Vertebrate embryonic cells will become nerve cells unless told otherwise

The past few years have witnessed a significant change in the understanding of how the vertebrate nervous system forms during embryogenesis. More than seventy years since Spemann and Mangold first demonstrated the phenomenon of neural induction, the molecular mechanisms underlying neural induction now appear to be at hand. Two independent approaches, one focusing on a ''default'' or ''ground-state'' model for neural induction and the other culminating in the discovery of secreted neural inducing factors (noggin, follistatin, and chordin), have not only brought to a successful conclusion the search for Spemann's neuralizing factor, but also illuminated its mechanism of action. It now appears that neuralization of embryonic cells occurs when cells do not receive other inducing signals telling them to form epidermis, mesoderm, or endoderm. This concept of neuralization allows for a reinterpretation of the classical views on both neural and epidermal specification. The secreted growth factor BMP4 (Bone Morphogenetic Protein) plays a pivotal role wherein BMP signaling induces epidermal differentiation. It is the absence of BMP signaling, accomplished by BMP antagonists including noggin, follistatin, and chordin, that leads to the formation of neural tissue.