Introduction
The neural tube is the embryonic structure that gives rise to the central nervous system—the brain and spinal cord—in chordates, including all vertebrates. Its formation begins with the appearance of a neural groove flanked by paired neural folds; as morphogenesis proceeds the groove deepens, the folds elevate and converge at the midline, and their fusion converts the open groove into a closed cylindrical tube. This sequence of groove formation, fold elevation, midline fusion and tube closure constitutes the fundamental stages of neurulation and establishes the substrate for the later regional specialization that produces distinct brain and spinal cord domains.
In human embryos, neural tube closure is generally completed by the fourth week of gestation (around day 28 post‑conception), representing a critical early event in central nervous system development. The same basic neurulation process is conserved across chordates, reflecting a shared embryological pattern within the phylum. Note: portions of this account have been flagged for additional verification; primary and secondary sources suitable for corroboration include peer‑reviewed literature accessible via Google Scholar and JSTOR, as well as authoritative texts and review articles.
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Development
Vertebrate neural tube formation arises by two morphogenetic modes—primary and secondary neurulation—which together segregate the embryonic ectoderm into three distinct populations: an internal neural tube primordium, an external epidermis, and neural crest cells produced at their interface that subsequently migrate to distant sites.
Primary neurulation begins with the neural plate whose lateral edges thicken and elevate to form neural folds while the central region remains depressed, creating a U-shaped neural groove aligned with the left–right midline. Medial convergence and fusion of these folds transform the flat plate into a hollow neural tube.
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Secondary neurulation involves a cohort of neural-plate–derived cells that condense into a solid, rod-like medullary cord within the embryo; this cord later undergoes cavitation to generate a continuous tubular lumen and thus a segment of neural tube.
Patterns of neurulation vary among taxa: teleost fishes form their neural tube solely by secondary neurulation; birds use primary neurulation for anterior (rostral) regions and secondary neurulation posteriorly; mammals employ both mechanisms, with secondary neurulation beginning near the level of the ~35th somite. In mammals the temporal relationship among neural crest emigration, neural tube closure, and closure of the overlying surface ectoderm differs regionally. In the cranial region neural crest cells emigrate first, followed by neural tube closure and then surface ectoderm closure; in the trunk this order is inverted, with ectodermal closure preceding neural tube closure and subsequent neural crest emigration.
Structure of the Neural Tube
The embryonic neural tube is organized along its length into four primary longitudinal domains—the prosencephalon (forebrain), mesencephalon (midbrain), rhombencephalon (hindbrain) and the spinal cord—established by regional specification and proliferation of neuroepithelial cells. The forebrain subdivides into the telencephalon, which gives rise to the cerebral hemispheres, and the diencephalon, which produces midline structures such as the optic vesicles and hypothalamic regions. The midbrain largely retains its mesencephalic identity without forming additional secondary vesicles, whereas the hindbrain partitions into the metencephalon (precursor of the pons and cerebellum) and the myelencephalon (precursor of the medulla oblongata).
During neurulation the tube remains transiently open at both extremities as cranial and caudal neuropores; these normally close by the fourth week of human gestation, and failure of closure underlies major defects such as anencephaly and spina bifida. Along the dorsoventral axis the tube becomes functionally regionalized: dorsal progenitor domains form the alar plate and give rise predominantly to sensory (afferent) circuits, while ventral domains form the basal plate and generate motor (efferent) neurons. The spinal cord emerges from the posterior neural tube as neuroepithelial cells proliferate and differentiate into the neurons and glia that assume these sensory and motor roles.
Dorsal–ventral patterning of the neural tube is organized by a positional-information system in which diffusible signals establish opposing concentration gradients that encode cell identity according to position and exposure history. Ventral identity is principally instructed by Sonic hedgehog (Shh), first released from the notochord and later from floor plate cells that arise at the ventral midline; this source-periphery arrangement generates a ventral→dorsal Shh gradient whose absolute level and duration of signaling together determine whether progenitors adopt floor-plate, motor-neuron, or ventral-interneuron fates. Three principal ventral cell classes—floor plate cells at the midline, motor neurons immediately dorsal to the floor plate, and multiple ventral interneuron classes—are specified by this graded Shh input, and experimental manipulations show that both concentration thresholds and exposure time shift progenitor outcomes toward progressively more ventral identities.
Molecularly, the Shh gradient is interpreted through a network of transcription factors divided into Shh-repressed (Class I) and Shh-induced (Class II) regulators, composed of homeodomain and bHLH proteins. Cross-regulatory interactions between these factors sharpen expression boundaries, producing discrete progenitor domains along the dorsal–ventral axis. Combinatorial codes of these transcription factors define distinct neuronal progenitor identities: in vitro experiments reproduce five molecularly distinct ventral neuronal groups whose induction thresholds predict their in vivo positions, and ventral interneurons are further classified into V0–V3 classes while motor neurons occupy distinct MN domains corresponding to specific progenitor signatures.
Dorsal patterning relies on bone morphogenetic proteins (BMPs) and Wnt signals, initially sourced from the overlying ectoderm and later reinforced by the roof plate to produce a dorsal→ventral BMP gradient that analogously specifies dorsal neuronal fates in a dose-dependent manner. Functional studies in zebrafish demonstrate that graded decreases in BMP signaling result in loss of dorsal sensory neurons and an expansion of interneuronal identities, underscoring BMP dose as a determinant of dorsal–ventral fate allocation and showing that perturbation of dorsal cues shifts tissue identity ventrally.
Finally, dorsal–ventral identity is refined by additional diffusible cues and interactions among signaling axes: fibroblast growth factors and retinoic acid contribute positional information across the neural tube, and retinoic acid acts with Shh ventrally to induce transcription factors such as Pax6 and Olig2 required for motor-neuron differentiation. Thus, opposing morphogen gradients, temporal dynamics of exposure, and an integrated transcriptional network together convert graded extracellular information into the discrete progenitor domains that generate the neural-tube neuronal repertoire.