The present invention makes available a method for inducing neuronal differentiation and preventing the death or degeneration of neuronal cells both in vitro and in vivo. The subject method stems from the unexpected finding that, contrary to traditional understanding of neural induction, the default fate of ectodermal tissue is neuronal rather than mesodermal and/or epidermal. In particular, it has been discovered that preventing or antagonizing a signaling pathway in a cell for a growth factor of the TGF-.beta. family can result in neuronal differentiation of that cell.