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Foundational Sciences · Embryology
Neural Tube & CNS Development
A Step 1 embryology lesson tracing CNS formation from notochord induction through neurulation, neuropore closure, and the 3→5 brain-vesicle sequence, then mapping each derivative and the board-favorite congenital malformations (NTDs, holoprosencephaly, Chiari II, Dandy-Walker) to their exact developmental mechanism and presentation.
13 min readHigh yield
The Developmental Sequence (Weeks 3–5)
The CNS is the first organ system to begin forming and one of the last to finish. The boards test a tight cause-and-effect chain, so anchor the timeline:
- Week 3 – induction. After gastrulation makes the trilaminar disc, the notochord (axial mesoderm) secretes signals that induce the overlying ectoderm to become neuroectoderm, forming the neural plate.
- Weeks 3–4 – neurulation. The neural plate invaginates into a neural groove flanked by two neural folds. The folds fuse (starting in the cervical region ~day 22) and "zipper" both cranially and caudally into the neural tube. Cells at the tips of the folds pinch off as neural crest.
- ~Day 25 / ~day 28 – neuropore closure. The tube's open ends close last: the anterior (rostral) neuropore ~day 25, the posterior (caudal) neuropore ~day 27–28. This fusion is folate-dependent — the single most testable risk factor.
- Weeks 4 → 5 – vesicles. The cranial tube balloons into 3 primary vesicles, then 5 secondary vesicles; the caudal tube becomes the spinal cord. The lumen persists as the ventricular system.
Neurulation, Neuropores & the Plates
- Neural tube (neuroectoderm) → CNS. The tube wall becomes neurons, ependyma, oligodendrocytes, and astrocytes; its lumen becomes the ventricles/central canal.
- Neural crest → PNS + "everything else." Migrates away from the tube; forms peripheral tissue (see origins table below).
- Anterior neuropore closes ~day 25; posterior ~day 28. Failure of closure = neural tube defect (NTD).
- Alar plate = dorsal = Sensory (afferent); basal plate = ventral = motor (efferent), divided by the sulcus limitans.
- Open NTDs (anencephaly, open spina bifida) raise maternal serum AFP and amniotic-fluid AFP + acetylcholinesterase.
- Risk factors for NTDs: low folate, valproate/carbamazepine (folate antagonists), and maternal diabetes / obesity.
Brain Vesicles → Adult Derivatives
| Primary (wk 4) | Secondary (wk 5) | Adult derivative | Cavity |
|---|---|---|---|
| Prosencephalon | Telencephalon | Cerebral hemispheres (cortex), basal ganglia | Lateral ventricles |
| Prosencephalon | Diencephalon | Thalamus, hypothalamus, retina & optic nerve, posterior pituitary, pineal | Third ventricle |
| Mesencephalon | Mesencephalon | Midbrain | Cerebral aqueduct |
| Rhombencephalon | Metencephalon | Pons + cerebellum | Upper 4th ventricle |
| Rhombencephalon | Myelencephalon | Medulla oblongata | Lower 4th ventricle |
Who Makes What: Tissue of Origin
- Neuroectoderm (neural tube): CNS neurons, astrocytes, oligodendrocytes, ependymal cells; retina, posterior pituitary, pineal gland.
- Neural crest: dorsal root, autonomic & enteric ganglia; Schwann cells; adrenal medulla (chromaffin cells); melanocytes; pia & arachnoid (leptomeninges); sensory ganglia of CN V, VII, IX, X; odontoblasts; parafollicular C cells; aorticopulmonary septum; facial/skull bones.
- Mesoderm — the classic exception: microglia are NOT neuroectoderm; they are yolk-sac/mesenchyme-derived macrophages. The dura mater is also mesoderm (vs. pia/arachnoid from crest).
Congenital Defects → Mechanism → Presentation
| Defect | Mechanism | Classic buzzwords |
|---|---|---|
| Anencephaly | Anterior neuropore fails to close | Absent forebrain + calvarium ("frog-like"); polyhydramnios (no swallowing center); ↑AFP; incompatible with life |
| Spina bifida occulta | Vertebral arches fail to fuse; dura intact, no herniation | Tuft of hair / dimple over lower spine; normal AFP (closed) |
| Meningocele | Meninges herniate through the bony defect | Cystic sac, no neural tissue inside |
| Myelomeningocele | Meninges + cord/nerve roots herniate (posterior neuropore failure) | Motor/sensory deficits, neurogenic bladder; ↑AFP; assoc. Chiari II |
| Holoprosencephaly | Prosencephalon fails to cleave into 2 hemispheres (~wk 5–6) | SHH mutation, trisomy 13, fetal alcohol syndrome; cyclopia, cleft lip/palate, single central incisor |
| Chiari II | Herniation of cerebellar vermis + tonsils + medulla through foramen magnum | Myelomeningocele + obstructive hydrocephalus (aqueductal stenosis); syrinx |
| Dandy-Walker | Agenesis of the cerebellar vermis | Cystic dilation of 4th ventricle filling an enlarged posterior fossa; hydrocephalus |
Vignette: The Newborn with a Lumbar Sac
A newborn has a fluid-filled sac over the lumbosacral spine covered by a thin membrane, with flaccid lower-limb weakness, absent ankle reflexes, and a distended bladder. Second-trimester maternal serum AFP was elevated, and prenatal ultrasound showed the "lemon" (frontal skull scalloping) and "banana" (curved cerebellum) signs. Brain MRI shows downward displacement of the cerebellar vermis and medulla through the foramen magnum with obstructive hydrocephalus. The mother took valproate for epilepsy and did not take periconceptional folate.
- Diagnosis: open myelomeningocele with Chiari II malformation.
- Mechanism: failure of the posterior neuropore to close (~day 28); the small posterior fossa forces hindbrain herniation, obstructing CSF → hydrocephalus.
- Prevention point: periconceptional folate supplementation; valproate is a folate antagonist and independent NTD risk factor.
Forebrain & Posterior-Fossa Malformations
- Holoprosencephaly = failed midline cleavage of the forebrain. Think SHH signaling defect, trisomy 13 (Patau), and fetal alcohol syndrome; severe form → cyclopia, mild form → cleft lip/palate, flat nose, or a single central maxillary incisor.
- Chiari I (milder, distinct from Chiari II): cerebellar tonsils herniate >3–5 mm below the foramen magnum; often asymptomatic until adulthood, then headaches; classically associated with syringomyelia — NOT with myelomeningocele.
- Dandy-Walker: vermis agenesis + cystic 4th ventricle + large posterior fossa; may co-occur with corpus callosum agenesis and hydrocephalus.
- Syringomyelia: a syrinx near the central canal (classically C8–T1) destroys crossing spinothalamic fibers in the anterior white commissure → bilateral "cape-like" loss of pain and temperature; associated with Chiari I.
Memory Hooks (real, high-yield)
- Anterior neuropore → Anencephaly (both start with A); the posterior/caudal neuropore → spina bifida (caudal defect).
- "SAME" + Alar/Basal: Sensory = Afferent, Motor = Efferent → Alar = Afferent (dorsal, sensory); Basal = motor (ventral).
- Holoprosencephaly → think 13: trisomy 13 (Patau) and the SHH gene; single midline everything (one ventricle, sometimes one eye).
- Chiari: "2 for 2" — Chiari II presents in the newborn (2) and pairs with myelomeningocele, while Chiari I hides until adulthood and pairs with syringomyelia.
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