Control of Cell Fate in a Vertebrate Neurogenic Lineage

The decision of an ectodermal cell to become a neuroepithelial cell initiates the building of a nervous system. The associated choice of which of the many different neuronal lineage pathways to follow begins the process of generating the vast numbers of different neuronal phenotypes that make up these exceptionally heterogeneous systems. Cell ablations and mutations in grasshoppers, flies, and worms have demonstrated a key role for cellular interactions in these events, particularly for neural induction as well as for a process of lateral inhibition-cells preventing the realization of their neighbors' potential for neuronal differentiation. Considerable excitement attends recent progress in the genetic dissection of cellular interactions underlying neuronal differentiation in the Drosophila retina. The importance of intercellular conversations comes as no surprise to vertebrate neuroembryologists, of course, for the role of target tissues in controlling neuronal survival, growth, and differentiation has been widely appreciated for a great many years. Indeed, the molecular basis for one such interaction has been known since the mid fifties-early sixties when Levi-Montalcini and her colleagues discovered the key role played by nerve growth factor (NGF)