What are the mechanism by which the basic neuronal pathways within the vertebrate CNS are laid down? Along what surfaces do axons of the early pathways grow? Are the first axons of any pathway important for its proper formation? Is the appropriate arrangement of these basic pathways required for normal development of those that follow? The goal of this research program is to examine the patterns and mechanisms involved in development of the early longitudinal pathways of the amphibian spinal cord. The relatively simple caudal spinal cord of embryonic and larval Xenopus laevis will be use. Studies under taken during the current grant period have identified the earliest descending fiber bundles of the tail spial cord and the neurons contributing to them, and have detemrined their developmental time course. Later appearing pathways that add to the initial bundles have been simularly described. The experiments proposed here will use the first two axon bundles of the spinal cord to pursue two main lines of investigation: 1) I will use retrograde tracers with combined light and electron microscopy to define the substrata along which axons of different stages in the sequence travel. I will describe the precise arrangement of identified axons and their growth cones with respect to their surroundings from the time pathways are initially laid out through the period when other pathways add to them, and 2) I will use photoelimination techniques to test the importance of the earliest axons to subsequent pathway development. In separate experiments I will delete the initial axons of the first two spinal pathways and the entire bundle of axons comprising each of these pathways. Results from these studies will increase our understanding of how fiber tracts in the vertebrate spinal cord are laid out during normal development and will furnish the basis for comparison of normal and regenerative spinal tract development.
