Although based on experimental evidence in animal models, most open neural tube defects (exencephaly, or anencephaly) result from failure of the neural tube to close, and some may be accounted for by postclosure rupture. Animal models have also illustrated that encephaloceles may be the result of abnormal closure. For example, delayed closure of the anterior neuropore appears to result in a rather tenuous closure with abnormally thin neuroepithelium and subsequent frontonasal encephalocele.
Because closure of the neural tube depends on a number of factors including the presence of normal neuroepithelium and its underlying mesenchyme and extracellular matrix, the types of and targets for insult that could result in failure of closure are also multiple. One of the most vulnerable periods for insult resulting in exencephaly in rodent embryos occurs before elevation of the cranial neural folds, when the embryos have approximately four to eight somite pairs (corresponding to the end of the third and beginning of the fourth week postfertiliza-tion in the human). At this time, premigratory neural crest cells appear to be particularly vulnerable to insult, their loss resulting in mesenchymal deficiency as well as interruption of the neuroepithelial integrity.60
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