ABSTRACT
During the third week of gestation, the notochordal process induces formation of the neural plate, which appears as a thickening in the epiblast on each side of the midline. This primary neural induction is believed to be caused by the action of two active diffusible signals (probably proteins). The initial signal induces formation of the rostral structures, and then induction proceeds caudally. However, these signals are not sufficient for all aspects of neural induction. Underlying tissue derived from the primitive streak is also required for neural tube formation. Nerve growth factor, epidermal growth factor, active inhibitors, noggins, follistatin and dorsalin have been implemented in the induction and maintenance of neural tissue. Many other proteins encoded by homologues of notch (xotch), Wnt and hedgehog genes have also been suggested to influence the early development of the nervous system.
