Neural Tube

The neural tube is the embryonic structure from which the brain and spinal cord develop. It forms from specialized ectoderm called neuroectoderm during neurulation, a major developmental process occurring primarily during the third and fourth weeks of embryonic development.

Formation of the neural tube begins with the neural plate, a thickened region of dorsal ectoderm induced by underlying axial tissues. The neural plate bends along its longitudinal axis, producing a central neural groove bordered by elevated neural folds. These folds approach one another and fuse in the dorsal midline, converting the initially flat neural plate into a closed neural tube.

The walls of the neural tube subsequently undergo extensive proliferation, regional patterning, and differentiation. Its cranial portion expands to form the developing brain, while its caudal portion forms most of the spinal cord. The lumen of the tube persists as the ventricular system of the brain and the central canal of the spinal cord.

Embryological Origin

The neural tube is derived from ectoderm, one of the three primary germ layers of the embryo.

A portion of dorsal ectoderm becomes specialized as neuroectoderm through neural induction. This neuroectoderm first forms the neural plate and subsequently undergoes folding and fusion to produce the neural tube.

The process links early germ-layer formation with development of the central nervous system.

Neural Induction

Neural induction establishes the region of ectoderm that will form the nervous system.

Signals from the organizer, notochord, prechordal mesoderm, and associated axial tissues modify signaling within the overlying ectoderm and promote neural differentiation.

The induced ectoderm becomes neuroectoderm and thickens to form the neural plate.

Neural Plate

The neural plate is the immediate precursor of the neural tube.

It appears as an elongated area of thickened neuroectoderm on the dorsal surface of the embryo. Its cranial portion is broader than its caudal portion, reflecting the future expansion of the cranial neural tube into the brain.

As development progresses, the plate begins to bend and change from a flat epithelial sheet into a three-dimensional structure.

Neural Groove

The central portion of the neural plate bends inward to form the neural groove.

This longitudinal depression becomes progressively deeper as neurulation proceeds.

The neural groove represents the open stage of the future lumen of the neural tube.

Neural Folds

The lateral margins of the neural plate elevate on either side of the neural groove to form the neural folds.

These folds progressively rise and converge toward the dorsal midline.

Their eventual fusion converts the open neural groove into the closed neural tube.

Primary Neurulation

Primary neurulation is the process by which the neural plate folds and closes to form the neural tube.

It involves coordinated changes in neuroepithelial cell shape, bending of the neural plate, elevation of the neural folds, convergence toward the midline, fusion of opposing folds, and separation of the newly formed neural tube from the surface ectoderm.

Primary neurulation produces the neural tube that develops into the brain and most of the spinal cord.

Median Hinge Point

Early neural plate bending involves a specialized region called the median hinge point.

Cells near the midline change shape and maintain a close relationship with the underlying notochord.

This bending contributes to formation and deepening of the neural groove.

Dorsolateral Hinge Points

Dorsolateral hinge points develop in portions of the neural plate and contribute to convergence of the neural folds.

Cellular changes at these sites allow the lateral portions of the neural plate to bend toward the midline.

The precise morphology of neural folding varies at different cranial-caudal levels.

Fusion of the Neural Folds

As the neural folds approach one another, their opposing surfaces make contact and fuse.

Fusion transforms the neural plate from an open sheet into a closed tube.

The neural tube subsequently separates from the surface ectoderm, which becomes continuous over the dorsal surface of the embryo.

Beginning of Neural Tube Closure

Neural tube closure does not occur simultaneously along the entire length of the embryo.

In humans, fusion begins in the future cervical region and proceeds in both cranial and caudal directions.

This progressive closure temporarily leaves the cranial and caudal ends of the neural tube open.

Neuropores

The temporary openings at the ends of the developing neural tube are called the neuropores.

The opening at the cranial end is the cranial neuropore, while the opening at the caudal end is the caudal neuropore.

Until they close, these openings allow communication between the neural tube lumen and the amniotic cavity.

Cranial Neuropore

The cranial neuropore is the temporary opening at the rostral end of the neural tube.

It normally closes during the fourth week, approximately around day 25 after fertilization, although developmental timing is expressed approximately rather than as an absolute fixed day.

Normal closure is essential for development of the brain and cranial vault.

Caudal Neuropore

The caudal neuropore lies at the caudal end of the neural tube.

It normally closes shortly after the cranial neuropore, approximately around day 27 after fertilization.

Abnormal closure in the caudal region is associated with spinal neural tube defects.

Separation from Surface Ectoderm

After neural fold fusion, the neural tube separates from the overlying surface ectoderm.

The surface ectoderm then becomes continuous across the dorsal aspect of the embryo and later contributes to epidermal structures.

The neural tube remains beneath the surface and continues its differentiation into the central nervous system.

Neural Crest Cells

Neural crest cells develop at the borders of the neural plate and dorsal neural folds.

During and after neural tube closure, these cells separate from the dorsal neural region and migrate extensively through the embryo.

Although closely associated with neural tube development, neural crest cells largely form structures outside the central nervous system.

Neural Crest Derivatives

Neural crest cells produce a wide range of structures, including important components of the peripheral nervous system.

Sensory ganglia
Autonomic ganglia
Enteric nervous system ganglia
Schwann cells
Melanocytes
Adrenal medullary chromaffin cells
Much of the craniofacial connective tissue and skeleton

The neural tube and neural crest therefore have closely related origins but markedly different developmental destinations.

Secondary Neurulation

The most caudal portion of the spinal cord develops through secondary neurulation.

Instead of forming directly by folding of the surface neural plate, cells within the caudal cell mass condense and subsequently undergo cavitation.

The resulting lumen becomes continuous with the neural tube produced by primary neurulation.

Primary and Secondary Neurulation

Feature	Primary Neurulation	Secondary Neurulation
Initial structure	Neural plate	Caudal cell mass
Major mechanism	Folding and fusion	Condensation and cavitation
Neural groove	Present	Not the principal mechanism
Major contribution	Brain and most spinal cord	Most caudal spinal cord

Neuroepithelium

The wall of the early neural tube consists largely of rapidly proliferating neuroepithelial cells.

These cells form a pseudostratified epithelium surrounding the neural tube lumen.

As development proceeds, neuroepithelial cells generate neural progenitor populations that ultimately produce neurons and macroglial cells of the central nervous system.

Neural Tube Lumen

The cavity enclosed by the neural tube becomes the internal fluid-containing system of the central nervous system.

Within the developing brain, the lumen expands and changes shape to form the cerebral ventricles and connecting channels.

Within the spinal cord, the lumen becomes the relatively narrow central canal.

Cranial Neural Tube

The cranial portion of the neural tube expands rapidly and develops a series of enlargements that form the early brain vesicles.

Its walls become highly specialized and undergo extensive regional growth, folding, and differentiation.

The cranial neural tube ultimately produces all major divisions of the brain.

Primary Brain Vesicles

The cranial neural tube initially becomes organized into three major primary brain vesicles.

Primary Vesicle	Common Name
Prosencephalon	Forebrain
Mesencephalon	Midbrain
Rhombencephalon	Hindbrain

Secondary Brain Vesicles

Further development subdivides the forebrain and hindbrain into secondary brain vesicles.

Primary Vesicle	Secondary Vesicles
Prosencephalon	Telencephalon and diencephalon
Mesencephalon	Mesencephalon remains undivided
Rhombencephalon	Metencephalon and myelencephalon

Major Brain Derivatives

Embryonic Division	Major Adult Derivatives
Telencephalon	Cerebral hemispheres and associated structures
Diencephalon	Thalamic and hypothalamic regions and associated structures
Mesencephalon	Midbrain
Metencephalon	Pons and cerebellum
Myelencephalon	Medulla oblongata

Spinal Neural Tube

The portion of the neural tube caudal to the developing brain forms the spinal cord.

Its walls become organized into distinct proliferative, neuronal, and axonal regions.

Dorsal and ventral patterning further establishes functional divisions associated with sensory and motor processing.

Ventricular Zone

The innermost region of the developing neural tube is associated with proliferating neuroepithelial and neural progenitor cells.

This region forms the ventricular zone adjacent to the lumen.

Cells generated here migrate outward and contribute to more peripheral layers of the developing central nervous system.

Mantle Layer

Developing neurons accumulate external to the ventricular zone to form the mantle layer.

Within the developing spinal cord, this region becomes the principal precursor of the gray matter.

Its organization becomes increasingly specialized as neuronal populations differentiate.

Marginal Layer

The outermost region of the developing spinal neural tube becomes the marginal layer.

It contains large numbers of axons extending from neurons within the mantle layer and other regions.

As these axonal pathways develop and become myelinated, the marginal layer contributes substantially to the white matter of the spinal cord.

Alar Plates

The dorsal regions of the developing spinal neural tube form the alar plates.

These regions become primarily associated with sensory processing.

They contribute to the dorsal gray matter of the spinal cord.

Basal Plates

The ventral regions of the developing neural tube form the basal plates.

These regions become primarily associated with motor functions.

They contribute to motor neuronal populations within the ventral and intermediate regions of the spinal cord.

Sulcus Limitans

The sulcus limitans is a longitudinal groove that separates the alar and basal plates within the developing neural tube.

It therefore marks an important boundary between predominantly sensory and motor territories.

This basic organization is also reflected in portions of the developing brainstem.

Roof Plate

The roof plate forms along the dorsal midline of the neural tube between the paired alar plates.

It serves important signaling functions in dorsal neural tube patterning.

The roof plate itself contains relatively few neuronal cell bodies compared with adjacent neural regions.

Floor Plate

The floor plate lies along the ventral midline of the neural tube between the basal plates.

Its development is strongly influenced by signals associated with the notochord.

The floor plate functions as an important signaling center for ventral patterning and axonal guidance.

Dorsal-Ventral Patterning

The neural tube develops distinct dorsal and ventral neuronal populations through interactions among signaling centers and molecular gradients.

Signals associated with the notochord and floor plate promote ventral identities, while signaling from dorsal tissues and the roof plate contributes to dorsal identities.

This organization establishes the fundamental sensory-motor pattern of the developing spinal cord and brainstem.

Sonic Hedgehog

Sonic hedgehog (SHH) is an important signaling molecule involved in ventral neural tube patterning.

Signals from the notochord help induce the floor plate, which subsequently becomes an additional source of SHH.

Different levels and durations of SHH signaling contribute to specification of distinct ventral neuronal populations.

Dorsal Patterning Signals

Dorsal neural tube development is influenced by signaling molecules associated with the overlying ectoderm and roof plate.

Members of the BMP and related signaling families contribute to specification of dorsal neural identities.

The interaction of dorsal and ventral signaling systems establishes organized neuronal domains across the neural tube.

Alar and Basal Plate Organization

Region	Position	Predominant Functional Association
Alar plate	Dorsal	Sensory processing
Basal plate	Ventral	Motor function
Roof plate	Dorsal midline	Patterning and guidance
Floor plate	Ventral midline	Patterning and guidance
Sulcus limitans	Between alar and basal plates	Boundary between sensory and motor territories

Development of the Ventricular System

The lumen of the cranial neural tube expands and becomes the ventricular system of the brain.

Different regions of the embryonic brain produce different portions of this interconnected cavity system.

The ventricles remain continuous with one another and contain cerebrospinal fluid after its production begins.

Neural Tube Lumen Derivatives

Embryonic Region	Major Lumen Derivative
Telencephalon	Lateral ventricles
Diencephalon	Third ventricle
Mesencephalon	Cerebral aqueduct
Metencephalon and myelencephalon	Fourth ventricle
Spinal cord	Central canal

Relationship to the Notochord

The notochord lies immediately ventral to the developing neural tube along much of the embryonic axis.

It participates in induction and ventral patterning of neural tissue.

Its signaling relationship with the neural tube is particularly important for formation of the floor plate and ventral neuronal populations.

Relationship to Somites

Somites develop on either side of the neural tube from paraxial mesoderm.

These segmented structures contribute to the axial skeleton, skeletal musculature, and dermis.

The spatial relationship among the neural tube, notochord, and somites is a fundamental feature of the embryonic trunk.

Developmental Timeline

Approximate Period	Developmental Event
Third week	Neural plate develops
Late third week	Neural groove and folds become prominent
Late third to fourth week	Neural folds fuse and neural tube forms
Approximately day 25	Cranial neuropore normally closes
Approximately day 27	Caudal neuropore normally closes
Fourth week onward	Rapid regional differentiation of brain and spinal cord

Major Derivatives

Neural Tube Region	Major Derivative
Cranial neural tube	Brain
Caudal neural tube	Most of spinal cord
Neural tube wall	Neurons and macroglial components of CNS
Cranial lumen	Ventricular system
Spinal lumen	Central canal

Clinical Significance

Neural Tube Defects

Neural tube defects are congenital abnormalities resulting from disturbances in normal neurulation and neural tube closure.

Their anatomy and severity depend on the location, extent, and nature of the developmental defect.

They most prominently affect the cranial and spinal regions.

Anencephaly

Anencephaly is a severe cranial neural tube defect associated with failure of normal closure of the cranial neural tube.

It results in profound abnormalities of the brain and cranial vault.

The condition is incompatible with prolonged postnatal survival.

Spina Bifida

Spina bifida describes a spectrum of abnormalities involving incomplete formation of the vertebral arches and, in more severe forms, abnormalities of the spinal neural tissues and meninges.

Its forms vary substantially in anatomical severity.

Open forms are associated with failure of normal neural tube closure in the spinal region.

Myelomeningocele

A myelomeningocele is an open neural tube defect in which neural tissue and meninges protrude through a defect in the vertebral column.

The involved spinal cord and nerve roots may be structurally abnormal.

Neurological impairment generally corresponds to the anatomical level and severity of the lesion.

Encephalocele

An encephalocele is a cranial defect through which intracranial contents may herniate.

The protruding sac can contain meninges, brain tissue, or both, depending on the lesion.

Its developmental mechanisms and anatomy differ among affected individuals.

Craniorachischisis

Craniorachischisis is an extensive and severe open neural tube defect involving both cranial and spinal regions.

Large portions of neural tissue remain exposed because of widespread failure of neural tube closure.

It represents one of the most severe abnormalities of neurulation.

Folate and Neural Tube Defects

Adequate maternal folate intake before conception and during early pregnancy reduces the risk of many neural tube defects.

The timing is important because neural tube closure occurs very early in embryonic development.

Folate supplementation around conception is therefore an important preventive measure.

Neural Tube Compared with Neural Crest

Feature	Neural Tube	Neural Crest
Origin	Neural plate neuroectoderm	Neural plate border region
Developmental behavior	Forms closed central tube	Cells migrate away from dorsal neural region
Major nervous system contribution	Central nervous system	Large portions of peripheral nervous system
Major neural structures	Brain and spinal cord	Sensory, autonomic and enteric ganglia, Schwann cells

Key Anatomical Features

Feature	Key Point
Embryological origin	Neuroectoderm
Immediate precursor	Neural plate
Developmental process	Neurulation
Formation	Fusion of neural folds
Major cranial derivative	Brain
Major caudal derivative	Most of spinal cord
Cranial lumen derivative	Ventricular system
Spinal lumen derivative	Central canal
Dorsal spinal region	Alar plate
Ventral spinal region	Basal plate
Dorsal midline	Roof plate
Ventral midline	Floor plate

Anatomical Importance

The neural tube is the fundamental embryological precursor of the central nervous system. Through progressive regionalization and differentiation, its relatively simple tubular organization is transformed into the extraordinarily complex anatomy of the brain and spinal cord.

Its walls generate the cellular architecture of the central nervous system, while its lumen persists as the ventricular system and central canal. Dorsal and ventral patterning establishes early sensory and motor territories, and expansion of the cranial tube produces the embryonic brain vesicles from which the major adult brain regions develop.

Normal formation and closure of the neural tube are therefore critical events in human development. Disturbances during neurulation can produce major congenital abnormalities, while normal development establishes the structural foundation for the brain, spinal cord, cerebral ventricles, brainstem organization, spinal gray matter, spinal white matter, and central canal.