Abstract
A central question in developmental neurobiology concerns the mechanisms that generate cellular diversity in the vertebrate nervous system. Cell lineage analyses have established that many progenitor cells in the developing nervous system are multipotent (Turner and Cepko 1987; Holt et al. 1988; Wetts and Fraser 1988). However, the mechanisms that control the differentiation of such progenitor cells are poorly understood. It is assumed that both the developmental history of a progenitor cell and its interactions with its local environment influence its final choice of developmental fate. Few experimental systems are available, however, in which it has been possible to dissect the relative contributions of these two factors to the differentiation program. In the vertebrate nervous system, only two well-defined multipotential progenitor cells have been isolated and studied in detail. These are the 02A progenitor, which gives rise to two types of glial cells in the central nervous system (Raff 1989),...