WordNet

a flexible rodlike structure that forms the supporting axis of the body in the lowest chordates and lowest vertebrates and in embryos of higher vertebrates
the area of mesoderm that forms the notochord (同)chordomesoderm

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2013/01/11 21:52:28」(JST)

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Notochord

Transverse section of a chick embryo of forty-five hours’ incubation.
Latin	notochorda
Gray's	subject #8 52
Precursor	chordamesoderm
Gives rise to	nucleus pulposus
Code	TE E5.0.1.1.0.0.8
MeSH	Notochord

The notochord is a flexible rod-shaped body found in embryos of all chordates. It is composed of cells derived from the mesoderm and defines the primitive axis of the embryo. In some chordates, it persists throughout life as the main axial support of the body, while in most vertebrates it becomes the nucleus pulposus of the intervertebral disc. The notochord is found ventral to the neural tube.

Notochords were the first "backbones" serving as support structures in chordates such as Haikouichthys.^[1] Notochords were advantageous to primitive fish-ancestors because they were a rigid structure for muscle attachment, yet flexible enough to allow more movement than, for example, the exoskeleton of the dominant animals of that time.^[2] Embryos of vertebrates have notochords today, as it retained a key role in signalling and coordinating development even as it was lost in most adults. In most tetrapods, they eventually develop into the nucleus pulposus of the intervertebral discs.

Development of the notochord

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.^{[citation needed]}

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.^{[citation needed]}

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 sellæ 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.^{[citation needed]}

Postembryonic vestige of the notochord is found in the nucleus pulposus of the intervertebral disks, but not in the vertebral bodies, from which notochordal cells usually regress entirely. In humans, by the age of 4, all notochord residue is replaced by a population of chondrocyte-like cells of unclear origin.^[3] Persistence of notochordal cells within the vertebra may cause a pathologic condition- persistent notochordal canal.^[4] They are also found to persist in the nasopharyngeal space and, in such an unusual instance, may give rise to Tornwaldt's cyst.

The notochord in neural development

Research into the notochord has played a key role in understanding the development of the central nervous system. By transplanting and expressing a second notochord near the dorsal neural tube, 180 degrees opposite of the normal notochord location, one can induce the formation of motoneurons in the dorsal tube. Motoneuron formation generally occurs in the ventral neural tube, while the dorsal tube generally forms sensory cells.^{[citation needed]}

The notochord secretes a protein called sonic hedgehog homolog (SHH), a key morphogen regulating organogenesis and having a critical role in signaling the development of motoneurons.^[5] The secretion of SHH by the notochord establishes the ventral pole of the dorsal-ventral axis in the developing embryo.

Evolution of the notochord

A dissected lungfish showing the notochord

The notochord is the defining feature of Chordates, and was present throughout life in many of the earliest chordates. Although the stomochord of hemichordates was once thought to be homologous, it is now viewed as a convergence.^[6] Pikaia appears to have a proto-notochord, and notochords are present in several basal chordates such as Haikouella, Haikouichthys, and Myllokunmingia, all from the Cambrian. The Ordovician oceans included many diverse species of agnathan fish which possessed notochords, either with attached bony elements or without, most notably the conodonts,^[7] placoderms^[8] and ostracoderms. Even after the evolution of the vertebral column in chondrichthyes and osteichthyes, these taxa remained common and are well represented in the fossils record. Several species (see list below) have reverted to the primitive state, retaining the notochord into adulthood, though the reasons for this are not well-understood.

The structure of the notochord

The notochord is composed primarily of a core of glycoproteins, encased in a sheath of collagen fibers wound into two opposing helices. The angle between these fibers determines whether increased pressure in the core will result in shortening and thickening versus lengthening and thinning.^[9]

Organisms which retain a post-embryonic notochord

Amphioxus
Tunicate larvae
Hagfish
Lamprey
Sturgeon
Coelacanth
African lungfish
Tadpoles
Ostracoderms (extinct)

Additional images

Surface view of embryo of Concolor gibbon (Hylobates concolor).
Diagram of a transverse section, showing the mode of formation of the amnion in the chick.
Section through the head of a human embryo, about twelve days old, in the region of the hind-brain.
Transverse section of human embryo eight and a half to nine weeks old.

References

^ Shu, D. G.; Morris, S. C.; Han, J.; Zhang, Z. F.; Yasui, K.; Janvier, P.; Chen, L.; Zhang, X. L. et al. (Jan 2003), "Head and backbone of the Early Cambrian vertebrate Haikouichthys", Nature 421 (6922): 526–529, Bibcode 2003Natur.421..526S, doi:10.1038/nature01264, ISSN 0028-0836, PMID 12556891 edit
^ http://faculty.vassar.edu/jolong/files/LongKoob2006.pdf
^ Urban, J. P. G. (2000). "The Nucleus of the Intervertebral Disc from Development to Degeneration". Integrative and Comparative Biology 40: 53. doi:10.1093/icb/40.1.53.
^ Christopherson, Lr; Rabin, Bm; Hallam, Dk; Russell, Ej (1 January 1999). "Persistence of the notochordal canal: MR and plain film appearance" (Free full text). AJNR. American journal of neuroradiology 20 (1): 33–6. ISSN 0195-6108. PMID 9974055. http://www.ajnr.org/cgi/pmidlookup?view=long&pmid=9974055.
^ Echelard, Y; Epstein, Dj; St-Jacques, B; Shen, L; Mohler, J; Mcmahon, Ja; Mcmahon, Ap (December 1993). "Sonic hedgehog, a member of a family of putative signaling molecules, is implicated in the regulation of CNS polarity". Cell 75 (7): 1417–30. doi:10.1016/0092-8674(93)90627-3. PMID 7916661.
^ Kardong, Kenneth V. (1995). Vertebrates: comparative anatomy, function, evolution. McGraw-Hill. pp. 55, 57. ISBN 0-697-21991-7.
^ http://www.palaeos.com/Vertebrates/Units/030Conodonta/030.000.html
^ http://www.palaeos.com/Vertebrates/Units/Unit060/060.000.html
^ http://icb.oxfordjournals.org/content/40/1/28.full

UpToDate Contents

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English Journal

Biochemical characterisation and assessment of fibril-forming ability of collagens extracted from Bester sturgeon Huso huso × Acipenser ruthenus.

Zhang X1, Ookawa M2, Tan Y2, Ura K2, Adachi S2, Takagi Y2.
Food chemistry.Food Chem.2014 Oct 1;160:305-12. doi: 10.1016/j.foodchem.2014.03.075. Epub 2014 Mar 24.
Collagens purified from Bester sturgeon organs were characterised biochemically, and their fibril-forming abilities and fibril morphologies formed in vitro clarified. Yields of collagens were 2.1%, 11.9%, 0.4%, 18.1%, 0.4%, 0.8% and 0.03% (collagen dry weight/tissue wet weight) from scales, skin, mu
PMID 24799243

The effects of henna (hair dye) on the embryonic development of zebrafish (Danio rerio).

Manjunatha B1, Wei-Bing P, Ke-Chun L, Marigoudar SR, Xi-Qiang C, Xi-Min W, Xue W.
Environmental science and pollution research international.Environ Sci Pollut Res Int.2014 Sep;21(17):10361-7. doi: 10.1007/s11356-014-2968-7. Epub 2014 May 27.
The powder of henna is extensively used as decorative skin paint for nail coloring and as a popular hair dye in Asian countries. Its human health risk is extensive, and it is frequently released as waste into the aquatic environment raising the concerns. Zebrafish (Danio rerio) embryos were employed
PMID 24859694

Thornwaldt Cysts.

Righi S1, Boffano P, Pateras D, Malvè L, Chiodo D, Boson M.
The Journal of craniofacial surgery.J Craniofac Surg.2014 Aug 21. [Epub ahead of print]
Thornwaldt cysts are rare and benign congenital cysts in the region of the pharyngeal bursa, formed by a communication between the notochord and the nasopharyngeal endoderm. They result from a mechanical obstruction, inflammation, or abscess of the pharyngeal bursa. They are superficial to the super
PMID 25148637

Japanese Journal

Developmental Stages in the Wild Medaka, Oryzias latipes

Iwamatsu Takashi
愛知教育大学研究報告. 自然科学編 60, 71-81, 2011-03-01
… The principal diagnostic features are the number and size of blastomeres, form of the blastoderm, extent of epiboly, development of the central nervous system, number and form of somites, optic and otic development, development of the notochord, heart development, blood circulation, the size and movement of the body, development of the tail, membranous fin（fin fold）development, and development of such viscera as the liver, gall bladder, gut tube, spleen and swim（air）bladder. …
NAID 120002954862

Identification of up-regulated genes in amphioxus neurula and the expression of AmphiFABP

Liu Xiaohui,Sun Yi,Zhang Juyong,Li Guang,Wang Yiquan
Genes & Genetic Systems 86(1), 37-46, 2011
… During the developmental process of amphioxus embryo, the neurula is a crucial stage because of neural tube and notochord formation as well as somite emergence at this stage. …
NAID 130000655285