25.6
Neurulation
Neurulation is the embryological process which forms the precursors of the central nervous system and occurs after gastrulation has established the th…
Neurulation is the process where components of the post-gastrulation embryo transform to create the future nervous system.
Following gastrulation, surface ectoderm at the embryos periphery is destined to generate skin components whereas more centrally located tissue, the neural ectoderm will produce the spinal cord and brain.
Protein signals cause this midline ectoderm to thicken and form the neural plate beneath which is a condensed rod of mesodermal cells known as the notochord. The center of the plate then affixes to this underlying structure.
Initially, notochord attached cells resemble columns. However, over time, they can tort into wedges, which, along with other factors causes the entire neural plate to bend upwards and inwards as if on a hinge. Concurrently, the plates edges become more prominent, generating structures called neural folds.
Eventually, two additional hinges arise. One beneath each neural fold and both fastened to the surface ectoderm causing the plates edges to turn inwards.
During this process of primary neurulation, the neural folds continue to move towards one another and ultimately touch, core less and detach from the embedding surface ectoderm. The result is a hollow ectoderm derived cylinder named the neural tube, which, depending on it's embryonic location will generate either the spinal cord or brain.
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Q1: What is neurulation and when does it occur during embryonic development?
Neurulation is the embryological process that forms the precursors of the central nervous system, occurring after gastrulation establishes the three primary cell layers. During neurulation, the central portion of the ectoderm—initially a flat sheet of cells—folds upward and inward, sealing to form a hollow neural tube. The anterior neural tube develops into the brain, while the remainder forms the spinal cord.
Q2: How does the neural plate transform into the neural tube?
Protein signals cause the midline ectoderm to thicken, forming the neural plate above a condensed mesodermal rod called the notochord. Notochord-attached cells transform from columns into wedges, causing the neural plate to bend upward and inward. Neural folds form at the plate's edges and move toward each other, eventually fusing and detaching from the surface ectoderm to create the hollow neural tube.
Q3: What are neural crest cells and where do they originate?
Neural crest cells are a population located at the junction between neural and surface ectoderm. As neural folds elevate and fuse during neurulation, neural crest cells appear at the fold tips, identifiable by markers like the Pax7 transcription factor. These cells subsequently migrate throughout the embryo via cell migration to generate diverse tissue types, including peripheral nervous system components like intestinal ganglia.
Q4: How do neural crest cells migrate away from the neural tube?
Neural crest cells downregulate specific adhesive proteins, allowing them to detach from other cells in the neural tube. This reduction in cell-cell adhesion enables neural crest cells to travel throughout the embryo, populating different regions to generate various tissue types including elements of the peripheral nervous system.
Q5: What is the difference between primary and secondary neurulation?
Primary neurulation forms most of the central nervous system in humans through folding of three distinct ectoderm cell sheets. Secondary neurulation, which forms a small posterior spinal cord region, involves loosely-packed cells that condense into a medullary cord, which then hollows out and merges with the anterior primary neural tube to form a continuous structure.
Q6: What developmental consequences can result from defects in secondary neurulation?
Although secondary neurulation plays a relatively minor role in forming the human central nervous system, defects in this process can cause developmental consequences such as certain types of spina bifida. These defects occur when the medullary cord fails to properly hollow out or merge with the primary neural tube.
Q7: How does the notochord influence neural plate folding during neurulation?
The notochord is a condensed rod of mesodermal cells positioned beneath the neural plate. Notochord-attached cells initially resemble columns but transform into wedges over time. This transformation, along with other factors, causes the entire neural plate to bend upward and inward as if on a hinge, driving the folding mechanism essential to neural tube formation.