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The notochord is a flexible rod made out of a material similar to cartilage. If a species has a notochord, it is, by definition, a chordate. The notochord lies along the anteroposterior ("head to tail") axis, is usually closer to the dorsal than the ventral surface of the animal, and is composed of cells derived from the mesoderm. The notochord has been observed to have many functional and developmental functions. The most commonly cited functions are as a site of muscle attachment, vertebral precursor, and as a midline tissue that provides signals to the surrounding tissue during development. Notochords are thought to be advantageous (both in an evolutionary and developmental context) because they provide(d) rigid structure for muscle attachment, but were still flexible. In some chordates, it persists throughout life as the main axial support of the body, while in most tetrapods it becomes the nucleus pulposus of the intervertebral disc.〔 The notochord plays a key role in signalling and coordinating development. Embryos of vertebrates still form transient notochord structures today during the gastrulation phase of development. The notochord is found ventral to the neural tube. ==Development== Notogenesis is the development of the notochord by the epiblasts that make up the floor of the amnion cavity ((Human Embryology )). The notochord arises from the bilaminar embryonic disk. The notochord forms during gastrulation and soon after induces the formation of the neural plate (neurulation), synchronizing the development of the neural tube. On the ventral aspect of the neural groove an axial thickening of the endoderm takes place. (In bi-pedal chordates, e.g. humans, this surface is properly referred to as the anterior surface). This thickening appears as a furrow (the chordal furrow) the margins of which anastomose (come into contact), and so convert it into a solid rod of polygonal-shaped cells (the notochord) which is then separated from the endoderm. It extends throughout the entire length of the future vertebral column, and reaches as far as the anterior end of the midbrain, where it ends in a hook-like extremity in the region of the future dorsum sellae of the sphenoid bone. Initially it exists between the neural tube and the endoderm of the yolk-sac, but soon becomes separated from them by the mesoderm, which grows medially and surrounds it. From the mesoderm surrounding the neural tube and notochord, the skull, vertebral column, and the membranes of the brain and medulla spinalis are developed. Postembryonic vestige of the notochord is found in the nucleus pulposus of the intervertebral discs. Isolated notochordal remnants may escape their lineage-specific destination in the nucleus pulposus and instead attach to the outer surfaces of the vertebral bodies, from which notochordal cells largely regress. In humans, by the age of 4, all notochord residue is replaced by a population of chondrocyte-like cells of unclear origin. Persistence of notochordal cells within the vertebra may cause a pathologic condition- persistent notochordal canal. They are also found to persist in the nasopharyngeal space and, in such an unusual instance, may give rise to Tornwaldt's cyst. 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「notochord」の詳細全文を読む スポンサード リンク
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