Mechanism of neurulation.
Abstract
Neural tube defects (NTDs) are variety of defects which result from abnormal closure of the neural tube during embryogenesis. Various factors are implicated in the genesis of neural tube defects, with contributions from both genetic and environmental factors. The clear understanding of the causes which leads to NTDs is lacking, but several non-genetic risk factors have been identified which can be prevented by maternal folic acid supplementation. Multiple genetic causes and several critical biochemical reactions have been identified whose regulation is essential for the closure of neural tube. Preventive therapies can be developed by identifying potential risk factors in the genesis of NTDs.
Keywords
- neural tube defects
- neurulation
- genetics
- neural plate
- folic acid
1. Introduction
Neural tube defects (NTDs) are birth defects which result from the abnormal closure of neural tube during embryogenesis [1]. Its etiology is multifactorial; nutritional, environmental, genetic, and exposure to various teratogenic drugs during pregnancy. The severity of NTDs varies from asymptomatic cutaneous manifestations to life threatening conditions where brain and spinal cord is completely exposed to the exterior.
Here authors describe the process of formation of central nervous system (brain and spinal cord) along with the possible etiologies of NTDs.
2. Neurulation
The initial process involved in the formation of nervous system (brain and spinal cord) is known as neurulation. It is further divided into:
Primary neurulation
Secondary neurulation
2.1 Primary neurulation
Primary neurulation is responsible for the future formation of brain and most part of the spinal cord. The thickened ectoderm, neural plate, elevates to form neural folds and subsequent fusion of neural folds give rise to neural tube (Figures 1 and 2). The fusion begins in cervical region and proceeds in cranial and caudal directions. Ends of the neural tube, neural pore, are closed first on the cranial side (21 days post fertilization) followed by the caudal side (28 days post fertilization) (Figure 3). Thus, neurulation is a process involved in the formation of neural tube and closure of neuropores by the end of the fourth week of embryo development [2, 3]. The defects resulting from the abnormalities in primary neurulation leads to open neural tube defects [4, 5].
2.2 Secondary neurulation
The formation of spinal cord distal to mid-sacral region is formed by the process of secondary neurulation. Some of the loosely packed cells of tail bud condense to form an epithelial rod, which later canalise and form a tubular lumen for the last part of sacral and coccygeal regions of spinal cord [6, 7, 8]. Studies of various pathways at molecular and cellular level in neurulation-stage embryos provide understanding of development of normal or abnormal neural tube [9]. The malformations resulting due to the abnormalities in secondary neurulation results in closed neural tube defects (Figure 4).
3. Mechanisms of neurulation
Shaping of the neural plate with mediolateral narrowing and rostro caudal elongation is needed to initiate closure of neural tube [10]. This elongation depends on Wnt signalling pathway via Frizzled (Fzd) membrane receptors [11]. Convergent extension of neural plate takes place through planar cell polarity (PCP) mediators via PCP genes functioning including
Functional failure of PCP mediator genes results in broad neural plate and craniorachischisis due to disruption in the closure of neural plate [14]. Occurrence of closure in the forebrain and part of the midbrain in craniorachischisis implies that PCP-dependent mechanism is not necessary for whole of the brain. Exencephaly developing in mutants of the genes
Tips of the neural folds approximate each other after bending of neuroepithelium to achieve closure. Median hinge points (MHP) and dorsolateral hinge points (DLHP) in a stereotypical manner bring out the bending. Regulating signals for the bending emanate from nonneural tissues around the neural folds. Notochord derived Shh causes induction of floor plate of the neural tube at the MHP. Factors enhancing Shh signals, for example, mutations in cilia-related genes such as
Complexity of cranial neurulation compared to the spinal neurulation appears due to more extensive and sensitive genetic mechanisms. As a result, exencephaly comprises three times of the cases as does spina bifida after induction by teratogens. Disruption of cranial neurulation is contributed by some specific factors; mesenchymal expansion under the neural folds, cytoskeleton disruption of actin filaments, and mutant genes (e.g.,
Meeting of the neural folds in dorsal midline give rise to two different types of epithelial layers, fledgling neural tube and overlying intact surface ectoderm, eventually after process of adhesion, fusion, and remodelling. Ephrin receptors, protease-activated receptors and
S. No. | Mechanism | Pathways/mediators/genes |
---|---|---|
1 | Shaping and convergent extension of neural plate | Wnt signalling pathway, PCP mediator genes- |
2 | Adhesion, fusion, and remodelling | Ephrin receptors, protease-activated receptors and |
3 | Cell proliferation | |
Neuronal differentiation | Notch pathway genes | |
Programmed cell death | ||
4 | Induction of floor plate | Notochord derived Shh, cilia-related genes- |
5 | Cranial Neurulation | mesenchymal expansion, genes for cytoskeleton components (e.g., cilium-dependent hedgehog signalling genes |
6 | Spinal Neural Tube Closure | by |
3.1 Type of neural tube defects
NTDs are classically divided into two types:
Open neural tube defects
Closed neural tube defects
3.1.1 Open neural tube defects
Open NTDs or spina bifida cystica are craniorachischisis, exencephaly-anencephaly and myelomeningoceles (Figure 5A–F). Open defects are characterised by the exposure of neural tissue through the skin as well as through the bony defect and is obvious at birth. These defects present with neurological deficit and carries poor prognosis. They can be identified easily during pregnancy due to high levels of α fetoprotein and acetylcholinesterase in amniotic fluid.
Craniorachischisis is the most serious and rare type of open NTD, which involves the defect in both cranial and spinal region. Their reported prenatal terminations range from 0.51 to 10.7 per 10,000 births in different regions of the world. Neural tube gets open from brain stem to spinal cord resulting in anencephaly and spina bifida simultaneously with external exposure of tissue in hindbrain and spinal cord on its posterior aspect. Death of the new-born is certain in craniorachischisis making it a lethal condition [21].
Anencephaly due to exencephaly involves non closure of only cranial part of the neural tube. Absence of forebrain and the vault of skull with intact skull base can be seen. Forebrain and midbrain are absent, brain stem is less severely involved and pituitary gland is hypoplastic in most of these cases. It is a lethal condition causing death of new-born within few days after birth.
Myelomeningocele results from defect in posterior part of spine usually in the lumbosacral region. Meningeal sac hernia takes place posteriorly, containing cerebro-spinal fluid and nervous tissue, through a bony defect in the vertebral arch. Myelocele is a similar open NTD involving the spinal cord without protrusion of meningeal sac. Spinal cord is typically divided into two halves giving an appearance of “open book,” which exposes ependymal layer to the surface.
Survival of the babies with open spina bifida depends on the severity and level of the lesion. Some other associated conditions including hydrocephalus, Chiari malformation type II, and vertebral abnormalities make them more complicated [21].
3.1.2 Closed neural tube defects
Closed NTDs or spina bifida occulta are encephalocele, meningocele, lipomeningocoele, diastematomyelia, and tethered filum terminale. Here, the underlying neural defect is masked by the intact overlying skin. The defect lies in the lower lumbar and sacral regions, and represents closed defects with deficient vertebral arches, sacral agenesis, and other skeletal defects. Presence of nevi, depigmentation, haemangiomas, localized hypertrichosis, and lumps including subcutaneous lipomas are some cutaneous stigmata of the lower back, may be the only signs of spina bifida occulta. Symptoms may not develop until late childhood and they possess comparatively better prognosis than open neural tube defects [21].
Encephalocoele is a round, soft, compressible, and nodular sac like protrusion of brain and/or its meningeal covering through an opening in the skull. Majority of encephaloceles pass through the midline calvarial defects and are classified according to the site of herniation; anterior, parietal, and occipital. Among these locations, occipital comprises 75% of the total number of encephaloceles. Hypertrichosis, and bluish translucency could be seen over the lesions during increased intracranial pressure. A comparatively better prognosis is observed if site is more rostral [22].
Meningocoele consists of meningeal herniation through the defect in the vertebral column. The spinal cord in these cases lies within the spinal canal in normal position. Atrophic epidermis of the skin usually covers the pedunculated and compressible lesion of herniated mass. They generally present with normal neurological examination and without any deformity (Figure 5A–F).
Lipomyelomeningocele is a form of occult spinal dysraphism where fat herniates through the bony defect. Diastematomyelia refers to a split in the spinal cord by a bony or a fibrous septum. Majority of these patients have cutaneous manifestations (Table 2) [23].
Open NTDs- Spina bifida cystica | ||||
---|---|---|---|---|
S.No | Type | Location of the defect | Findings of the defect | Prognosis |
1 | Craniorachischisis | Cranial and Spinal | Open neural tube from brain stem to spinal cord | Death of the new born, lethal condition |
2 | Exencephaly-Anencephaly | Cranial | Absent forebrain and skull, thick and flat skull base | Death of the new born, lethal condition |
3 | Myelomeningoceles | Posterior part of spine, lumbar region | Meningeal sac hernia containing CSF and nervous tissue Associations with hydrocephalus, Chiari malformation type II, and vertebral abnormalities | Survival depends on severity and level of the lesion |
Closed NTDs | ||||
1 | Encephalocele | Cranial | Hernia through small opening in the skull | Depends on site, lesion more rostral with better prognosis |
2 | Meningocele | Spinal | Meningeal herniation covered by the skin without its appendages | normal neurological examination and functions of the body |
3 | Lipomeningocoele | Spinal | Fat along with meninges herniates through the bony defect | Usually have normal neurological function. |
4 | Diastematomyelia | Spinal | Spinal cord splitting by bony or fibrous septum | Neurological deficit with bowel and bladder involvement |
5 | Tethered filum terminale | Spinal | Conus medullaris is tethered by filum terminal | Usually become symptomatic in the late childhood |
4 Aetiology of neural tube defects
NTDs prevalence range from 0.5 to 10 per 1000 pregnancies, thereby poses significant public health problem [1]. Variations in the incidence are due to large variety of risk factors such as:
Environmental factors
Nutritional deficiencies
Genetic causes
4.1 Environment factors
Environmental exposure to air pollution, extremes of temperature, and exposure of toxins to the expectant mothers are some of the known risk factors contributing to the aetiology of NTDs [24]. Study on various animals suggests the effect of teratogens in development of NTDs. Anticonvulsant drug valproic acid and a fungal product fumonism are known teratogens to develop NTDs in humans.
Hyperglycemia in embryos of cultured rodents, maternal obesity and diabetes mellitus are recognised risk factors for NTDs. Increased oxidative stress, change in Pax3 gene functions, apoptosis of neuroepithelial cell, activation of apoptosis signal-regulating kinase 1(ASK1) enzyme are some effects brought about by the maternal and embryonic hyperglycemia resulting in NTDs (Table 3) [25].
Multifactorial (50%) | |||
---|---|---|---|
A | S.No. | Non-genetic causes | Examples |
1 | Environmental | Air pollution Extremes of temperature Exposure of toxins to the expectant mothers Teratogens; anticonvulsant-valproic acid, fungal product fumonism Hyperglycemia in embryos Maternal obesity and diabetes mellitus | |
2 | Nutritional | Poor nutritional status and folate deficiency in mothers | |
B | S.No. | Genetic causes | Examples |
1 | Gene-gene interactions | Supplementary sequel of heterozygous mutations Variable phenotypic expressions of | |
2 | Effect of modifier genes | Variation in | |
3 | Implications through experimental models | PCP genes mutations- | |
4 | Folate one-carbon metabolism in mitochondria | suboptimal levels of folate in maternal blood interact with mutated | |
5 | Histone modifications | Mutations in histone demethylases Mutations in histone deacetylases Teratogens- Valproic acid and trichostin A | |
6 | Syndromes | Trisomy 13, Trisomy 18 and Triploidy | |
Unknown factors (50%) |
4.2 Nutritional deficiencies
Folate is a well recognized vitamin B supplement implicated in the causation of neural tube defects. Poor socioeconomic status with high risk of congenital anomalies focus the scientists to find out the nutritional deficiencies in such cases. In mothers of NTD fetuses, folate was found to be deficient. Mechanism of folic acid in prevention of NTDs was considered when it was seen that blood folic acid levels in some mothers of affected foetuses were normal. It was believed that some suboptimal levels of folate in maternal blood interact with mutated genes, such as
4.3 Genetics causes
Genetic mutations in the aetiology of NTDs always depend on polygenic and multifactorial inheritance. The causations by gene variants are complicated by the multiple genes, modifier genes, epigenetic factors, and environmental effects. Some recognised mutations of different genes obtained by experiments on animals were found to be the causes in minimal number of NTD cases in humans. Increasing understanding of development of neural tube and NTDs on molecular and cellular level still needs more precision to identify genetic basis of occurrence in individual cases. More than 200 mutations in the genes, and association of the environmental risk affect folate metabolism to causes NTDs. Significance to focus more on individual genes by scientists comes from the fact of having very less percentage of NTD cases in syndromes of chromosomal aberrations as compared to the isolated cases of NTDs. Now a days, data analysis from large-scale genome sequencing of NTD patients is more promising and practicable to mark the contribution of various genes in the patients and mutational burden of associated risks.
4.3.1 Gene interactions and modifier genes
Three wide range mechanisms to explain gene interactions in development of NTDs are; 1—Functional incompetency of two non-comparable genes for example
4.3.2 Genetic implications through experimental models
Mutations in PCP genes—
4.3.3 Genetic factors relation with environmental risk factors
A known environmental risk factor when interacts with genetic alteration in the embryo, could eventually instigate the risk of NTDs. Folate one-carbon metabolism in mitochondria is highly studied category for finding cause of NTDs in such cases and genes related to folate metabolism enlighten the ambience of maternal folate levels.
Enhanced risk of developing spina bifida by the ‘risk’ genes
4.3.4 Gene-regulatory mechanisms and NTDs
Multigenic involvement of NTDs makes it more complicated and difficult to identify due to irregular expressions of genes. Such as, insufficient or excess expression of
5. Prevention of neural tube defects
Primary prevention is quite effective in reducing the birth defects related to NTDs. It has been suggested that folic acid supplementation in a dose of 0.4 mg per day prevents large number of NTDs. There is three fold reduction in NTDs recurrence with an intake of 4 mg folic acid per day by the expectant mothers. The exact mechanism of this prevention is yet to be elucidated, but folate plays an important role in numerous chemical reactions, including thymidine and purine production and S- adenosylmethionoine synthesis, which is the methyl donor for DNA, lipids and proteins.
6. Conclusion
The causes of NTDs are multifactorial in humans, including genetic and non-genetic factors. The combination of these factors leads to defective closure of neural tube and subsequent development of malformed fetal appearance. Folic acid supplementation during pregnancy and its awareness through various platforms are necessary to prevent further occurrence of NTDs in children.
Acknowledgments
I am thankful to my family for the support they have provided in writing this chapter.
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