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Basic Science
Anatomy
13 notes available
1 Last Minute Review 2 Embryology & Developmental Anatomy 3 Vascular Anatomy 4 Cerebral Cortex 5 Subcortical Nuclei (BG, Thalamus, Hypothalamus) 6 Limbic System 7 Cerebellum 8 Brainstem 9 Cranial Nerves 10 Spinal Cord 11 Motor System 12 Sensory System 13 Peripheral Nerves and Muscles
Basic Science › Anatomy

Embryology & Developmental Anatomy

Embryology & Developmental Anatomy

What You'll Learn

  • Neurulation — ectoderm → neural plate → neural folds → neural tube; closure begins at the future cervical region (3rd–5th somite) and proceeds bidirectionally. Anterior neuropore closes day 25 (failure → anencephaly); posterior neuropore closes day 27–28 (failure → spina bifida).
  • Three primary vesicles (week 4): prosencephalon, mesencephalon, rhombencephalon → five secondary vesicles: telencephalon, diencephalon, mesencephalon, metencephalon, myelencephalon.
  • Neural crest = peripheral and autonomic nervous system + Schwann cells + sensory ganglia (DRG, CN V/VII/IX/X sensory) + adrenal medulla + melanocytes + odontoblasts + branchial arch derivatives.
  • Dysraphisms — spina bifida (occulta, meningocele, myelomeningocele), anencephaly. Folate before conception (0.4 mg/d, or 4 mg/d if prior NTD or VPA/CBZ exposure) reduces NTD risk ~70%. Maternal serum AFP elevated in open NTDs.
  • Posterior fossa malformations — Chiari I (tonsils ≥5 mm below foramen magnum), Chiari II (myelomeningocele + hindbrain herniation), Chiari III (encephalocele), Dandy-Walker (vermis hypoplasia + cystic 4th ventricle + enlarged posterior fossa).
  • Holoprosencephaly — failure of forebrain cleavage; ranges from alobar (severe, fused single ventricle, cyclopia) to lobar; associated with SHH pathway defects, trisomy 13, maternal diabetes.
  • Migration disorders — lissencephaly (smooth brain, no gyri; LIS1, DCX), heterotopia (periventricular nodular — filamin A, subcortical band — DCX females), polymicrogyria, schizencephaly.
  • Syringomyelia — CSF-filled cavity in spinal cord; classic association with Chiari I; "cape distribution" loss of pain/temperature (central cord crossing fibers); preserved fine touch/vibration.
HighYield Pearls
  • Anterior neuropore closes day 25, posterior day 27–28: failure → anencephaly (rostral) and spina bifida / myelomeningocele (caudal); folate 0.4 mg/d preconception (4 mg/d if prior NTD or on VPA/CBZ) cuts NTD risk ~70%.
  • Maternal serum AFP elevated in OPEN NTDs (anencephaly, open spina bifida) but normal in closed/skin-covered defects; acetylcholinesterase in amniotic fluid is confirmatory.
  • 3 → 5 vesicles: prosencephalon → telencephalon (hemispheres, lateral ventricles, basal ganglia) + diencephalon (thalamus, hypothalamus, 3rd ventricle, retina/CN II); mesencephalon → midbrain + cerebral aqueduct; rhombencephalon → metencephalon (pons + cerebellum, upper 4th vent) + myelencephalon (medulla, lower 4th vent + central canal).
  • Neural crest = PNS + Schwann cells + DRG + autonomic/enteric ganglia + adrenal medulla + melanocytes + odontoblasts + craniofacial mesenchyme + leptomeninges; oligodendrocytes (including optic nerve) are CNS-derived, NOT neural crest.
  • Basal (motor, ventral) vs alar (sensory, dorsal) plates separated by the sulcus limitans; cerebellum is alar-plate–derived.
  • Cortical migration is INSIDE-OUT — earliest-born neurons populate deep layers (V/VI), later-born populate superficial layers (II/III); Reelin (Cajal-Retzius cells) + DCX + LIS1 drive migration.
  • Lissencephaly: LIS1 (Miller-Dieker, 17p13.3) classic agyria/pachygyria; DCX X-linked → lissencephaly in males, "double cortex" subcortical band heterotopia in heterozygous females.
  • Periventricular nodular heterotopia → FLNA X-linked dominant, female-predominant epilepsy; FCD type IIb (balloon cells + transmantle sign) is the most surgically favorable cortical dysplasia.
  • Myelination CNS: peaks 3rd trimester → ~2 yr, proceeds caudal → rostral, central → peripheral, posterior → anterior; subcortical U-fibers myelinate LAST → U-fiber SPARING is a classic leukodystrophy clue (e.g., X-ALD, MLD).
  • Holoprosencephaly = SHH pathway failure; alobar form has single ventricle + fused thalami + cyclopia; associated with trisomy 13 and maternal diabetes.
  • Chiari I = tonsils ≥5 mm below foramen magnum (± syrinx); Chiari II = myelomeningocele + small posterior fossa + tectal beaking + hydrocephalus; Chiari III = occipital encephalocele.
  • Dandy-Walker = vermian hypoplasia + cystic 4th ventricle + enlarged posterior fossa with elevated torcula; Joubert = molar tooth sign + episodic apnea/hyperpnea + abnormal eye movements + ciliopathy.
  • Aqueductal stenosis → L1CAM (X-linked HSAS: hydrocephalus, stenosis, spasticity, adducted thumbs); hydranencephaly = bilateral ICA infarction in utero (cortex absent, brainstem/thalami preserved).
  • Septo-optic dysplasia (de Morsier) = absent septum pellucidum + optic nerve hypoplasia + pituitary dysfunction (HESX1, SOX2/3); Aicardi syndrome = agenesis/dysgenesis of corpus callosum + infantile epileptic spasms + chorioretinal lacunae; almost exclusively females, rare affected males usually 47,XXY; presumed de novo X-linked/male-lethal mechanism, gene unknown.
  • Agenesis of corpus callosum imaging: "racing car" sign, parallel ventricles, colpocephaly, Probst bundles.
  • Premature brain injury: germinal matrix / IVH (grades I–IV) in the subependymal zone of premies; periventricular leukomalacia (PVL) → spastic diplegia.
🔍 Quick ReferenceEmbryology / vesicles · Malformations · Genes / pearls
Embryology / vesicles / derivatives
  • Anterior neuropore closes day 25 → failure = anencephaly
  • Posterior neuropore closes day 27–28 → failure = spina bifida / myelomeningocele
  • Secondary neurulation → caudal (sacrococcygeal) spinal cord
  • Telencephalon → cerebral hemispheres + basal ganglia + lateral ventricles
  • Diencephalon → thalamus + hypothalamus + 3rd ventricle + retina/CN II
  • Mesencephalon → midbrain + cerebral aqueduct
  • Metencephalon → pons + cerebellum + upper 4th ventricle
  • Myelencephalon → medulla + lower 4th ventricle + central canal
  • Neural crest → Schwann cells + DRG + autonomic/enteric ganglia + adrenal medulla + melanocytes + odontoblasts + craniofacial bone
  • Sulcus limitans → divides basal (motor, ventral) from alar (sensory, dorsal) plates
  • Cerebellum → alar-plate derivative (rhombic lip)
  • Inside-out cortical migration → layer VI born first, layer II last; Reelin / DCX / LIS1
  • U-fibers myelinate last → U-fiber sparing = leukodystrophy clue
Malformations / imaging signs
  • Smooth brain / agyria-pachygyria → lissencephaly (LIS1, DCX)
  • "Double cortex" subcortical band heterotopia in a female → DCX heterozygote
  • Cobblestone lissencephaly + muscular dystrophy + eye anomalies → Walker-Warburg / muscle-eye-brain (POMT1/2, FKTN)
  • Bilateral perisylvian small folded gyri → polymicrogyria
  • Full-thickness CSF cleft cortex → ventricle → schizencephaly
  • One enlarged dysplastic hemisphere + drug-resistant epilepsy → hemimegalencephaly (consider hemispherotomy)
  • Transmantle sign + balloon cells → FCD type IIb (surgically favorable)
  • Single ventricle + fused thalami + cyclopia → alobar holoprosencephaly
  • Racing-car sign / parallel ventricles / colpocephaly / Probst bundles → agenesis of corpus callosum
  • Absent septum pellucidum + optic nerve hypoplasia + pituitary dysfunction → septo-optic dysplasia (de Morsier)
  • Chorioretinal lacunae + infantile spasms + ACC in a female → Aicardi syndrome
  • Cerebellar tonsils ≥5 mm below foramen magnum ± syrinx → Chiari I
  • Myelomeningocele + small posterior fossa + tectal beaking + hydrocephalus → Chiari II
  • Occipital encephalocele → Chiari III
  • Vermian hypoplasia + cystic 4th ventricle + elevated torcula → Dandy-Walker
  • Molar tooth sign + episodic apnea + ciliopathy → Joubert
  • Triventricular hydrocephalus with normal 4th ventricle → aqueductal stenosis
  • Cortex absent, brainstem/thalami preserved (in utero bilateral ICA infarct) → hydranencephaly
  • Subependymal germinal matrix bleed in a premie → IVH grades I–IV; periventricular T2 hyperintensity / cystic change → PVL → spastic diplegia
Genes / pearls
  • LIS1 (17p13.3) → Miller-Dieker lissencephaly
  • DCX (X-linked) → lissencephaly in males, double-cortex band heterotopia in females
  • FLNA (X-linked dominant) → periventricular nodular heterotopia (female-predominant, epilepsy)
  • RELN → lissencephaly with cerebellar hypoplasia
  • SHH / ZIC2 / SIX3 / TGIF → holoprosencephaly (also trisomy 13, maternal diabetes)
  • HESX1, SOX2, SOX3 → septo-optic dysplasia (de Morsier)
  • L1CAM (Xq28) → X-linked hydrocephalus (HSAS) with aqueductal stenosis + adducted thumbs + spasticity
  • HLXB9 (MNX1) → Currarino triad (sacral agenesis + anorectal malformation + presacral mass)
  • POMT1/POMT2 / FKTN / FKRP → cobblestone (type II) lissencephaly / Walker-Warburg / muscle-eye-brain disease
  • Folate 0.4 mg/d preconception (4 mg/d if prior NTD or on VPA/CBZ/MTX) → cuts NTD risk ~70% (MRC Vitamin Study)
  • Valproate, carbamazepine, methotrexate, trimethoprim, sulfasalazine → folate antagonists → ↑ NTD risk
  • Maternal serum AFP ↑ + acetylcholinesterase + in amniotic fluid → open NTD
  • TORCH + Zika → microcephaly + periventricular calcifications
  • Hirschsprung disease → failed neural crest migration to distal colon (RET mutations)
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