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Neurulation is the process by which the neural plate (a region of ectoderm induced by signals from the underlying notochord) folds into the neural tube, the precursor of the brain and spinal cord. The neural plate borders elevate to form neural folds, which converge at the midline and fuse, creating a hollow tube that detaches from the overlying ectoderm. At the junction where neural and non-neural ectoderm meet, the neural crest cells delaminate and migrate throughout the embryo, contributing to a remarkable diversity of cell types including peripheral neurons, melanocytes, and craniofacial cartilage. Failure of neural tube closure produces severe birth defects: anencephaly (failure to close at the head) and spina bifida (failure to close at the tail).
After gastrulation establishes the three germ layers, the first organ to begin forming is the nervous system. Neurulation transforms a flat sheet of ectodermal cells into the neural tube — the embryonic precursor of the entire central nervous system. The brain develops from the anterior end of the tube, the spinal cord from the posterior, and the hollow interior becomes the ventricles and central canal. Understanding this process is both scientifically fascinating and medically critical, as neural tube defects are among the most common birth defects worldwide.
Neural induction begins during gastrulation, when the notochord (dorsal mesoderm) signals to the overlying ectoderm. Contrary to early assumptions, the signal is not "become neural" but rather "stop being epidermal." BMP signaling promotes epidermal fate; the notochord secretes BMP antagonists (Chordin, Noggin, Follistatin) that create a BMP-free zone in the overlying ectoderm. In this zone, the ectoderm's default fate — neural — is permitted to manifest. The result is the neural plate, a thickened region of ectoderm directly above the notochord, distinguished from the surrounding presumptive epidermis by the absence of BMP signaling.
The neural plate then undergoes dramatic morphogenesis. The cells at the plate's midline (the medial hinge point) change shape — becoming wedge-shaped through apical constriction — causing the plate to bend. The lateral edges of the plate elevate to form neural folds, which move toward each other at the dorsal midline. When they meet, they fuse, creating a closed tube that separates from the overlying ectoderm. Closure begins in the middle of the embryo and proceeds both anteriorly and posteriorly (in humans, this process takes about a week, from days 21 to 28 of gestation). Failure of closure at the anterior end produces anencephaly (absence of brain); failure at the posterior end produces spina bifida. Folic acid supplementation reduces the incidence of these defects by supporting the rapid cell proliferation and gene regulation required for successful closure.
At the border where the neural plate meets the non-neural ectoderm, a special population of cells arises: the neural crest. These cells undergo an epithelial-to-mesenchymal transition, delaminate from the neural folds, and migrate throughout the embryo to form a remarkable diversity of derivatives: peripheral neurons and glia, melanocytes, craniofacial bone and cartilage, smooth muscle, and the adrenal medulla. The neural crest is so developmentally significant that it is sometimes called the "fourth germ layer." Defects in neural crest migration or differentiation produce a range of clinical conditions collectively called neurocristopathies, including Hirschsprung disease, Waardenburg syndrome, and many craniofacial anomalies.