Neural tube formation: notochord induction and closure
Neurulation is the formation of the neural tube, which becomes the brain and spinal cord, from ectoderm, the outermost layer of the embryo.
Remember
The notochord, a rod in the midline of the embryo, instructs ectoderm to become nervous tissue, and the neural tube then closes from the middle towards both ends.
Memory aid
a zip that closes from the middle outwards
Picture the neural tube closing like a zip that starts in the middle and works towards both ends.
The nervous system arises from the ectoderm, which is the outermost layer of the embryo, and begins to form in the third week. Underneath it, along the midline, runs the notochord, a rod made of mesoderm. Mesoderm forms structures such as muscle and bone. The notochord releases signals that induce (instruct) the ectoderm above it to become neuroectoderm, which is ectoderm specialised to form nervous tissue.
The neuroectoderm thickens into the neural plate on the back of the embryo. By week four a groove forms in the plate, its edges rise and fold together, and the plate separates from the surface to become the neural tube. The cells along the border of the plate become neural crest cells, which migrate away from the tube.
Closure of the tube begins in the middle of the embryo and spreads towards both ends. Folding depends on signals from the notochord that spread by diffusion, so cells near the signal are affected first. The openings left at either end are the neuropores: the cranial one at the head end and the caudal one at the tail end. They are the last to close, so neural tube defects arise when either stays open.
Induction means a neighbouring tissue is instructed to change its fate: the inducing notochord is mesoderm, while the neural tube is ectoderm. Endoderm, which forms the lining of the gut and organs such as the liver and lungs, has no part in forming the neural tube.
Key facts
- Inducer
- the notochord, a rod of mesoderm in the midline, instructs ectoderm to become neuroectoderm, which then forms the neural plate.
- Source layer
- the neural plate and the neural tube are ectoderm, even though the inducing notochord is mesoderm.
- Closure
- it takes approximately 10 days to complete.
- Direction of closure
- from the middle of the embryo towards both the cranial and caudal ends.
- Neuropores
- the cranial and caudal openings that are the last to close, so neural tube defects occur if either stays open.
- Neural crest
- cells that arise from the border of the plate and migrate away from the tube.
Exam traps
- The nervous system does not come from mesoderm just because the notochord that induces it is made of mesoderm: the neural tube itself arises from ectoderm.
- Surface ectoderm (the ectoderm that becomes the skin surface) does not induce the neural plate, because the inducing signals come from the notochord: the ectoderm that responds to them forms the neural plate and the nervous system, not the skin surface.
- Neural crest cells cannot induce the neural plate, because they arise from its border once it has formed.
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