Q.99 Closure of neural tube during neurulation is the process in which neural tube eventually
forms a closed cylinder that separates from surface ectoderm. Which one of the following
terms is linked to the failure in the closure of entire neural tube i.e. brain and spinal cord,
over the body axis?
(A) Craniorachischisis (B) Spina bifida
(C) Anencephaly (D) Cleft palate
Answer: (A) Craniorachischisis
Craniorachischisis represents the complete failure of neural tube closure along the entire body axis, affecting both the brain and spinal cord during neurulation. This severe neural tube defect (NTD) occurs when the neural folds fail to fuse from the cranial to caudal ends, leaving the neural plate open and exposed. Other options involve partial failures at specific regions.
Option Analysis
(A) Craniorachischisis
This term describes the most extensive NTD, combining anencephaly (cranial failure) with rachischisis (complete spinal cord exposure over the vertebral axis). It matches the question’s description of “failure in the closure of entire neural tube i.e. brain and spinal cord, over the body axis.”
(B) Spina bifida
Spina bifida refers to incomplete vertebral arch fusion in the lumbosacral spine, often with meninges or neural tissue protrusion (e.g., myelomeningocele). It affects only the caudal neural tube, not the entire axis including the brain.
(C) Anencephaly
Anencephaly results from failed anterior neuropore closure around day 25, leading to absence of the cranial vault and forebrain. It spares the spinal cord, limiting the defect to the brain region.
(D) Cleft palate
Cleft palate arises from failed fusion of palatal shelves in the secondary palate during weeks 6-9, involving facial ectoderm and mesenchyme. It does not relate to neural tube closure.
Neural tube closure failure during neurulation leads to severe birth defects like craniorachischisis, where the entire neural tube—brain and spinal cord—remains open along the body axis. This process, critical around days 22-28 post-fertilization, involves neural fold fusion into a closed cylinder separating from surface ectoderm. For CSIR NET aspirants, distinguishing craniorachischisis from spina bifida or anencephaly is essential for mastering developmental biology.
Neurulation Process Overview
Neurulation transforms the neural plate into a tube via primary (days 18-28) and secondary phases. Anterior neuropore closes by day 25 (somites 18-20); posterior by day 27-28 (somite 25-30). Folate deficiency, genetic factors (e.g., MTHFR mutations), and teratogens disrupt zippering, causing NTDs.
Types of Neural Tube Defects
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Craniorachischisis: Total axis involvement; lethal, with exposed neural tissue from cranium to sacrum. Incidence ~1/10,000; often results in miscarriage.
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Spina bifida: Caudal-specific; forms like occulta (skin-covered) or aperta (myelomeningocele). Surgical correctable post-birth.
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Anencephaly: Cranial-only; incompatible with life due to brain degeneration.
| Defect | Affected Region | Closure Site Failure | Prognosis |
|---|---|---|---|
| Craniorachischisis | Entire axis (brain + spine) | Both neuropores + full length | Lethal |
| Spina bifida | Lumbosacral spine | Posterior neuropore | Variable; treatable |
| Anencephaly | Cranial vault/forebrain | Anterior neuropore | Lethal |
| Cleft palate | Secondary palate | Palatal shelves | Surgical repair |
Prevention and CSIR NET Relevance
Folic acid (400-800 mcg daily preconception) reduces NTD risk by 70%. Questions test defect specificity: entire axis failure = craniorachischisis. Practice differentiating via neuropore timing and axis extent.
