Neurulation is a part of organogenesis in vertebrate embryos. Steps of neurulation include the formation of the dorsal nerve cord, and the eventual formation of the central nervous system. The process begins when the notochord induces the formation of the CNS (central nervous system) by signaling the ectoderm germ layer above it to form the neural plate. The neural plate folds in upon itself to form the neural tube, which will eventually form the vertebrate CNS.
Neural plate and tube
The neural plate is a thick and flat bundle of ectoderm formed after induction by the notochord. Mesoderm surrounding the notochord at the sides will develop into the somites (future muscles, bones, and limbs of the vertebrate). The neural plate will develop into the neural tube which gives rise the CNS, comprised by the spinal cord and the brain. Masses of tissue called the neural crest separate from the neural tube and form the cord's sensory neurons, eventually coming to be the PNS (peripheral nervous system) and a few other types of cells.
Different portions of the neural tube form by two different processes, called primary and secondary neurulation, in different species. In primary neurulation, the neural plate creases inward until the edges come in contact and fuse. In secondary neurulation, the tube forms by hollowing out of the interior of a solid precursor.
Primary neurulation occurs in response to soluble growth factors secreted by the notochord. After Shh from the notocord induces its formation, the floor plate of the incipient neural tube also secretes Shh. After closure, the neural tube forms a basal plate and an alar plate in response to the combined effects of Shh and factors including BMP4 (bone morphogenic protein 4) secreted by the roof plate. The basal plate forms most of the ventral portion of the nervous system, including the motor portion of the spinal cord and brain stem; the alar plate forms the dorsal portions, devoted mostly to sensory processing.
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Neural tube defects in humans
Normally the closure of the neural tube occurs around the 30th day after fertilization. However, if something interferes and the tube fails to close properly, a neural tube defect will occur. Among the most common tube defects are anencephaly, encephalocele, and spina bifida. The incidence of neural tube defects is 2.6 in 1,000 worldwide.
Pregnant women taking medication for epilepsy have a higher chance of having a child with a neural tube defect. Research has shown that women with folic acid deficiences also have a higher chance of having a child with a neural tube defect, but this is only one factor. Taking folic acid does not completely negate the risk of neural tube problems, but markedly reduces the risk.
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