Embryonic Nervous System Development

• Neurolation is the process where the neural tube forms and develops into the spinal cord and brain.
• Complications during neurolation can lead to neural tube defects.
• The nervous system's development starts in the third week of gestation.
• Primary neurulation forms the functional central nervous system.

Development of the Embryonic Nervous System

• By day 18, the ectodermal germ layer is disc-shaped with cranial and caudal ends.
• Cross-sections reveal three germ layers: ectoderm, mesoderm, and endoderm.
• The central notochord induces the ectoderm to thicken, forming the neural plate.
• By the end of the third week, neural folds extend upwards from the neural plate edges.
• The depressed mid-region becomes the neural groove.
• Neural folds fuse at the midline, converting the neural groove into a closed neural tube.
• Neural crest cells separate from the neural folds and migrate throughout the body.
• Neural crest cells form Schwann cells, meninges, endocardial cushions, parafollicular cells, and the adrenal medulla.
• Before full fusion, the cephalic and caudal ends communicate with the amniotic cavity through cranial and caudal neuropores.
• The cranial neuropore closes around day 25.
• The cranial end develops into three primary vesicles: the forebrain (prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon).
• These three develop into five secondary vesicles: telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon.
• The caudal neuropore closes around day 27.
• Failure of the neural tube to close properly results in neural tube defects.
• Neural tube defects can occur as part of syndromes, with chromosomal disorders, or due to environmental exposure.
• Folic acid deficiency during pregnancy and folic acid antagonists increase the risk of neural tube defects.

Neural Tube Defects

• Anencephaly is the failure of the neural tube to close at the cranial end, preventing the brain from developing and is incompatible with life.
• Anencephaly results in high alpha-fetoprotein levels and polyhydramnios.
• Spina bifida is the failure of the neural tube to close at the caudal end.
• In spina bifida, the vertebra overlying the defect do not fully develop, leaving the vertebral arch open.
• Spina bifida occulta is an asymptomatic defect where the vertebral arch fails to fuse at the midline.
• Spina bifida occulta's only evidence may be a small tuft of hair over the defect.
• In spina bifida occulta, the spinal cord remains intact, and alpha-fetoprotein levels are normal during pregnancy.
• In spina bifida with meningocele, the meninges protrude through the vertebral defect, but the spinal cord does not.
• In spina bifida with meningomyelocele, both the meninges and spinal cord protrude through the vertebral defect, and is usually associated with Chiari II malformation and hydrocephalus.
• Alpha-fetoprotein levels are increased in spina bifida with meningomyelocele.
• In myeloschisis, the spinal cord is exposed because the skin does not cover the defect.
• Myeloschisis is the most severe type of spina bifida, and alpha-fetoprotein levels are increased during pregnancy.

The development of the embryonic nervous system begins in the third week of gestation. The ectoderm thickens, forming the neural plate, which folds to create the neural groove. This groove then closes to form the neural tube, a critical step in forming the brain and spinal cord.