PROGENITOR DISPERSAL AND THE ORIGIN OF EARLY NEURONAL PHENOTYPES IN THE CHICK-EMBRYO SPINAL-CORD

Using DiI and fluorescent dextrans, we have created fate maps of the n eural plate and early neural tube describing the extent of progenitor cell dispersal and the spatial origin of morphologically distinct neur onal cell types along the dorsoventral axis of the developing chick sp inal cord. Nonuniform dispersal and mixing of progenitors occur within the early neuroepithelium, with the degree of dispersal being determi ned by the initial position of the cells along the mediolateral axis o f the neural plate. Dispersal is greatest in the midregions of the ven tricular epithelium and decreases toward the dorsal and ventral midlin es. Phenotypically diverse classes of neurons are born at specific dor soventral locations in the neural tube. Motor neurons are the most ven tral cell type generated followed, at progressively more dorsal positi ons, by distinct classes of interneurons. Several genes show dorsovent rally restricted patterns of expression within the neural tube and the fate maps were used to investigate the relationship between one of th ese genes, Pax3, and progenitor cell dispersal and fate. The results i ndicate that the dorsoventral pattern of Pax3 expression is not mainta ined by restrictions to cell mixing and are consistent with a role for this transcription factor in specifying the identity of neurons with contralateral descending axons. (C) 1998 Academic Press.