pathway formation.

During nervous system formation nerve cells extend axons in order to form precise patterns of neuronal connectivity. These connections are often established after the neuronal growth cones have pioneered or navigated through complex pathways to their target area both within the CNS and to and from the periphery. Recent studies have provided evidence that the process of specific pathway formation may rely on a number of molecular guidance mechanisms and cues such as selective adhesion, growth cone avoidance, surface gradients, guidepost cells, and chemotropism. Analysis of the molecular basis for these guidance mechanisms show that the molecules involved often belong to distinct multigene families and that they can provide both short- and long-range attractive as well as repulsive cues. Many of these molecules have a modular structure that is made up of different tandemly arranged domains that allow for multiple functional interactions with a range of other molecules. This allows the same molecule to be multifunctional, for example, by attracting certain neurons while repelling others. This review is an overview of the molecular structure, as it relates to function and mechanisms of action of some of the major gene families thought to be mediating specific axonal guidance and pathway formation.