A neuropore is one of the temporary openings at either end of the developing neural tube in a human embryo. During the third and fourth weeks after conception, a flat sheet of cells called the neural plate folds inward and zips shut to form the tube that will become the brain and spinal cord. The two ends of that tube are the last parts to seal: the anterior (front) neuropore, which closes around day 24-25 of gestation, and the posterior (rear) neuropore, which closes a couple of days later. These brief openings matter far more than their fleeting existence might suggest, because failure of either one to close properly is a major cause of some of the most common and serious birth defects in the world, affecting roughly 300,000 pregnancies each year.
How the Neural Tube Zips Shut
For decades, textbooks taught a simple “zipper model” of neural tube closure: folding begins somewhere around the future neck and proceeds smoothly in both directions, like pulling a zipper up toward the head and down toward the tail. The anterior neuropore was the last cranial bit to close, the posterior neuropore the last caudal bit, and that was that. The problem with this tidy picture is that it could not explain certain patterns of birth defects, particularly cases where a neural tube defect appears in the middle of the spine rather than at one of the endpoints.
Research on human embryos has since shown that closure actually starts at multiple sites along the embryo’s length. One widely accepted initiation point, sometimes called Site A, sits in the future cervical (neck) region. But a second initiation point, Site B, has been identified at the boundary between the developing midbrain and hindbrain. Closure from Site B spreads in both directions: its backward extension meets the forward wave from Site A somewhere over the hindbrain, while its forward extension meets yet another closure wave originating from the very front of the neural groove (Site C) over the forebrain. That final meeting point over the forebrain is where the anterior neuropore seals shut. Meanwhile, the closure wave from Site A continues all the way down the embryo’s back to form the posterior neuropore at the caudal end.1PubMed. Neural tube closure in humans initiates at multiple sites: evidence from human embryos and implications for the pathogenesis of neural tube defects
This multi-site model explains why neural tube defects can appear in locations other than the two neuropores. If any of these closure waves stalls or fails to meet its counterpart, the gap can occur wherever the wave stopped, not only at the endpoints. The old zipper model, which predicted defects only at the two neuropores, simply could not account for that.2BJR|Case Reports. Multiple neural tube defects: rare developmental anomaly with an elusive embryological explanation
Anterior Versus Posterior Neuropore
The two neuropores are not interchangeable openings that just happen to sit at opposite ends. They differ in timing, in what structures they give rise to, and in the consequences when they fail to close.
The anterior neuropore is the last cranial opening to seal, and its closure is essential for proper brain formation. When it does not close, the result is anencephaly, a condition in which major portions of the brain and skull fail to develop. Anencephaly is invariably fatal, either before or shortly after birth. The anterior neuropore closes around day 24 to 25 after fertilization, which is often before many people even know they are pregnant.
The posterior neuropore closes a few days later, during what embryologists call Carnegie stage 12, when approximately 25 pairs of somites (the blocks of tissue that form the spine’s vertebrae) are present. The site of its final closure corresponds roughly to the second sacral vertebral level, which is in the lower back.3PubMed. The development of the human brain, the closure of the caudal neuropore, and the beginning of secondary neurulation at stage 12 Failure of the posterior neuropore to close is the classic cause of open spina bifida, in which part of the spinal cord or its surrounding membranes protrude through a gap in the spine. Unlike anencephaly, spina bifida is survivable, but it can lead to paralysis, bladder and bowel dysfunction, and hydrocephalus (fluid buildup in the brain) depending on where and how severe the defect is.
Below the level where the posterior neuropore closes, the rest of the spinal cord actually forms by a different process called secondary neurulation, in which a solid cord of cells hollows out rather than folding from a flat plate. Defects in secondary neurulation tend to produce subtler, often closed (skin-covered) forms of spina bifida in the very lowest part of the spine.
The Physical Forces That Pull the Tube Shut
Closing the neuropores is not just a matter of gene signals flipping on and off. It involves real mechanical forces acting on living tissue, and researchers have spent considerable effort figuring out how embryonic cells physically pull the neural folds together.
One key player is a network of actin filaments and myosin motors sitting at the tops (apical surfaces) of the cells lining the neural plate. These proteins work like tiny purse strings: they contract, narrowing the tops of the cells and causing the neural plate to bend inward.4Development. Neural tube closure: cellular, molecular and biomechanical mechanisms The bending brings the two neural folds closer together until they meet at the midline and fuse.
At the posterior neuropore specifically, mouse studies have revealed something interesting: as closure nears completion, an actin cable forms a continuous ring around the shrinking opening. As this ring tightens, a new zippering point appears that moves from the tail end forward, essentially closing the last gap from both directions at once.5PubMed Central. Biomechanical coupling facilitates spinal neural tube closure in mouse embryos This dual-direction final squeeze appears to be a conserved feature: comparative work in chick embryos has found a similar link between tension generation and cell patterning around closing neuropores, even though the specific structures differ between species.6PubMed Central. Anisotropic Cell Shape and Motion Coordinate Hindbrain Neuropore Morphogenesis
Fluid pressure adds another mechanical layer. Once part of the neural tube has sealed, it traps cerebrospinal fluid inside. That trapped fluid exerts pressure on the walls of the tube from within. Research in chicken embryos has shown that the developing hindbrain actually expands by thinning its dorsal tissue under this positive pressure, while the spinal cord resists the same pressure and keeps its shape.7bioRxiv. Differential tissue deformability underlies shape divergence of the embryonic brain and spinal cord under fluid pressure This difference in tissue stiffness helps explain why the brain and spinal cord end up with such different shapes, even though they start as the same tube.
Molecular Signals That Guide Closure
Behind the physical forces, a web of molecular signaling pathways tells cells when and how to move. Two of the most studied are the planar cell polarity (PCP) pathway and the Sonic Hedgehog (Shh) pathway.
The PCP pathway helps coordinate the movement of large groups of cells in the same direction, a process called convergent extension that narrows the neural plate and lengthens it. When components of the PCP pathway are disrupted in animal models, the neural plate stays too wide for the folds to reach each other, and the tube fails to close.8PubMed Central. Roles of planar cell polarity pathways in the development of neural tube defects In humans, variants in PCP-related genes have been linked to neural tube defects, though the genetics are complex and no single gene accounts for most cases.
Sonic Hedgehog signaling has a more nuanced relationship with neural tube closure. Too little Shh activity is associated with holoprosencephaly, a midline brain defect where the forebrain fails to divide into two hemispheres. Too much Shh activity, on the other hand, is linked to exencephaly (brain tissue protruding from an open skull) and spina bifida. Making this even more complicated, the Shh pathway depends on tiny hair-like cellular structures called cilia, and mutations that impair cilia function often disrupt Shh signaling in ways that prevent the neural tube from closing properly.9PubMed Central. The relationship between sonic Hedgehog signaling, cilia, and neural tube defects
More recently, attention has turned to the non-coding parts of the genome. Long non-coding RNAs and circular RNAs appear to interact with the molecular machinery that modifies how DNA is packaged and read, influencing the gene-expression profiles needed during neural tube closure. When these non-coding RNA networks are disrupted, the risk of neural tube defects rises.10Human Gene. Exploring non-coding RNAs contribution to neural tube defects Separately, bioinformatic work has identified microRNAs that target genes in the BMP signaling pathway, another cascade central to normal neural tube development.11PubMed. MicroRNA-Mediated Regulation of BMP Signaling in the Developing Neural Tube The picture that emerges is one of many overlapping regulatory layers, none of which is solely responsible for closure and any of which can contribute to failure.
What Happens After the Neuropore Closes
Closure of a neuropore is not a clean, instantaneous event followed by silence. Live-imaging studies of the mid-hindbrain neuropore in animal models have revealed unexpected collective cell behavior at the closing point. As the neuropore seals, surrounding cells first gather toward the closure site, then appear to be released from it and redistribute outward. This relocation step depends on both programmed cell death (apoptosis) and enzymes that break down the material between cells. Blocking either process impairs the cell relocation, suggesting that apoptosis-driven remodeling of the tissue around the neuropore is part of the final cleanup after closure.12BMC Developmental Biology. Caspases and matrix metalloproteases facilitate collective behavior of non-neural ectoderm after hindbrain neuropore closure
Meanwhile, the cells on the surface of the embryo, the non-neural ectoderm that flanks the neural folds, need to fuse over the top of the sealed tube so that skin will eventually cover the spinal cord and brain. Disruptions to this surface-layer fusion can leave the neural tube technically closed but still exposed, which can cause some of the tissue damage seen in open spina bifida even when the underlying neural tube architecture is partially intact.
When Neuropore Closure Fails
Neural tube defects are among the most common congenital malformations worldwide, with an incidence that varies widely by region, from about 0.2 to as many as 11 per 1,000 live births depending on geography, genetics, and dietary factors.13PubMed Central. Neural tube defects: Different types and brief review of neurulation process and its clinical implication The type of defect depends on where along the tube closure failed and during which developmental window.
The most severe cranial defect, anencephaly, results from failure of the anterior neuropore. It accounts for a large share of neural tube defect diagnoses and is always lethal. Encephalocele, in which brain tissue herniates through a skull defect, represents a somewhat less extreme failure of cranial closure. On the spinal side, myelomeningocele (open spina bifida) is the most common serious form, involving protrusion of the spinal cord and meninges through a vertebral gap. Milder variants include meningocele, where only the meninges protrude, and spina bifida occulta, a usually asymptomatic gap in the vertebral arch with no herniation of neural tissue.
Both genetic and environmental factors feed into the risk. Maternal diabetes and obesity are recognized risk factors. High blood sugar in the embryo can trigger oxidative stress, alter the function of key developmental genes, and increase abnormal cell death in the neuroepithelium, all of which can interfere with closure.14Frontiers in Spinal Neurosurgery. Neurulation and the Possible Etiologies of Neural Tube Defect Certain medications, particularly some anti-seizure drugs, also raise the risk. The best-established protective factor is folic acid supplementation before and during early pregnancy, which has been shown to prevent a substantial fraction of neural tube defects. Many countries now mandate folic acid fortification of grain products for this reason.
Detecting and Treating Neuropore-Related Defects Before Birth
Because the neuropores close so early, the window of vulnerability is essentially over before the first routine ultrasound. Most neural tube defects are detected later in pregnancy, either through elevated maternal serum alpha-fetoprotein levels in a blood test or through an anatomy scan ultrasound, typically around 18 to 20 weeks. In some countries, routine first-trimester screening can raise suspicion earlier, but detailed diagnosis usually requires the second-trimester scan.
For open spina bifida specifically, fetal surgery has become an option in specialized centers. The landmark Management of Myelomeningocele Study (MOMS trial) demonstrated that repairing the defect before birth could reduce the need for a shunt to drain excess brain fluid and improve motor outcomes in the child’s legs compared to postnatal repair. Two surgical approaches now exist: open fetal surgery, which involves a full incision into the uterus, and fetoscopic surgery, which uses small instruments through tiny ports. A systematic review of fetoscopic repair studies found that outcomes for shunt placement and reversal of hindbrain herniation were similar between the two approaches. However, fetoscopic repair carried a higher rate of preterm membrane rupture (about 79% compared with 36% for open surgery) and a greater need for additional postnatal repair of the closure site (roughly 30% versus 7%). On the other hand, the risk of uterine wall separation was essentially zero with fetoscopy, compared with about 8% in open fetal surgery.15PubMed Central. Fetal surgery for open spina bifida The choice between approaches depends on the specific clinical scenario and the expertise of the surgical team.
Organoids and the Future of Neuropore Research
One persistent challenge in studying human neuropore closure is that it happens so early in development that direct observation is essentially impossible. Animal models have taught us a great deal, but differences between species, such as the number of closure initiation sites, mean that not every finding in mice or chickens translates directly to humans.
A newer approach involves growing neural tube organoids from human stem cells. These miniature, three-dimensional structures can recapitulate key features of neural tube formation in a dish, giving researchers a way to study human-specific aspects of closure in real time. Recent work has generated organoids with an elliptical shape that resembles the actual human neural tube, carrying cranial-to-upper-spinal identity, which allows researchers to evaluate how different nutrients and metabolic factors affect closure at the cellular level.16PubMed Central. Effectiveness of Multivitamins vs Folic Acid on Prevention of Neural Tube Defects in Mouse Genetic Models and Human Organoids Organoid technology has been described as an excellent model system for studying both the normal formation of the human neural tube and the occurrence of defects.17PubMed Central. Organoids as a new model system to study neural tube defects
These models are already being used to test whether specific vitamin combinations differ in their ability to prevent closure failure, and to probe the roles of individual genes and signaling pathways in a human context. For a process that finishes within the first month of pregnancy and has consequences that last a lifetime, the ability to watch it unfold in a lab setting is a significant step forward. How quickly these organoid findings translate into changes in clinical advice remains to be seen, but the technology is filling a gap that animal models alone could not.