DIFFERENTIAL PROGENITOR DISPERSAL AND THE SPATIAL ORIGIN OF EARLY NEURONS CAN EXPLAIN THE PREDOMINANCE OF SINGLE-PHENOTYPE CLONES IN THE CHICK HINDBRAIN

Clonal analysis of the chick embryo hindbrain has shown that during th e first 48 hr of neurogenesis the large majority of neural progenitor cells generate clones of neurons of only a single major phenotype or o f only closely related phenotypes. This is despite considerable spatia l intermixing of diverse neuronal phenotypes at these stages of develo pment and suggests that phenotype may he decided early in mitotic prec ursors and remembered through several subsequent rounds of division an d dispersal (Lumsden et al. [1994] Development 120:1581-1589). Here we have used fate-mapping and clonal analysis to study neuroepithelial c ell dispersal and mixing in the early hindbrain and discuss this data in relation to the generation of single phenotype neuronal clones. We find that dispersal is not uniform throughout the dorsoventral axis of the neural tube, hut is highly dependent on position along that axis. Neuronal identity is related to the spatial origin and, hence, enviro nment of the cell, and the spatial intermixing of diverse neuronal phe notypes at HH stage 20 is largely the result of circumferential neuron al migration as medially born branchial motor neurons migrate laterall y while the more laterally born mlf neurons migrate medially. Constrai nts on the dispersal of clonally related progenitors, in particular th ose that lie adjacent to the floor plate, may serve to restrict the fa te of these cells to the generation of only one major neuronal phenoty pe, i.e., motor neurons. (C) 1998 Wiley-Liss, Inc.