How do precise connections develop between neurons and targets? Experimental and descriptive studies of a simple vertebrate system, the development of muscle innervation in the chick embryo, have shown that selective pathfinding by the growing of individual neurites (the "growth cones") plays an essential role in the genesis of the precise and orderly connections that are required for the nervous system of function. As yet, little is known about the cellular and molecular mechanisms of growth cone guidance. The proposed studies contribute to the resolution of this major unsolved problem in neurobiology. The central aim of the proposed studies is to analyze the temporal and spatial distribution of navigational cues and to characterize guidance at the level of individual growth cones. Specific neural populations are labeled with a non-toxin fluorescent dye that allows visualization of active growth cones with image enhancement technology. The central studies analyze the interactions of individual growth cones with navigational cutes as they grow within living sections of the chick embryo in culture. This culture paradigm largely preserves the embryonic environment and yet allows analysis of growth cone navigation with a greater degree of spatial and temporal resolution than has heretofore been possible. Studies in the first specific aim address the navigation of epaxial motoneurons. These motoneurons are known to require a long- distance cue from the epaxial muscles in order to grow out. The proposed studies will characterize how growth cones respond to the cue, determine the spatial-temporal distribution, source and effective lifetime of the cue, show if the somata are essential for growth cones to respond to the cue elucidate changes in the internal structure of active growth cones and show how axons choose one among two possible epaxial muscle targets. Studies in the second specific aim address three tissues that border axon pathways and appear to channel axon outgrowth by acting as barriers to axon advancement. The proposed studies will chart the temporal-spatial development of the barrier function, probe for molecules that define this function and determine the cellular mechanism of guidance. The third specific aim is to obtain monoconal antibodies to the target-derived epaxial cue, the barrier tissues or to subpopulations of motoneurons. The studies proposed are likely to provide provocative information on the cellular and molecular nature of axonal guidance cues. Analysis of the cellular and molecular mechanisms that are effective is specific neuronal pathfinding in the chick should give insights into normal and abnormal development of the human nervous system. Factors important to the development of the complexly interdependent neuromuscular system may eventually be manipulated to treat deficiencies in nerve-muscle interactions that are responsible for human disease states. Finally, and understanding of the processes conferring specificity on developing embryonic connections is relevant to an understanding and proper treatment of the relatively poor specific nerve regeneration in humans.