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In the developing chordate (including vertebrates), the neural tube is the embryo's precursor to the central nervous system, which comprises the brain and spinal cord. The neural groove gradually deepens as the neural folds become elevated, and ultimately the folds meet and coalesce in the middle line and convert the groove into the closed neural tube. The ectodermal wall of which forms the rudiment of the nervous system. The centre of the tube is the ''neural canal''. ==Development== The neural tube develops in two ways: ''primary neurulation'' and ''secondary neurulation''. Primary neurulation divides the ectoderm into three cell types: * The internally located neural tube * The externally located epidermis * The neural crest cells, which develop in the region between the neural tube and epidermis but then migrate to new locations Primary neurulation begins after the neural plate forms. The edges of the neural plate start to thicken and lift upward, forming the neural folds. The center of the neural plate remains grounded, allowing a U-shaped neural groove to form. This neural groove sets the boundary between the right and left sides of the embryo. The neural folds pinch in towards the midline of the embryo and fuse together to form the neural tube.〔Gilbert, Scott F. Developmental Biology Eighth Edition. Sunderland, Massachusetts: Sinauer Associates, Inc., 2006.〕 # In ''secondary neurulation'', the cells of the neural plate form a cord-like structure that migrates inside the embryo and hollows to form the tube. Each organism uses primary and secondary neurulation to varying degrees. * Neurulation in ''fish'' proceeds only via the secondary form. * In ''avian'' species the posterior regions of the tube develop using secondary neurulation and the anterior regions develop by primary neurulation. * In ''mammals'', secondary neurulation begins around the 35th somite. Mammalian neural tubes close in the head in the opposite order that they close in the trunk. * In the head: :#Neural crest cells migrate :#Neural tube closes :#Overlying ectoderm closes * In the trunk: :#Overlying ectoderm closes :#Neural tube closes :#Neural crest cells migrate 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「neural tube」の詳細全文を読む スポンサード リンク
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