## Embryological Origin of the Neural Tube **Key Point:** The neural tube is derived from the **neuroectoderm** (dorsal ectoderm), NOT the notochord. The notochord induces the overlying ectoderm to form the neural plate, but the notochord itself is mesodermal in origin and does not directly form the neural tube. ## Correct Statements on NTDs ### Anencephaly - Failure of closure of the **cranial neuropore** at approximately **week 3–4** of gestation - Results in absence of major portions of the brain and skull vault - Incompatible with life; most infants are stillborn or die shortly after birth ### Spina Bifida Occulta - **Bony defect** of the vertebral arch (lamina) only - Spinal cord and meninges remain **intact** and do not herniate - Often asymptomatic; may present with skin dimple, tuft of hair, or lipoma over the defect - Most common form of spinal dysraphism ### Caudal Regression Syndrome - Arises from failure of the **caudal eminence** (primitive streak derivatives) to differentiate properly - Can result in sacral agenesis, lumbosacral maldevelopment, and lower limb abnormalities - Associated with maternal diabetes ## The Notochord's True Role **High-Yield:** The notochord is a **mesodermal structure** that: 1. Induces the overlying ectoderm to form the neural plate (inductive signal) 2. Is NOT itself the source of neural tube tissue 3. Regresses during development; remnants form the **nucleus pulposus** of intervertebral discs Option 3 incorrectly states the notochord is the **source** of the neural tube, when in fact it is only the **inducer**. **Mnemonic:** **NOTOCHORD = NOT the source** — it's the **inducer** of neural tissue. [cite:Langman's Embryology 14e Ch 4]
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