• Neurulation includes the formation of the neural plate (day 18-19), neural folds (day 20-21), and the neural tube (day 22-26); the latter will develop into the future brain and spinal cord
• The neural plate forms as a central strip of surface ectoderm cells, just above the notochord. It is induced to become neuroectoderm through changes in cell shape (cuboidal to columnar) and proliferation; the cells develop N-CAM adhesion molecules, disorganized filaments and elongated tubules; this flat layer of cells is referred to as the neural plate

• The layer of cells between the regular surface ectoderm and the neuroectoderm possess characteristics of both cell types, such as the possession of both L-CAM (ectoderm) and N-CAM (neuroectoderm) adhesion molecules, and are called neural crest cells.
• At the neural plate, the cells continue to morph into pyramidal shaped cells with straight neuro-tubules, neuro-filaments concentrated at the apex of the cell, connected by desmosomes
• The change in cell shape and a decrease in cell number cause the neuroectodermal cells to gradually fold inward, forming neural folds (d 20-21)

• The neural tube continues to grow in this manner, and eventually breaks free of the surface ectoderm which will be sealed off. Up to this point, it was supplied continuously with amniotic fluid.

• As the neural tube forms, the closing process is critical, occurring from the cranial to the caudal end as the anterior neuropore closes around day 24, the posterior around day 26. This is a critical event, as defects in closure may result in spina bifida or other neural tube defects. The risk of a neural tube defect can be decreased by folic acid supplements, a campaign led by the March of Dimes.
• The neuroepithelial cells at this stage are bipotential, capable of forming neurons or neuroglial cells.
• Once the neural tube has closed completely, vertebral structures develop around it, as do meninges and finally, skin from the surface ectoderm

Review: The notochord is of mesenchymal origin. During formation, it is continuous with embryonic endoderm, allowing for the neurenteric canal between the amniotic and yolk sac cavities. The notochord then detaches from the endoderm to form a closed tube in the mesoderm. On the other hand, the neural tube originates from the ectodermal layer, when the surface ectoderm is induced by the notochord to form neuroectoderm. Thus, the neural tube begins its development intercalated in the primitive ectoderm but, like the notochord, eventually detaches to form a closed tube.

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