Making a Neural Tube: Neural Induction and Neurulation

The future central nervous system is derived from an unspecified sheet of ectoderm, with fate being instructed by signals emanating, in the main, from a specialized region of the early embryo, the organizer. The organizer secretes signals that have the net effect of inhibiting the BMP pathway, be it by extra-cellular antagonism or by intracellular modulation of the ability of the cell to perceive BMP signals. Other factors also play a role in neural induction, for example, the FGF family of molecules, but their exact role in neural induction remain unknown. As more players are identified in what undoubtedly will be a signaling network leading to neural induction, the exact molecular mechanism of neural induction can be established. Once induced, the neuroepithelium rolls into the neural tube. One model, and one that has gained widespread acceptance, is the hinge point model. In this model, both extrinsic (i.e., outside the neural plate) and intrinsic forces cooperate and synergize in bending the neural plate. Although the cellular behaviors of much of this process have been well characterized, the molecular bases for these behaviors have so far proved elusive. The relationship between induction of the neuroepithelium and its subsequent morphological movements is of particular interest to the developmental neurobiologist.