new relation is the process of neural tube formation and its development into the spinal cord and the brain any complication during this complex process can result in neural tube defects the development of the nervous system begins during the third week of gestation primary neural ation is the process that forms the functional central nervous system let's begin with the development of the embryonic nervous system by day 18 of fetal development the ectodermal germ layer has the shape of a disc with a cranial and caudal end the cross-section on the right illustrates three germ layers the ectoderm mesoderm and endoderm at this point the central nodal cord has induced the overlying ectoderm to thicken and form the neural plate the cross-section on the right will help visualize the folding of the neural plate leading to the formation of the neural tube by the end of the third week the edges of the neural plate extend upward to become neural folds the depressed mid region is now the neural groove each neural fold then grows toward each other to fuse at the midline converting the neural groove into a complete and closed and hollow neural tube at the same time as the neural folds fuse a population of cells separates from the tips of the neural folds to become a mass of his ankle cells called the neural crest the neural crest cells migrate throughout the body to form a wide variety of cell types including Schwann cells the meninges endocardial cushions power follicular cells and the adrenal medulla before the fusion of the neural tube is complete the cephalic and caudal ends communicate with the amniotic cavity through the cranial and caudal neuro pores the cranial neural pore closes on day 25 after closure the cranial end of the neuro tube develops into three primary vesicles the forebrain or pros and Cephalon the midbrain or mesencephalon and the hindbrain or rhombencephalon these three develop later into five secondary vesicles the forebrain develops into the telencephalon and diencephalon the midbrain develops into the mesencephalon the hind brain develops into Menton Cephalon and the myelin Cephalon the caudal neuro poor then closes on day 27 during early embryonic stages of development failure of the neuro tube to close properly results in neural tube defects one of the most common causes of congenital abnormalities neural tube defects can occur as part of syndromes and associated with chromosomal disorders or as a result of an environmental exposure folic acid deficiency during pregnancy and folic acid antagonists such as carbamazepine phenytoin and trimethoprim increased the risk of neural tube defects anencephaly is the failure of the neuro tube to spontaneously close at the cranial end hence the brain does not develop this condition is incompatible with life anencephaly causes high alpha-fetoprotein levels and polyhydramnios during pregnancy spina bifida is the failure of the neural tube to spontaneously close at the caudal end vertebra overlying the defect do not fully develop causing the vertebral arch to remain open spina bifida occulta is an asymptomatic defect caused by failure of the two halves of the vertebral arch to fuse at the midline the only evidence of its presence may be a small tuft of hair over the defect on the right illustration we can see that spina bifida occulta only affects the vertebral arch leaving the spinal cord intact it's important to know that in spina bifida occulta there is no increase of alpha-fetoprotein levels during pregnancy in spina bifida with been in jail seal the meninges protrude through the vertebral defect without protrusion of the spinal cord in spina bifida with meningeal Milo seal both the meninges and the spinal cord protrude through the vertebral defect notice that the skin covers the neural tube defect it is usually associated with Kyary - malformation and hydrocephalus alpha-fetoprotein levels will be increased in spina bifida with milo species the spinal cord can be seen externally since the skin does not cover the protruding defect this is the most severe type and will also cause alpha-fetoprotein levels to be increased during pregnancy you