DESCRIPTION (From the Applicant's Abstract): Stem cells are self-renewing multipotent progenitors with the broadest developmental potential in a given tissue at a given time (Morrison et al. Cell 88:287). There is great interest in neural stem cells because of their importance in neural development and their therapeutic potential. Neural crest stem cells (NCSCs) migrate out of the neural tube in early to mid-gestation, and give rise to the peripheral nervous system (PNS) as well as other tissues. In order to understand PNS formation, the process by which NCSCs differentiate into the diverse cell types of the PNS must be understood. Although neural crest cells were thought to differentiate within days of migrating, we have recently discovered that NCSCs persist into late gestation by self-renewing within peripheral nerves (Morrison et al. Cell 96:737). This observation suggests that PNS development may be more dynamic than previously thought and raises several specific questions. First, why would the self-renewal of NCSCs be promoted within peripheral nerves if, as current models suggest, the neural crest only gives rise to Schwann cells in nerves? Aim #1 is to use CRE-recombinase fate mapping to test whether the neural crest actually gives rise to multiple lineages of cells within nerves. If so, nerve development would require multilineage differentiation by stem cells rather than just overt differentiation by Schwann precursors. This would fundamentally change models of nerve development. Aim #2 is to test whether postmigratory NCSCs also persist in other areas of the fetal PNS. Aim #4 is to test whether there are cell-intrinsic differences between these NCSC populations from different regions of the PNS in terms of the types of neurons they can form. If there are cell-intrinsic differences between postmigratory NCSC populations then perhaps differences between stem cell lineages interact with environmental differences to generate neural diversity. Aim #3 is to test whether rare NCSCs also persist postnatally. The discovery of NCSCs in postnatal tissues might fundamentally alter approaches to regeneration in the PNS. Answers to these questions could change the way we think about PNS development, injury, and disease.