Motor axon pathfinding in the peripheral nervous system

Functional motor performance is dependent upon the correct assemblage of ne ural circuitry, a process initiated during embryonic development. How is th e complicated neural circuitry that underlies functional behavior formed? D uring early stages of development, motor neurons extend their axons in a pr ecise manner to their target destinations where they form fine synaptic con nections. This process is not random but rather, highly stereotyped and spe cific. Results of recent studies indicate that positive and negative molecu les influence particular steps in the navigation of motor axons to their ta rgets. These molecules include, but are not limited to, members of the Sema phorin family and their receptors, Neuropilins and Plexins, Slits and their Robe receptors, members of the Eph family, extracellular matrix molecules, Hepatocyte Growth Factor/Scatter Factor, peanut agglutinin-binding glycopr oteins, and neural cell adhesion molecule. The developing avian peripheral nervous system has served as an excellent model system for many years for s tudies of the basic cellular interactions that underlie motor axon pathfind ing. The principal advantage for the experimental use of the avian embryo i s the ease of access to early developmental events. Fine microsurgical mani pulations, difficult at best in mouse embryonic development, are readily ac complished in avian embryos and have provided a powerful approach to unrave ling the cellular interactions that govern motor axon pathfinding. These ap proaches, combined in recent years with molecular biology, have begun to pr oduce critical insights into the mechanisms that sculpt cellular architectu re during neural development. (C) 2001 Elsevier Science Inc.