Embryology of the Nervous System

Induction and Neurulation (Weeks 3-4)

• Neural plate formation: The notochord (derived from the primitive node) secretes sonic hedgehog (Shh), bone morphogenetic proteins (BMPs), and other morphogens that induce the overlying epiblast to differentiate into the neural plate—a thickened region of ectoderm fated to become nervous tissue. This process is mediated by FGF (fibroblast growth factor) and Wnt signaling inhibition.
• Neural fold elevation and tube closure: The neural plate undergoes a process of primary neurulation wherein its lateral edges (neural folds) elevate and fuse in the midline to form the neural tube. This fusion begins at the cervical region (around week 3.5) and proceeds bidirectionally—rostral fusion (toward the brain) and caudal fusion (toward the spine). The anterior neuropore closes around day 25 and the posterior neuropore around day 27. Failure of fusion results in open neural tube defects (NTDs) such as anencephaly (failure of rostral closure) and spina bifida (failure of caudal closure).
• Neural crest cell delamination: As the neural tube closes, specialized cells at the junction between neural and non-neural ectoderm—the neural crest cells—undergo epithelial-to-mesenchymal transition (EMT) and delaminate from the dorsal neural tube. These cells migrate extensively and give rise to most PNS structures including all dorsal root ganglia (sensory neurons), sympathetic chain ganglia, the adrenal medulla, Schwann cells, and meninges of the PNS. Neural crest cells also contribute to craniofacial structures, melanocytes, and other non-neural tissues.

CNS Regionalization (Weeks 3-8)

• Primary vesicle formation: The early neural tube immediately undergoes regionalization into three primary brain vesicles: the prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain). This regionalization is controlled by transcription factors including Otx2 (anterior), Gbx2 (posterior), and organizer regions that express morphogens like Wnt and Fgf.
• Secondary vesicle subdivision: By week 5, the prosencephalon subdivides into the telencephalon (which generates the cerebral cortex, hippocampus, amygdala, and basal ganglia) and the diencephalon (which generates the thalamus, hypothalamus, epithalamus, retina, and optic nerve). The mesencephalon remains unsegmented. The rhombencephalon divides into the metencephalon (which generates the cerebellum and pons) and the myelencephalon (which generates the medulla oblongata). The region between the midbrain and hindbrain (the isthmic organizer) contains important signaling centers.
• Flexures: During week 4, the neural tube develops three characteristic flexures—the cephalic flexure (midbrain region), the cervical flexure (rhombencephalon-spinal cord junction), and the pontine flexure (within the hindbrain)—that bend the developing brain into characteristic C-shaped anatomy.

Proliferation, Migration, and Differentiation (Weeks 5-24+)

• Neuroepithelial proliferation: The ventricular zone of the neural tube serves as a germinal matrix containing multipotent neural stem cells and radial glia that undergo rapid mitosis to generate neurons and glia. Neurons are generated primarily between weeks 8-16 (with some ongoing neurogenesis in specific regions); glial cells (astrocytes and oligodendrocytes) are generated primarily after week 16. The timing of neurogenesis determines cortical layer identity—inside-out layering results because earlier-generated neurons migrate past existing neurons, so the deepest layers (VI) contain the oldest neurons and superficial layers (II/III) contain the youngest.
• Neuronal migration: Newly born immature neurons (neuroblasts) migrate from the ventricular zone to their final destinations via radial migration (along radial glial fibers) and tangential migration (along fiber tracts and through the intermediate zone). GABAergic interneurons originating in the medial ganglionic eminence (MGE) and caudal ganglionic eminence (CGE) migrate tangentially into the cortex. Errors in migration produce cortical malformations including polymicrogyria (excessive small folds), lissencephaly (absent gyri from migration failure), heterotopias (ectopic neurons), and schizencephaly (clefts through cortex).
• Axonogenesis and synaptogenesis: Growing axons navigate toward their targets using growth cones that respond to chemoattractant molecules (e.g., netrin, Shh) and chemorepellent molecules (e.g., slit, ephrin). Major fiber tracts including the corpus callosum, commissures, and major ascending/descending tracts form during weeks 12-20. Synaptic connections form exuberantly initially then are pruned through activity-dependent competitive elimination; this refinement continues into postnatal development and puberty.
• Myelination: Oligodendrocytes (CNS) and Schwann cells (PNS) begin wrapping axons in myelin around month 5 of gestation for large caliber axons, with myelination continuing through early adulthood. CNS myelination proceeds from ventral to dorsal regions and from rostral to caudal; the corticospinal tracts myelinate well into infancy.

Ventricular System and Cerebrospinal Fluid Formation (Weeks 4-8)

• Ventricle development: The neural tube's central canal expands into the four cerebral ventricles (two lateral ventricles in the telencephalon, the third ventricle in the diencephalon, and the fourth ventricle in the hindbrain). The choroid plexus—specialized ependymal tissue with high metabolic activity—begins producing cerebrospinal fluid (CSF) by week 4. Obstruction of CSF flow through developmental malformations produces congenital hydrocephalus.

Spinal Cord Development (Weeks 3-8)

• Neural tube closure: The spinal cord develops from caudal portions of the neural tube and the more caudal portion differentiates through a process called secondary neurulation (involving a solid neural cord that cavitates). The notochord lies ventral to the spinal cord and eventually becomes the nucleus pulposus of the intervertebral discs. Failure of neural tube closure results in spina bifida, while notochord remnants can form chordomas.
• Spinal cord segmentation and fiber tract formation: The spinal cord develops cervical, thoracic, lumbar, and sacral segments. Alar plates (dorsal neural tube) become sensory components including dorsal columns and dorsal horns, while basal plates (ventral neural tube) become motor components including ventral horns with motor neurons. By week 8, major ascending tracts (spinothalamic, dorsal columns) and descending tracts (corticospinal) begin forming.
• Segmental organization and spinal nerves: The spinal cord is segmentally organized; dorsal root ganglia (containing sensory neuron cell bodies derived from neural crest) form adjacent to each spinal cord segment, while ventral roots carry motor axons from spinal cord motor neurons. Meninges (derived from neural crest) envelop the spinal cord.

Cranial Nerve Development

• Rhombomeric segmentation: The hindbrain (rhombencephalon) is divided into eight rhombomeres (r1-r8) separated by rhombomeric boundaries. Each rhombomere generates specific cranial nerve nuclei and nerves. Neural crest cells from specific rhombomeres form the cranial ganglia that become sensory ganglia for CN V (trigeminal), CN VII (facial), CN IX (glossopharyngeal), and CN X (vagus).
• Cranial nerve organization: CN I (olfactory) and CN II (optic) are not true cranial nerves but rather extensions of the forebrain. CN III-XII (oculomotor through hypoglossal) develop as somatic motor and branchiomotor components innervating muscles and as sensory components subserving various modalities.

Meningeal Development

• The meninges derive from neural crest cells (primarily) and mesoderm. The dura mater derives largely from mesoderm, the arachnoid and pia mater from neural crest cells. The subarachnoid space fills with CSF.

Genetic Factors

• Gene mutations: Mutations in genes controlling neurulation (e.g., VANGL1, CELSR1, SCRIB in planar cell polarity pathway), neural crest migration (e.g., EDN3, GDNF, SOX10), and axonal guidance (e.g., netrin, slit, semaphorin genes) cause malformations. Holoprosencephaly results from mutations in SHH, ZIC2, SIX3, and other genes controlling midline development.
• Chromosomal abnormalities: Trisomy 13, trisomy 18, and 22q11 deletion syndrome frequently present with CNS malformations.

Maternal Nutritional Deficiencies

• Folate deficiency: Low folic acid intake during the periconceptional period (weeks 2-4) increases NTD risk approximately 2-3 fold. Homocysteine elevation (folate or B12 deficiency) correlates with NTD risk. Periconceptional folic acid supplementation (0.4-5 mg daily) reduces NTD risk by approximately 50%.
• Other nutritional deficiencies: Vitamin B12 and vitamin A deficiencies increase NTD risk.

Maternal Medications and Chemicals

• Antiepileptic drugs (AEDs): Phenytoin, valproic acid (VPA), phenobarbital, and carbamazepine all increase NTD risk, with VPA carrying the highest risk (1-2% incidence). Carbamazepine and phenytoin cause a recognizable pattern of birth defects (fetal anticonvulsant syndrome).
• Retinoids: All-trans retinoic acid and other vitamin A analogues are highly teratogenic, causing holoprosencephaly, microcephaly, and other CNS malformations; absolute contraindication in pregnancy.
• Other medications: Methotrexate (antifolate), lithium (increases Ebstein anomaly and other CNS defects), topiramate (increases cleft lip/palate and possible NTD increase).
• Alcohol: Fetal alcohol spectrum disorders (FASD) result from maternal alcohol exposure and cause microcephaly, intellectual disability, corpus callosum abnormalities, and other CNS defects. No safe level of alcohol consumption in pregnancy exists.
• Tobacco: Maternal smoking correlates with increased NTD risk.
• Teratogenic chemicals: Pesticides, organic solvents, lead exposure increase malformation risk.

Maternal Metabolic Disorders

• Maternal diabetes: Poorly controlled maternal diabetes (particularly type 1) increases NTD risk approximately 2-3 fold and also increases risk of caudal regression syndrome (sacral agenesis/spinal dysgenesis spectrum).
• Hyperthermia: Maternal fever (>38.9°C) in the first trimester, particularly from herpes simplex virus (HSV) infection, increases NTD and other CNS defect risk.

Maternal Infections

• Cytomegalovirus (CMV): In utero CMV infection causes microcephaly, cortical malformations, polymicrogyria, and other CNS manifestations.
• Zika virus: Maternal infection, especially during the first trimester, causes microcephaly and other CNS malformations including cortical abnormalities and brainstem hypoplasia.
• Rubella, syphilis, toxoplasmosis: Each causes CNS manifestations when acquired in utero.

Environmental Exposures

• Radiation: High-dose ionizing radiation during the critical period (weeks 3-4) increases neural tube defects and microcephaly.

Open Neural Tube Defects (NTDs)

Anencephaly (Failure of Rostral Neural Tube Closure)

• Cardinal presentation: Absence of major portions of the brain, skull, and scalp. The condition is incompatible with life, and affected fetuses die in utero or within hours of birth.
• Prenatal findings: Markedly elevated maternal serum alpha-fetoprotein (MSAFP) (typically >7-10 multiples of the median [MoM]), elevated amniotic fluid AFP, and characteristic ultrasound findings of absent cranial vault.
• Physical exam findings at birth: Absence of skull vault and scalp; exposed neural tissue; facial features remain intact; polyhydramnios often present (due to inability to swallow amniotic fluid).

Spina Bifida (Failure of Caudal Neural Tube Closure)

• Cardinal presentation: Incomplete closure of the vertebral column and spinal cord, with severity ranging from occult (no visible defect) to open (exposed neural tissue). The neurological deficit depends on the level (higher = worse) and severity of the malformation.
• Neurological deficits: Include paraplegia or paresis (depending on level), bowel/bladder dysfunction (neurogenic bladder leading to urinary retention or incontinence), sensory loss below the lesion level, loss of reflexes below the level, and variable lower limb deformities (club foot, hip dislocation).
• Physical exam findings:
• Visible defect with exposed meninges (meningocele) or spinal cord (myelomeningocele)
• Tuft of hair, dimple, or skin tag overlying the lesion
• Flaccid paralysis and sensory loss in stocking distribution below the lesion
• Anal wink reflex absent if sacral involvement
• Orthopedic deformities including clubfoot, knee flexion contractures, hip dislocation
• Associated findings: Chiari malformation (hindbrain herniation through foramen magnum, often causing hydrocephalus), syringomyelia (fluid-filled cavity within spinal cord), tethered spinal cord (spinal cord anchored by filum terminale or scar tissue causing progressive neurological deterioration).

Prenatal screening (first step)

• Maternal serum alpha-fetoprotein (MSAFP): drawn at roughly 15–20 weeks. AFP is a fetal-derived protein that leaks directly into amniotic fluid and then maternal circulation when neural tissue is not covered by skin, so it is elevated only in open defects. ACOG recommends that all pregnant patients be offered screening for neural tube defects, either by MSAFP or by ultrasound.
• Common false-positive causes: underestimated gestational age, multiple gestation, abdominal wall defects (gastroschisis/omphalocele), and fetal demise — recheck dating before pursuing invasive testing.

Confirmatory imaging (the practical gold standard)

• Targeted (level II) obstetric ultrasound: directly visualizes the defect and its level. Cranial signs of the associated Chiari II malformation are what examiners test — the lemon sign (scalloped frontal bones) and the banana sign (cerebellum wrapped around the brainstem with an effaced cisterna magna). Ventriculomegaly is often present.
• Fetal MRI: reserved for anatomic clarification, for candidacy assessment before in-utero repair, and when ultrasound is limited.
• Amniocentesis: amniotic fluid AFP plus acetylcholinesterase (AChE); a positive AChE distinguishes true open NTD from maternal/fetal blood contamination. Karyotype or chromosomal microarray is offered because NTDs cluster with trisomy 13 and 18.

Postnatal evaluation

• Anencephaly is a clinical diagnosis at delivery — absent calvarium and scalp with exposed dysplastic neural tissue.
• Myelomeningocele: neurologic level is established by segmental motor and sensory exam and anal wink; head ultrasound through the anterior fontanelle screens for hydrocephalus, with serial occipitofrontal circumference plotted on growth curves.
• Occult dysraphism (sacral dimple, hair tuft, lipoma): spinal ultrasound is the initial study in infants before posterior element ossification; MRI of the spine is definitive and is the test for tethered cord or syringomyelia at any age.
• No named severity score is in routine use; prognosis is stratified by anatomic level of the lesion.

Immediate neonatal stabilization (open myelomeningocele)

• Sac protection: nurse prone or lateral; cover the defect with sterile, non-adherent saline-moistened gauze to prevent desiccation and rupture. Avoid pressure on the sac.
• Latex-free environment from the delivery room onward: repeated surgical and catheter exposure makes latex sensitization and anaphylaxis a lifelong risk in this population.
• Antibiotics: broad-spectrum perioperative coverage until the defect is closed, since exposed neural tissue is a direct conduit to ventriculitis.

Definitive surgical management

• Postnatal closure: neurosurgical repair within the first day or two of life is standard; it prevents infection and prevents further mechanical injury but does not reverse established deficits.
• Prenatal (in-utero) repair: the Management of Myelomeningocele Study (MOMS) showed that mid-gestation fetal closure reduces the need for CSF shunting and improves motor function relative to postnatal repair, at the cost of preterm birth and uterine dehiscence. ACOG supports offering it at experienced fetal centers to carefully selected candidates; obesity, multiple gestation, prior preterm birth, and coexisting unrelated anomalies exclude patients.
• CSF diversion: ventriculoperitoneal shunt, or endoscopic third ventriculostomy with choroid plexus cauterization, for progressive hydrocephalus.
• Posterior fossa decompression for symptomatic Chiari II, and detethering for symptomatic tethered cord.

Longitudinal medical management

• Neurogenic bladder: clean intermittent catheterization with urodynamic surveillance, plus an antimuscarinic (oxybutynin) for detrusor overactivity — renal preservation drives long-term survival.
• Bowel program, orthopedic/physiatry care, and shunt surveillance through a multidisciplinary spina bifida clinic.

Prevention and contraindications

• Folic acid: USPSTF recommends daily supplementation for all persons planning or capable of pregnancy; CDC/ACOG advise a substantially higher dose for a prior affected pregnancy, begun before conception.
• Avoid valproate and, where possible, other high-risk antiepileptics in people who may conceive.
• Anencephaly has no corrective therapy; ACOG frames care as counseling, comfort measures, and termination or perinatal palliative options.

Emergencies — recognize these first

• Shunt malfunction or infection: obstruction raises ICP acutely. Infants show bulging fontanelle, vomiting, rapidly rising head circumference, and sunsetting eyes (upward gaze palsy from tectal compression); older children show headache, lethargy, and papilledema. Fever with shunt symptoms suggests ventriculitis, most often coagulase-negative staphylococci. Requires urgent imaging and shunt tap/revision.
• Symptomatic Chiari II malformation: caudal displacement of the medulla and cerebellar vermis compresses lower cranial nerve nuclei — inspiratory stridor, apnea, dysphagia, and aspiration in infancy. A leading cause of early death; requires emergent decompression or shunt evaluation.
• Latex anaphylaxis: IgE-mediated after repeated surgical exposure; presents intraoperatively as hypotension and bronchospasm. Treat with intramuscular epinephrine 0.3 mg (adult dosing; weight-based in children).
• Meningitis/ventriculitis from an unrepaired open defect: direct bacterial seeding of CSF.

Chronic complications of the malformation

• Tethered cord syndrome: the conus is anchored by scar after repair; longitudinal traction with growth produces progressive scoliosis, new gait deterioration, back or leg pain, and worsening continence — the signal is loss of a previously achieved milestone.
• Syringomyelia: altered CSF dynamics create a central cord cavity; classic cape-like dissociated loss of pain and temperature with preserved dorsal column function.
• Neurogenic bladder with upper tract damage: high detrusor pressures cause vesicoureteral reflux, hydronephrosis, recurrent pyelonephritis, and chronic kidney disease — historically the chief cause of late mortality.
• Insensate skin: pressure ulcers and painless fractures below the sensory level.
• Neuroendocrine and cognitive sequelae: precocious puberty, obesity, and a nonverbal learning profile associated with hydrocephalus and callosal dysgenesis.

Treatment-related

• Shunt overdrainage: slit-ventricle syndrome and subdural hematoma.
• In-utero repair: preterm birth, PPROM, uterine dehiscence, and mandatory cesarean in subsequent pregnancies.

• Neuropore timing drives the phenotype: the anterior neuropore closes near day 25 and the posterior near day 27 — rostral failure gives anencephaly, caudal failure gives spina bifida. Both precede the first missed period in many patients, which is why folic acid must be preconceptional.
• AFP logic: elevated MSAFP plus positive amniotic acetylcholinesterase = open NTD. Spina bifida occulta and any skin-covered (closed) defect have normal AFP — this is the single most common distractor.
• Ultrasound buzzwords: lemon sign (frontal scalloping) and banana sign (cerebellum draped around the brainstem) both reflect Chiari II, which accompanies myelomeningocele, not meningocele.
• Single best next step in a neonate with an exposed sac: sterile saline-soaked non-adherent dressing, prone positioning, latex-free precautions, and antibiotics — before imaging, before neurosurgical consultation is completed.
• The association examiners love: valproate carries the highest antiepileptic NTD risk, and poorly controlled maternal diabetes links to both NTDs and caudal regression syndrome (sacral agenesis, flaccid legs).
• Loss of an achieved milestone — new scoliosis, worsening gait, or new incontinence years after repair — means tethered cord until MRI proves otherwise; the same presentation with headache and vomiting means shunt failure.
• Neural crest is the money derivative list: dorsal root ganglia, sympathetic chain, adrenal medulla, Schwann cells, melanocytes, and pia/arachnoid. Dura is mesodermal.
• Distractor to avoid: Dandy-Walker (vermian hypoplasia with a cystic posterior fossa and enlarged fourth ventricle) is not the malformation of myelomeningocele — Chiari II is. Likewise, do not attribute holoprosencephaly (single ventricle, cyclopia, trisomy 13) to a neurulation failure; it is a forebrain cleavage/midline patterning defect.