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Neurulation is the stage of development where the neural plate forms the neural tube. The events that occur during neurulation mark the beginning of the formation of the central nervous system.
The first event in neurulation is the formation of a thickened area of cells, called the neural plate.
Approximately 19 days after fertilization, the notochord induces the ectodermal cells lying just cranial to the primitive node to differentiate into columnar neuroepithelial cells, in a process called neural induction.
The neural plate forms at the cranial end of the embryo, and grows in a cranial to caudal direction. The cranial end of the neural plate indicates the region of the eventual brain, and the narrower caudal end represents the eventual region of the spinal cord.
By the end of the third week of development, the lateral edges of the neural plate become elevated, and fold to form the neural fold.
The groove created by the folding of the neural plate is called the neural groove. Gradually, the two neural folds approach each other, beginning around the level of the fourth somite.
Approximately 25 days after fertilization, the folds fuse together, and the neural plate transforms into the neural tube, the precursor to the central nervous system.
Fusion of the neural tube usually begins in the cervical region, extending in both cranial and caudal directions. The openings that are formed at the cranial and caudal ends of the embryo are called cranial and caudal neuropores.
During the closure of the neural tube, cells on the lateral side of the neural plate detach forming a new cell population, called the neural crest.
The neural crest cells migrate and displace the cells of the ectoderm to enter the underlying mesoderm. These cells contribute to the formation of the peripheral nervous system, including the formation of neurons and glial cells of the sympathetic, parasympathetic, and sensory nervous systems.
Once the neural tube has completely fused, the process of neurulation is complete.