Motor axon pathfinding in the peripheral nervous system

Functional motor performance is dependent upon the correct assemblage of neural circuitry, a process initiated during embryonic development. How is the complicated neural circuitry that underlies functional behavior formed? During early stages of development, motor neurons extend their axons in a precise manner to their target destinations where they form fine synaptic connections. This process is not random but rather, highly stereotyped and specific. Results of recent studies indicate that positive and negative molecules influence particular steps in the navigation of motor axons to their targets. These molecules include, but are not limited to, members of the Semaphorin family and their receptors, Neuropilins and Plexins, Slits and their Robo receptors, members of the Eph family, extracellular matrix molecules, Hepatocyte Growth Factor/Scatter Factor, peanut agglutinin-binding glycoproteins, and neural cell adhesion molecule. The developing avian peripheral nervous system has served as an excellent model system for many years for studies of the basic cellular interactions that underlie motor axon pathfinding. The principal advantage for the experimental use of the avian embryo is the ease of access to early developmental events. Fine microsurgical manipulations, difficult at best in mouse embryonic development, are readily accomplished in avian embryos and have provided a powerful approach to unraveling the cellular interactions that govern motor axon pathfinding. These approaches, combined in recent years with molecular biology, have begun to produce critical insights into the mechanisms that sculpt cellular architecture during neural development.