neural fold

During neurulation (week 3-4 of development), the notochord induces the overlying ectoderm to thicken and form the neural plate. The lateral edges of this plate elevate to form the neural folds, while the central region depresses to form the neural groove. The neural folds move toward the midline and fuse, beginning in the cervical region and proceeding bidirectionally (like a zipper) toward the cranial and caudal ends, forming the neural tube. The cranial neuropore closes around day 25 and the caudal neuropore around day 27-28. Cells at the tips of the neural folds, known as neural crest cells, delaminate and migrate to form a wide variety of structures including dorsal root ganglia, autonomic ganglia, Schwann cells, melanocytes, adrenal medulla, and craniofacial cartilage/bone (via pharyngeal arch derivatives). Failure of the cranial neuropore to close results in anencephaly, while failure of the caudal neuropore to close results in spina bifida (ranging from spina bifida occulta to myeloschisis). Folate deficiency is a major risk factor for neural tube defects, and maternal folic acid supplementation (400 mcg/day) prior to and during early pregnancy significantly reduces this risk. Elevated maternal serum alpha-fetoprotein (AFP) and amniotic fluid AFP are classic screening markers for neural tube defects, with acetylcholinesterase in amniotic fluid being a more specific confirmatory test.