Notochord

The notochord, called ( " notochord " from Latin chorda or Greek CHORD " gut ", " gut " and Latin dorsum " back "), even notochord or axis bar is the original, mesodermal, inner axial skeleton of all chordates and is the eponymous feature for this. It is located as an elastic rod with original chordates such as amphioxus and in early developmental stages of vertebrates ( Vertebrata ) in the back (dorsal ) between the neural tube and the gut. It serves as an endoskeleton and thus gives the lancelet protection, strength and serves as a starting point for the muscles.

In all vertebrates, the embryonic notochord is created and formed in the course of individual development ( ontogeny ) usually completely or to rudiments inside of their replacements spine back. It induces by its location but also with the chordates, where she is re-formed, the formation of the neural tube, which develops into the central nervous system from the spinal cord and brain.

Construction

The notochord is in its original form a long, flexible rod made of specialized cells, the so-called chordagenen tissue, dar. These are single cells with a large vacuole, by their internal pressure ( turgor ) the notochord is replaced by its stiffness. The Chordastab is surrounded by a connective tissue sheath, the Chordascheide. In the lancelet, this structure is replaced by monetary roll- consecutive and specialized muscle cells and in some tunicates ( Tunicata ) is, to glykogenreiche cells with yolk inclusions.

Education and ontogeny

The notochord arises in the early stages of embryonic development in the formation of the germ layers ( gastrulation ) as a mesodermal pinch from the primary node. The resulting structure is similar to the beginning of a hollow cylinder, but evolves with time into a solid cylinder. This Notochordvorgänger fused (approximately on the 20th day of embryonic development in humans ) into the underlying endoderm to form the chordal plate. This transition structure is only short-lived, she finds (for example, already on 22 - 24th day of embryonic development in humans ) an end by the constriction of the notochord from the primitive gut ( archenteron ). More often is - mistakenly - regarded only the last part of the process, after which the notochord is then interpreted as entodermales instead mesodermal tissue. But it is also true that few Entodermalzellen can be included in the notochord permanently by this procedure.

The acrania and other animal groups in which the notochord acts as part of the musculoskeletal system, a powerful collagen casing was designed to compete Chordastab. The acrania even the chordagenen cells are replaced by muscle cells and are in direct contact with the overlying neural tube. Both the Ascidienlarven as well as in the free-swimming Appendikularien the gelatinous tissue is contained and stabilized the rod by the internal pressure of the cells. Even in vertebrates, the notochord is applied in the form of large vakuolenreicher cells but remains only in the cyclostomes and some fish such as the cartilaginous fishes, contain the troublemakers and Latimeria life in the mold.

For all other vertebrates the notochord is only embryonic created and replaced by the spinal column or concentrated on individual, small regions of the same. Next to her, initially, developing the somites, later, the definitive vertebrae. With the emergence of the spine, the regression of the notochord begins. Whether a chorda dorsalis - radical is situated within the vertebral body of birds and that the gelatinous nucleus pulposus is a chorda dorsalis - residue in the intervertebral discs ( disc ) of mammals, is not fully understood. These gelatinous cores are today, however, interpreted mainly as a neoplasm.

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