Neural Tube Formation and Closure

Neural tube formation is the morphogenetic event that converts the flat neural plate into the closed tube from which the brain and spinal cord arise. The plate's edges rise as neural folds, bend at defined hinge points, and meet and fuse along the dorsal midline; closure is completed in a regionally staggered sequence that leaves transient openings, the cranial and caudal neuropores, which seal last.

Definition

Neural tube formation and closure is the process by which the neural plate folds along hinge points and its apposed neural folds fuse dorsally to form a closed neural tube, sealing at the cranial and caudal neuropores to complete primary neurulation, with secondary neurulation forming the most caudal segment.

Scope

The entry covers primary neurulation (folding and fusion of the neural plate) and secondary neurulation (canalization of a caudal cell condensation), the cellular and biomechanical drivers of closure, the discrete closure sites, and the developmental basis of neural tube defects. It is an educational reference to the developmental anatomy and is not clinical guidance.

Key concepts

• Neural plate and neural folds
• Median and dorsolateral hinge points
• Apical constriction
• Convergent extension
• Cranial and caudal neuropores
• Primary versus secondary neurulation
• Closure initiation sites
• Planar cell polarity signalling

Mechanisms

In primary neurulation the neural plate narrows and lengthens through convergent extension while bending at a median hinge point over the notochord and at paired dorsolateral hinge points; apical constriction of neuroepithelial cells helps the folds elevate and curve toward the midline, where they appose and fuse. Closure does not proceed as a single zipper but begins at discrete initiation sites and spreads bidirectionally, so the cranial and caudal neuropores are the last regions to seal. Coordinated regulation of cytoskeletal contractility, cell adhesion, and planar cell polarity signalling underlies these movements. In secondary neurulation, the most caudal neural tube forms by condensation and canalization of mesenchyme rather than by folding. Failure of these steps at the cranial or caudal end leaves the neural tube open and produces the corresponding neural tube defects.

Clinical relevance

Incomplete closure underlies neural tube defects such as anencephaly (failure of cranial closure) and open spina bifida (failure of caudal closure), and the timing of these events places them in the early embryonic period. This entry explains the developmental anatomy behind such anomalies for reference and education and is not a basis for individual diagnosis, screening decisions, or treatment.

Epidemiology

Neural tube defects are among the most frequent structural malformations of the central nervous system, and their occurrence is modified by genetic background and by periconceptional nutritional factors, with folate status repeatedly associated with risk at the population level.

History

Descriptions of neural fold elevation and fusion date to classical embryology, but the recognition that closure begins at multiple discrete sites and proceeds bidirectionally, and the dissection of its biomechanical and molecular drivers, are products of later experimental and genetic work in model organisms summarised in modern reviews.

Debates

How many closure initiation sites exist and how conserved are they?

Closure begins at several discrete sites rather than a single point, but the exact number and their correspondence between species, including humans, has been debated and refined as evidence accumulates.

Key figures

• Andrew J. Copp
• Nicholas D. E. Greene
• Evanthia Nikolopoulou

Seminal works

• nikolopoulou-2017
• copp-2013
• greene-2014

Frequently asked questions

Does the neural tube close like a single zipper from one end?

No. Closure begins at several discrete initiation sites and spreads in both directions from each, so the cranial and caudal neuropores are the last regions to seal.

What is the difference between primary and secondary neurulation?

Primary neurulation forms most of the neural tube by folding and fusing the neural plate, whereas secondary neurulation forms the most caudal portion by condensation and canalization of mesenchymal cells without folding.