Neural Induction and Neurulation

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Studies with molecular markers suggest that a neural plate is established by HH stage 5 (20-22 h). After this stage, the streak and node begin to regress posteriorly, a process that continues until about the 20 somite stage, after which they are no longer visible and their remnants become incorporated into the tail bud.

At stage 5, the first formation of the "head process" becomes apparent. It is a short aggregation of mesoderm directly anterior to the node and continuous with prechordal mesoderm, and it derives from the deep part of the node (primitive pit). The head process condenses to form the notochord, which is readily apparent anterior to the node as the latter starts to regress at subsequent stages. The first pair of somites condense either side of the notochord at HH stage 7, and these, and the subsequent four pairs, lie beneath part of the neural plate fated to form the hind-brain. The remainder underlie prospective spinal cord. The cardiac primordium begins to form at the anterior end of the embryo at HH stage 8 (four somites).

The neural plate comprises a pseudostratified epithelium. Between its caudal tip and the prospective infundibulum, its midline overlies the notochord, which induces the overlying neuroepithelial cells to become the floorplate. Anterior to the infundibulum the neural plate overlies prechordal mesoderm. Lateral to the midline, the neural plate overlies paraxial mesoderm: the segmented somitic mesoderm posteriorly (behind the prospective otocyst) and unsegmented cranial paraxial mesoderm anteriorly.

Neural tube closure (neurulation) begins at stage 8 at the level of the mid-brain and extends both anteriorly and posteriorly. Closure at the rostral extremity (anterior neuropore) is complete by stage 10, whereas the posterior neuropore remains open until the tail bud develops. Soon after closure, neural crest emerges from the midbrain and hindbrain (HH stages 10-12; 36 h) and, later, from the spinal cord.

Simultaneously with neurulation, the embryo also folds ventrally to enclose the gut and bring the two heart primordia together to fuse.

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