Embryology & Developmental Malformations

What You'll Learn

Ectoderm gives rise to the entire nervous system — neural tube (CNS) and neural crest (PNS, melanocytes, adrenal medulla, and a substantial contribution to craniofacial/anterior cranial meninges; mesoderm contributes much of the posterior cranial and spinal meninges — meningeal origin is mixed)
Neural tube closure occurs days 22–28: anterior neuropore closes day 25, posterior neuropore day 27 — failure causes anencephaly (anterior) or spina bifida (posterior)
Three primary brain vesicles → five secondary vesicles → adult brain structures and ventricular cavities
Globus pallidus is derived from diencephalon (not telencephalon like the rest of the basal ganglia) — classic board question
Neuronal migration: radial (inside-out cortical layering) and tangential (interneurons from ganglionic eminences) — defects cause lissencephaly, heterotopia, polymicrogyria
Chiari malformations: Type I = tonsils >5 mm below foramen magnum; Chiari II = vermis + brainstem + 4th ventricle herniation through foramen magnum (± tonsils); associated with myelomeningocele. Vermis/brainstem are defining, not tonsils.
Holoprosencephaly = failure of forebrain cleavage; associated with SHH gene mutations, trisomy 13, cyclopia
Alpha-fetoprotein (AFP): elevated in neural tube defects, decreased in Down syndrome

HighYield Pearls

Folate prevention: preconception folic acid 400 µg/day (4 mg/day if prior affected pregnancy) cuts NTD risk ~70% — must start before conception
Chiari II defining feature: downward herniation of vermis + brainstem + 4th ventricle through foramen magnum (not tonsils) — obligate with myelomeningocele + tectal beaking + hydrocephalus
Holoprosencephaly = SHH pathway failure to cleave forebrain → alobar (single ventricle + fused thalami + cyclopia/proboscis); strong trisomy 13 association
Joubert → "molar tooth sign" (elongated superior cerebellar peduncles + deep interpeduncular fossa) + vermis hypoplasia + episodic apnea + abnormal eye movements; ciliopathy
Dandy-Walker triad: vermis hypoplasia + cystic 4th ventricle + enlarged posterior fossa with elevated tentorium — hydrocephalus & ataxia
Lissencephaly genetics: Type I "agyria-pachygyria" → LIS1 (Miller-Dieker) and DCX (X-linked — males lissencephaly, female heterozygotes get subcortical band/"double cortex"); Type II "cobblestone" = α-dystroglycanopathy (Walker-Warburg, MEB, Fukuyama)
FCD type IIb — balloon cells + transmantle sign on MRI — most surgically amenable cortical dysplasia in drug-resistant epilepsy
Periventricular nodular heterotopia → FLNA (X-linked, women, normal IQ, epilepsy & cardiac valve disease); males usually lethal
L1CAM → HSAS (Hydrocephalus + Stenosis of Aqueduct of Sylvius + Spasticity) — X-linked, adducted thumbs + congenital aqueductal stenosis
Hydranencephaly ≠ anencephaly: bilateral ICA-territory destruction in utero → cortex replaced by CSF but brainstem/thalami/cerebellum preserved; transilluminates
Congenital CMV = most common congenital cause of SNHL → periventricular calcifications + microcephaly + chorioretinitis (vs Toxo = diffuse/scattered calcifications + hydrocephalus)
Aicardi syndrome: presumed X-linked dominant (causative gene not yet identified); occurs almost exclusively in females, with rare reported males (often mosaic or 47,XXY) — agenesis of corpus callosum + infantile spasms + chorioretinal lacunae

🔍 Quick ReferenceClinical / imaging · Genetics / etiology · Associated syndromes

Clinical / imaging

"Molar tooth sign" on axial MRI → Joubert syndrome
"Racing car" sign + parallel ventricles + colpocephaly + Probst bundles → agenesis of the corpus callosum
Cystic 4th ventricle + absent/hypoplastic vermis + enlarged posterior fossa + elevated tentorium (torcular-lambdoid inversion) → Dandy-Walker malformation
Tectal beaking + small posterior fossa + low-lying torcula + lumbar myelomeningocele → Chiari II
Cerebellar tonsils >5 mm below foramen magnum + tussive headache + syrinx → Chiari I
"Cobblestone" cortex with eye + muscle disease → Walker-Warburg / type II lissencephaly
"Double cortex" / subcortical band heterotopia → DCX mutation in female heterozygote
Full-thickness gray-matter-lined CSF cleft (open- or closed-lip) → schizencephaly
Transmantle sign + balloon cells → focal cortical dysplasia type IIb (Taylor)
Cortex replaced by CSF with intact brainstem & thalami; head transilluminates → hydranencephaly
Single midline ventricle + fused thalami + cyclopia/proboscis → alobar holoprosencephaly
Periventricular calcifications + microcephaly + SNHL + chorioretinitis → congenital CMV
Diffuse intracranial calcifications + hydrocephalus + chorioretinitis → congenital toxoplasmosis

Genetics / etiology

SHH (sonic hedgehog) pathway mutation → holoprosencephaly (also trisomy 13)
LIS1 (17p13.3) deletion → Miller-Dieker / classic lissencephaly
DCX (Xq22) — X-linked → lissencephaly in males, subcortical band heterotopia in females
FLNA (filamin A, Xq28) → periventricular nodular heterotopia (women + cardiac valve disease)
L1CAM mutation → HSAS / X-linked hydrocephalus with adducted thumbs
POMT1/POMT2/FKTN/FKRP (α-dystroglycanopathy) → type II "cobblestone" lissencephaly (Walker-Warburg, MEB, Fukuyama)
Folate deficiency ± MTHFR / valproate / carbamazepine exposure → neural tube defects
HLXB9 (MNX1) mutation → Currarino triad (sacral agenesis + presacral mass + anorectal malformation)
ICA-territory in utero infarction → hydranencephaly
Trisomy 13 (Patau) → holoprosencephaly + midline facial defects

Associated syndromes / pearls

Aicardi syndrome → agenesis of corpus callosum + infantile spasms + chorioretinal lacunae (presumed X-linked dominant, gene not yet identified; almost exclusively female, with rare reported males)
Septo-optic dysplasia (de Morsier) → absent septum pellucidum + optic nerve hypoplasia + hypopituitarism
Walker-Warburg syndrome → type II lissencephaly + eye anomalies + congenital muscular dystrophy (most severe α-dystroglycanopathy)
Miller-Dieker syndrome → classic lissencephaly + dysmorphic facies (17p13.3 deletion including LIS1)
VACTERL association → vertebral + anal + cardiac + tracheoesophageal + renal + limb anomalies; tethered cord risk
Congenital Zika syndrome → microcephaly + subcortical calcifications + ventriculomegaly + arthrogryposis
Congenital rubella → cataracts + PDA/cardiac defects + sensorineural deafness + "blueberry muffin" rash
Bilateral perisylvian polymicrogyria → pseudobulbar palsy + epilepsy + cognitive impairment
Hemimegalencephaly → one enlarged dysplastic hemisphere + drug-resistant epilepsy → hemispherotomy
Premature infant pearls → germinal matrix / IVH (grade I–IV) + periventricular leukomalacia → spastic diplegic CP
Mowat-Wilson (ZEB2), Kabuki (KMT2D), Rubinstein-Taybi (CREBBP) → multisystem syndromes with ACC / brain malformations

Neural Tube Formation

Neurulation Overview

Notochord (mesodermal structure) induces overlying ectoderm to form the neural plate (day 17–18)
Neural plate folds → neural groove → neural folds → fusion into neural tube (weeks 3–4, days 22–28 — primary neurulation complete by day 28)
Neural crest cells migrate from the dorsal edges of the neural folds before and during closure
Neural tube lumen → ventricular system and central canal of spinal cord

Primary Neurulation

Fusion begins day 22 at the 3rd–5th somite level (future cervical region)
Closure proceeds bidirectionally — cranially and caudally (zipper-like fashion)
Anterior neuropore closes day 25 → failure causes anencephaly
Posterior neuropore closes day 27 → failure causes spina bifida / myelomeningocele
Primary neurulation forms the brain and spinal cord down to the upper sacral level

Secondary Neurulation

Forms the caudal neural tube (sacral and coccygeal segments), days 28–32
Occurs by canalization of a solid cord of cells (not folding of a neural plate)
Defects in secondary neurulation → occult spinal dysraphisms (lipomyelomeningocele, tethered cord, dermal sinus tract)

Alpha-Fetoprotein (AFP)

Elevated maternal serum AFP → open neural tube defects (anencephaly, myelomeningocele), also elevated in omphalocele, gastroschisis, twin pregnancy
Decreased maternal serum AFP → Down syndrome (trisomy 21), Edwards syndrome (trisomy 18)
Elevated amniotic fluid AFP + acetylcholinesterase → confirms open neural tube defect

Board Pearl

Anterior neuropore = day 25; posterior neuropore = day 27. Anterior failure → anencephaly. Posterior failure → myelomeningocele. Folic acid supplementation (0.4 mg/day; 4 mg/day if prior affected pregnancy) reduces neural tube defect risk by ~70% (MRC Vitamin Study). Must be started before conception.

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