CNS Embryology & Development

The human nervous system begins as a flat sheet of cells on the back of a three-week embryo. From this sheet, by a sequence of foldings, divisions, and migrations that span the next thirty-eight weeks, the entire central nervous system is built. The process is so reliable that the major anatomical features are present in essentially every healthy newborn. The process is also fragile enough that disturbances at specific developmental windows produce specific malformations — neural tube defects, hydrocephalus, holoprosencephaly, agenesis of the corpus callosum, cerebellar malformations, cortical dysplasias. Understanding the embryology is therefore not just an academic exercise; it is the framework for recognizing and classifying congenital neurological disease.

This page covers the key events of CNS embryology, the relationships between developmental compartments and adult structures, and the malformations that arise when development goes awry. The point is to give the trainee enough developmental anatomy to read an MRI of a congenital malformation and place the lesion in the correct embryological framework.

Neurulation: The Neural Plate and Tube

The first event of CNS development is neurulation — the conversion of a flat plate of ectodermal cells into a closed tube. The process begins around day 18 after conception, when a strip of dorsal ectoderm thickens to form the neural plate, induced by signals from the underlying notochord. The neural plate develops a longitudinal groove — the neural groove — flanked by two rising folds. By day 22, the folds meet in the midline and fuse, beginning at the level of the future cervical cord and progressing both rostrally and caudally over the next several days.

Closure proceeds in both directions from the initial fusion point:

• Rostral (anterior) neuropore: closes around day 25. Failure of closure produces anencephaly, an absence of the forebrain and overlying cranial vault that is incompatible with extended life.
• Caudal (posterior) neuropore: closes around day 27. Failure of closure produces spina bifida in its various forms — myelomeningocele, meningocele, spina bifida occulta.

The neural tube is the closed structure that results. Its cavity becomes the ventricular system; its walls become the brain and spinal cord. Cells at the dorsal aspect of the closing tube break off to form the neural crest, which gives rise to peripheral sensory ganglia, autonomic ganglia, Schwann cells, the adrenal medulla, melanocytes, parts of the skull and face, and other derivatives. The neural crest is the basis of an entire family of clinical conditions — neurofibromatosis (involving Schwann cell precursors), congenital melanocytic abnormalities, Hirschsprung disease (failed enteric ganglion migration), and others.

Neural Tube Defects

Failure of neural tube closure produces the spectrum of neural tube defects (NTDs). Folate deficiency is the major modifiable risk factor; the introduction of folic acid fortification of grain in many countries has reduced NTD incidence substantially. The specific lesions:

• Anencephaly: absent forebrain and calvarium. Stillbirth or death shortly after birth.
• Encephalocele: herniation of brain tissue through a calvarial defect, typically occipital or frontal.
• Myelomeningocele: open spinal cord with the neural tissue exposed at the surface. The most common viable NTD; associated with Chiari II malformation and hydrocephalus in nearly all cases.
• Meningocele: a fluid-filled sac of meninges protruding through a vertebral defect, without neural tissue involvement. Better prognosis than myelomeningocele.
• Spina bifida occulta: a small vertebral defect with intact overlying skin and no protrusion. Often asymptomatic; may be associated with a tethered cord, intraspinal lipoma, or dermal sinus.

The Three Primary Vesicles

By day 28, the rostral end of the neural tube has developed three swellings — the primary brain vesicles. Each will give rise to a major division of the adult brain:

• Prosencephalon (forebrain): becomes the cerebral hemispheres and the diencephalon.
• Mesencephalon (midbrain): remains as the midbrain.
• Rhombencephalon (hindbrain): becomes the pons, medulla, and cerebellum.

Caudal to the vesicles, the rest of the neural tube becomes the spinal cord.

The Five Secondary Vesicles

By the end of the fifth week, the three primary vesicles have subdivided into five secondary vesicles:

• Telencephalon (from prosencephalon): becomes the cerebral hemispheres — cortex, basal ganglia, hippocampus, amygdala, olfactory bulbs.
• Diencephalon (from prosencephalon): becomes the thalamus, hypothalamus, epithalamus, subthalamus.
• Mesencephalon: becomes the midbrain.
• Metencephalon (from rhombencephalon): becomes the pons and cerebellum.
• Myelencephalon (from rhombencephalon): becomes the medulla.

Holoprosencephaly

Failure of the prosencephalon to divide into two cerebral hemispheres produces holoprosencephaly, a spectrum of malformations ranging from alobar (single ventricle, fused hemispheres) through semilobar to lobar (partial separation). Severity correlates with the degree of failed cleavage. Facial malformations often accompany the brain malformation, with cyclopia and proboscis at the extreme severe end. Causes include genetic syndromes (trisomy 13, 18p− syndrome, mutations in SHH and related signaling genes) and teratogenic exposures (maternal diabetes, alcohol). The brain malformation is irreversible; severity determines prognosis.

The Flexures

As the neural tube grows, the rostral portion bends at three points to fit within the cranial vault:

• Cephalic (mesencephalic) flexure: at the midbrain. Persists in the adult brain.
• Cervical flexure: at the medulla-cord junction. Largely straightens out as development proceeds.
• Pontine flexure: at the pons, opposite in direction to the other two. Brings the pons forward and lifts the cerebellum into its adult position.

The flexures shape the adult brain’s geometry — particularly the way the brainstem bends forward and the way the cerebellum sits on top of the pons.

The Cerebral Hemispheres Expand

The telencephalon expands dramatically through the second and third trimesters, sweeping forward (to form the frontal lobes), backward (to form the occipital lobes), and downward and forward (to form the temporal lobes). The growing hemispheres roll back over the diencephalon and brainstem, eventually covering them completely. The white matter pathways follow this expansion — fibers that initially run in straight lines must curve as the hemispheres curve.

The expansion of the cerebrum and the curvature of the temporal lobe in particular explains the C-shape of several structures:

• The lateral ventricle is C-shaped, sweeping forward (frontal horn), backward (occipital horn), and downward and forward (temporal horn).
• The caudate nucleus follows the curve of the lateral ventricle.
• The fornix and stria terminalis follow similar curves.
• The hippocampus, which begins on the medial surface of the hemisphere, ends up tucked into the medial temporal lobe by the same curving motion.

Recognizing this C-shape in adult anatomy is one of the most useful conceptual organizers in neuroanatomy.

Gyrification

Early in development, the cerebral surface is smooth (lissencephalic). Beginning around 20 weeks gestation, the cortex folds into gyri and sulci — a process driven by tangential expansion of the upper cortical layers relative to the lower layers, and by mechanical forces of the developing white matter. The primary sulci (central, lateral, calcarine) appear first; secondary and tertiary sulci appear later.

Disorders of gyrification produce specific malformations:

• Lissencephaly: smooth brain, absent or markedly reduced gyri. Patients have profound intellectual disability and intractable seizures. Several genetic causes (LIS1, DCX, ARX mutations).
• Pachygyria: broad, flattened gyri with thickened cortex. A milder form of the lissencephaly spectrum.
• Polymicrogyria: too many small gyri with abnormal cortical architecture. Variable cognitive and motor consequences; often associated with epilepsy.
• Schizencephaly: a cleft extending from the cortical surface to the lateral ventricle, often filled with CSF. Open-lipped (clefts open to ventricles) or closed-lipped forms.
• Focal cortical dysplasia: localized abnormality of cortical lamination, often a cause of focal epilepsy. Identified on MRI as cortical thickening, blurred gray-white junction, or transmantle sign.

Migration of Cortical Neurons

Cortical neurons are not born where they end up. They originate in the ventricular zone (the lining of the lateral ventricle) and migrate outward along the processes of radial glia. The migration occurs in waves between about 8 and 24 weeks of gestation. Each successive wave passes through the previously deposited layers, with the earliest-born neurons ending up deepest in the cortex and the latest-born neurons reaching the outermost layers. The result is a six-layered cortex with characteristic cytoarchitecture.

Disorders of migration include:

• Heterotopia: neurons arrested mid-migration, forming nodules of gray matter in abnormal locations (periventricular nodular heterotopia, subcortical band heterotopia).
• Lissencephaly: severe migration failure producing the smooth-brain phenotype.
• Focal cortical dysplasia: localized migration and lamination abnormalities.

Cerebellar Development

The cerebellum develops from the dorsal portion of the metencephalon. The cerebellar primordium grows out as two lateral swellings (the future hemispheres) connected by a midline structure (the vermis). The growth is dramatic — the cerebellum expands more than 30-fold during fetal development, with the most rapid growth in the third trimester and the early postnatal period.

Disorders of cerebellar development:

• Dandy-Walker malformation: agenesis or hypoplasia of the vermis with cystic dilation of the fourth ventricle and an enlarged posterior fossa. Often associated with hydrocephalus.
• Joubert syndrome: vermal hypoplasia with the characteristic “molar tooth” appearance of the midbrain on axial MRI. Multiple genetic causes.
• Chiari II malformation: caudal displacement of the cerebellar vermis and tonsils through the foramen magnum. Always associated with myelomeningocele.
• Chiari I malformation: caudal displacement of the cerebellar tonsils (more than 5 mm below the foramen magnum) without associated myelomeningocele. May present at any age; commonly causes occipital headache exacerbated by Valsalva, syringomyelia.

Spinal Cord Development

The spinal cord develops from the caudal portion of the neural tube. Initially the cord extends the full length of the vertebral canal. As the embryo grows, the vertebral column grows faster than the cord, so that the conus medullaris (the caudal end of the cord) progressively rises relative to the vertebral levels. At birth, the conus is at about L3; in the adult, it is at about L1-L2. The nerve roots that originate at the cord level but exit at lower vertebral levels stretch out as the cauda equina.

This developmental migration is the basis for several clinical facts:

• Lumbar puncture in the adult is safely performed below L2 because the cord ends above this level.
• Tethered cord syndrome — failure of the cord to ascend normally during development — leaves the conus low and traction on the cord produces neurological signs.
• Spina bifida occulta with a tethered cord may present in adolescence or adulthood with progressive lower extremity weakness, sensory loss, bladder dysfunction, or back pain.

Myelination

Myelination begins in the third trimester and continues into the third decade of life. Different pathways myelinate at different times — generally, sensory pathways before motor, primary cortices before association cortices, and brainstem before cerebrum. The corticospinal tract is largely myelinated by age 2, while the prefrontal cortex continues to myelinate into the late twenties. This protracted timeline explains both the relatively poor motor coordination of the young infant and the protracted development of executive function through adolescence.

Disorders of myelination are covered in the Glia and Myelin page. Hypomyelinating leukodystrophies (Pelizaeus-Merzbacher disease, vanishing white matter disease) and demyelinating diseases (multiple sclerosis, neuromyelitis optica) reflect failure or destruction of this protracted process.

Ventricular System Development

The central cavity of the neural tube becomes the adult ventricular system. The cavity of the telencephalon becomes the paired lateral ventricles, communicating with the third ventricle (cavity of the diencephalon) through the foramina of Monro. The third ventricle communicates with the fourth (cavity of the metencephalon/myelencephalon) through the cerebral aqueduct (cavity of the mesencephalon). The fourth ventricle communicates with the subarachnoid space through the foramina of Magendie (midline) and Luschka (lateral).

Disorders of ventricular development:

• Aqueductal stenosis: narrowing or obstruction of the cerebral aqueduct produces non-communicating hydrocephalus. Can be congenital or acquired (post-infectious, post-hemorrhagic, tumor-related).
• Dandy-Walker malformation: enlarged fourth ventricle with cystic communication to a large posterior fossa.
• Holoprosencephaly: a single ventricle from failed prosencephalon cleavage.
• Hydrocephalus ex vacuo: enlargement of the ventricles from atrophy of surrounding brain tissue rather than from obstruction of CSF flow.

Corpus Callosum Development

The corpus callosum is the largest interhemispheric commissure. It develops from rostral to caudal between about 8 and 20 weeks of gestation. The genu develops first, then the body, then the splenium, and finally the rostrum. This developmental sequence means that partial agenesis of the corpus callosum typically affects the splenium and rostrum (the last to form) while the genu remains intact.

Agenesis of the corpus callosum can be isolated or part of a syndrome. The condition is sometimes asymptomatic and discovered incidentally on imaging done for other reasons; in other cases, it is associated with cognitive impairment and seizures. The combination of agenesis with infantile spasms, hypsarrhythmia on EEG, and chorioretinal lacunae defines Aicardi syndrome (X-linked, almost exclusively in girls).

🔍 Did You Know?

The neural crest is sometimes called the “fourth germ layer” because of the extraordinary range of tissues it produces — peripheral sensory ganglia, sympathetic ganglia, parasympathetic ganglia, enteric ganglia, Schwann cells, adrenal medulla, melanocytes, much of the bone and connective tissue of the face, the dental papillae, the smooth muscle of the great vessels, the cardiac outflow tract, and many other structures. The disorders of neural crest development — neurocristopathies — therefore span an extraordinary clinical spectrum: from Hirschsprung disease (failed enteric migration) to neurofibromatosis (Schwann cell tumors) to congenital melanocytic abnormalities to the malformations of CHARGE syndrome.

Pitfalls and Pearls

• Neural tube defects are preventable by adequate periconceptional folate. The standard recommendation is 0.4 mg daily for women planning pregnancy; 4 mg daily for women with a prior NTD-affected pregnancy.
• The conus medullaris in the adult is at L1-L2, not the bottom of the vertebral canal. Lumbar puncture below L2 (typically L3-L4 or L4-L5) is safe.
• Chiari II malformation accompanies essentially every myelomeningocele. The hindbrain herniation often produces obstructive hydrocephalus requiring shunting in infancy.
• Chiari I malformation may present at any age with occipital headache exacerbated by Valsalva, syringomyelia, or downbeat nystagmus. The cerebellar tonsils descend more than 5 mm below the foramen magnum.
• Partial agenesis of the corpus callosum typically affects the splenium and rostrum, reflecting the rostral-to-caudal development sequence.
• Migration disorders are a major cause of intractable epilepsy. MRI may reveal lissencephaly, pachygyria, polymicrogyria, focal cortical dysplasia, or heterotopia.
• Holoprosencephaly correlates with facial malformation severity. Cyclopia at the severe end; hypotelorism, midline cleft lip and palate, or a single maxillary central incisor at the milder end.
• Tethered cord may present in adolescence or adulthood with progressive lower extremity neurological symptoms or back pain. Look for cutaneous markers (hair tuft, dimple, lipoma, hemangioma) over the lumbosacral area.
• The neural crest produces the peripheral nervous system and many non-neural derivatives. Neurocristopathies span an extraordinary clinical spectrum.

References

1. Sadler TW. Langman’s Medical Embryology. 14th ed. Wolters Kluwer; 2018.
2. Mtui E, Gruener G, Dockery P. FitzGerald’s Clinical Neuroanatomy and Neuroscience. 8th ed. Elsevier; 2020.
3. Barkovich AJ. Pediatric Neuroimaging. 5th ed. Lippincott Williams & Wilkins; 2012.
4. Volpe JJ. Volpe’s Neurology of the Newborn. 6th ed. Elsevier; 2017.
5. Copp AJ, Greene ND. Genetics and development of neural tube defects. J Pathol. 2010;220(2):217-230.
6. Guerrini R, Dobyns WB. Malformations of cortical development: clinical features and genetic causes. Lancet Neurol. 2014;13(7):710-726.