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neural tube defect
Neural tube defects (NTDs) are a group of conditions in which an opening in the spinal cord or brain remains from early in human development. In the 3rd week of pregnancy called gastrulation, specialized cells on the dorsal side of the embryo begin to change shape and form the neural tube. When the neural tube does not close completely, an NTD develops.
Specific types include: spina bifida affects the spine, anencephaly results in little to no brain, encephalocele affects the skull, and iniencephaly which results in severe neck problems.〔(【引用サイトリンク】url=http://www.nichd.nih.gov/health/topics/ntds/conditioninfo/Pages/default.aspx )〕
NTDs are one of the most common birth defects, affecting over 300,000 births each year worldwide. For example, spina bifida affects approximately 1,500 births annually in the USA, or about 3.5 in every 10,000 (0.035% of US births), which has decreased from around 5 per 10,000 (0.05% of US births) since folate fortification was started.〔 The number of deaths in the USA each year due to neural tube defects also declined from 1,200 before folate fortification was started to 840.
==Types==
There are two types of NTDs: open, which are more common, and closed. Open NTDs occur when the brain and/or spinal cord are exposed at birth through a defect in the skull or vertebrae (back bones). Examples of open NTDs are anencephaly, encephaloceles, hydranencephaly, iniencephaly, schizencephaly, and spina bifida. Rarer types of NTDs are called closed NTDs. Closed NTDs occur when the spinal defect is covered by skin. Common examples of closed NTDs are lipomyelomeningocele, lipomeningocele, and tethered cord.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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