neural groove

**Embryology/Process (neurulation, ~days 18–28):** - Notochord and prechordal mesoderm induce overlying ectoderm (via sonic hedgehog from notochord, and inhibition of BMP-4 by noggin, chordin, follistatin) to thicken into the neural plate (neuroectoderm). - The neural plate invaginates along its midline to form the neural groove, flanked by the neural folds. - The neural folds elevate, approach each other, and fuse in the midline, beginning at the future cervical region (around somite 4–6) and proceeding cranially and caudally like a zipper, forming the neural tube. - Open ends are the cranial (anterior) neuropore and caudal (posterior) neuropore. The anterior neuropore closes ~day 25 (about day 24–26) and the posterior neuropore ~day 27–28 (about day 26–28). - Neural crest cells arise from the crest of the neural folds, undergo epithelial-to-mesenchymal transition, and migrate. They give rise to the PNS ganglia (dorsal root and autonomic), Schwann cells, melanocytes, adrenal medulla chromaffin cells, odontoblasts, parafollicular (C) cells, aorticopulmonary septum, and facial skeleton/cartilage, among others. - Neural tube lumen becomes the ventricular system and central canal. Neural tube gives rise to CNS neurons, ependyma, astrocytes, oligodendrocytes (neuroepithelial cells). Microglia derive from mesoderm (yolk sac). - Alar plate (dorsal, sensory) and basal plate (ventral, motor) are separated by the sulcus limitans.

**Clinical significance:** - Failure of neural fold closure produces neural tube defects (NTDs): - Anencephaly: failure of anterior neuropore closure; absent forebrain/calvarium; polyhydramnios (no swallowing), "frog-like" appearance. - Spina bifida occulta: intact skin, tuft of hair or dimple over the defect, usually asymptomatic. - Meningocele: meninges herniate through the defect. - Myelomeningocele: meninges plus spinal cord/nerve roots; associated with Chiari II malformation and hydrocephalus. - Myeloschisis/rachischisis: open neural tissue exposed. - Screening: elevated maternal serum AFP and acetylcholinesterase in amniotic fluid; ultrasound. (Low AFP is associated with Down syndrome.) - Prevention: folic acid supplementation, 0.4 mg/day for all women of childbearing age, 4 mg/day for those with a prior NTD-affected pregnancy, started at least 1 month before conception and continued through the first trimester. - Risk factors: folate deficiency, valproate and carbamazepine, maternal diabetes, obesity, hyperthermia. - Neural crest-related disorders: Hirschsprung disease, neurofibromatosis, DiGeorge, and others.

**High-yield pearls:** - Neural plate → neural groove → neural folds → neural tube. - Notochord induces the neural plate; it persists as the nucleus pulposus of the intervertebral disc. - Neural tube is derived from surface ectoderm-derived neuroectoderm; neural crest is a separate ectoderm-derived population. Surface ectoderm gives rise to epidermis, lens, and anterior pituitary (Rathke pouch), and others. - Cranial neural tube closure defects are more often fatal; lumbosacral defects are the most common site for spina bifida.