25.6
神経胚形成
神経細胞は、胚の外胚葉、中胚葉、内胚葉の3つの細胞層が形成された後、中枢神経系の前駆体を形成する発生学的過程です。ヒトの場合、中枢神経系の大部分は一次神経形成によって形成されます。一次神経形成では、もともと平らなシート状の細胞であった外胚葉の中央部分が上や内側に折り畳まれ、封鎖されて中空の神経管が形…
神経化は、原腸形成後の 胚変換のうちの一つで，神経系を作り出すプロセスです。原腸形成後、胚の周辺の表面外胚葉は皮膚成分を 生成するようになりますが，より中心に位置する組織である神経外胚葉は 脊髄と脳を産生します。タンパク質シグナルは、この正中線外胚葉を肥厚化させ、その下に、脊索として知られる 中胚葉細胞を凝縮した神経板を 形成するようになります。神経板の中心はこの 基礎構造に固定されます。最初、脊索付着細胞は柱のような形をしています。しかし，時間の経過によってくさび型に曲がり，ほかの要素とともに，蝶番のように 神経板を上方へ，内側へ曲げます。同時に、神経板の淵は目立つようになり，神経襞と呼ばれる構造を生み出します。最終的に，蝶番は2つに増えます。1つは神経襞の下にあり、どちらも表面外胚葉に固定されているため、神経板の端を内側に向けています。この一次神経形成の過程の間、神経襞は互いに向かって移動し続け、最終的には隣接する表面外胚葉に接触，合体し，外胚葉から分離します。その結果、神経管と呼ばれる 中空の外胚葉由来の円筒ができ、その胚の位置に応じて、脊髄や脳が生成されます。
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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.