Development of central nervous system and neural tube defects
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Central nervous system develops from the neural tube, formed by neurulation in the 3rd and 4th weeks.
Neural crest cells, breaking away from the neural fold tips during neurulation, form nearly the entire peripheral nervous system, distinct from the tube's CNS derivatives.
Development of Spinal Cord
Spinal cord develops from the caudal neural tube; its wall thickens into three zones: ependymal (matrix), mantle (grey matter), and marginal (white matter).
Classical theory: ependymal zone cells proliferate into neuroblasts (become neurons) and spongioblasts (become neuroglia), which migrate outward into the mantle and marginal zones.
Newer theory: the tube wall has one pluripotent neuroepithelial cell type showing interkinetic nuclear migration; the zonal look reflects phases of one proliferative cycle, not separate populations.