"During spinal cord regeneration in the axolotl, it has been shown that all of the neurons in the regenerate derive from cells near the site of amputation. It is unknown whether the new neurons derive from a population of multipotent progenitor cells in the mature tissue, or whether committed neuronal progenitors also contribute. Focusing on committed neuronal progenitors expressing the basic helix-loop-helix (bHLH) family of proneural transcription factors, I will identify the expression patterns of these proteins in the developing, mature, and regenerating spinal cord and determine whether a population of committed neural progenitors persists in the adult axolotl tissue. Using a Cre/loxP reporter system to permanently label bHLH-expressing cells, I will track the fates of these cells during normal development and regeneration. This technique will also allow me to determine whether spinal cord regeneration involves de-differentiation of committed neuronal progenitors to a more embryonic-like state. Transplantation of labeled cells will allow me to determine whether positional cues direct the fate of a committed progenitor, or whether expression of specific bHLH proteins is itself enough to determine the fate of the cell. These experiments will provide a foundation for future experiments to identify the signaling molecules that induce progenitor cells to proliferate and regenerate the spinal cord after injury."