Wikipedia

Notochord

Thenotochordis an elastic, rod-like structure found inchordates.In chordatevertebratesthe notochord is an embryonic structure that disintegrates, as the vertebrae develop, to become thenucleus pulposusin theintervertebral discsof the vertebral column. In non-vertebrate chordates a notochord persists.

Notochord
Transverse section of a chick embryo of forty-five hours' incubation.
Details
PrecursorAxial mesoderm
Gives rise toNucleus pulposus
Identifiers
Latinnotochorda
MeSHD009672
TEE5.0.1.1.0.0.8
FMA85521
Anatomical terminology
Position of notochord and axochord in bilaterians. (A) Zebrafish notochord. (B) Ascidian notochord. (C) Lancelet notochord. Notochord is positioned just ventral to the neural tube and dorsal to the gut, flanked by myotome. (D) Notochord homolog in annelid. Cross-section showing the position of the proposed axochord to the ventral mesentery, blood vessel, and nerve chord. Axochord is found to be dorsal to the nerve chord and ventral to gut of the animal. Red: notochord; Magenta: axochord; Green: nerve chord; Blue: epidermis; Yellow: mesoderm.

The notochord is derived from theembryonicmesodermand consists of an inner core ofvacuolatedcellsfilled withglycoproteins,covered by twohelicalcollagen-elastinsheaths. It lies longitudinally along therostral-caudal(head to tail)axisof the body,dorsalto thegut tube,andventralto thedorsal nerve cord.Some chordate invertebrates, such astunicates,develop a notochord during thelarvalstage but lose it through subsequent stages into adulthood.

The notochord is important for signaling the dorso-ventral patterning of cells coming from the mesodermal progenitors. This helps form the precursors needed for certain organs and the embryo to develop. In summary, the notochord plays essential roles in embryonic development.

The notochord provides a directional reference to the surrounding tissue as a midline structure duringembryonic development,acts as a precursor for vertebrae and a primitive axialendoskeleton.In aquatic animals it can facilitate tail motion when swimming.[1]

Presence

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Incephalochordates(lancelets), the notochord persists throughout life as the main structural support of the body.

Intunicates,the notochord is present only in the larval stage, becoming completely absent in the adult animal, and the notochord is not vacuolated.[2]

In allvertebratesother than thehagfish,the notochord is present only during earlyembryonic developmentand is later replaced by thebonyand/orcartilaginousvertebral column,with its original structure being integrated into theintervertebral discsas thenucleus pulposus.[3][4]

Structure

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The notochord is a long, rod-like midline structure that develops dorsal to thegut tubeand ventral to theneural tube.The notochord is composed primarily of aglycoproteinscore that is encased in a sheath ofcollagenfibers. This is wound into two opposinghelices.The glycoproteins are stored in vacuolated, turgid cells, which are covered withcaveolaeon their cell surface.[5]The angle between these fibers determines whether increased pressure in the core will result in shortening and thickening versus lengthening and thinning.[6]

Alternating contraction ofmuscle fibersattached to each side of the notochord result in a side-to-side motion resemblingstern sculling,which allowslocomotion.The stiffened notochord prevents movement throughtelescoping motionsuch as that of anearthworm.[7]

Role in signaling and development

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The notochord plays a key role in signaling and coordinating development.Embryosof modern vertebrates form transient notochord structures duringgastrulation.The notochord is foundventralto theneural tube.

Notogenesisis the development of the notochord byepiblaststhat form the floor of theamnioncavity.[8]The progenitor notochord is derived from cells migrating from theprimitive nodeand pit.[9]The notochord forms duringgastrulationand soon after induces the formation of theneural plate(neurulation), synchronizing the development of theneural tube.On theventralaspect of the neural groove, an axial thickening of theendodermtakes place. (In bipedal chordates, e.g. humans, this surface is properly referred to as theanteriorsurface). 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]

In vertebrates, it extends throughout the entire length of the future vertebral column, and reaches as far as the anterior end of themidbrain,where it ends in a hook-like extremity in the region of the futuredorsum sellaeof thesphenoid bone.Initially, it exists between the neural tube and the endoderm of the yolk-sac; soon, the notochord becomes separated from them by themesoderm,which grows medially and surrounds it. From the mesoderm surrounding the neural tube and notochord, theskull,vertebral column, and themembranesof thebrainandmedulla spinalisare developed.[10]Because it originates from the primitive node and is ultimately positioned with the mesodermal space, it is considered to be derived from mesoderm.[11]

A postembryonic vestige of the notochord is found in thenucleus pulposusof the intervertebral discs. Isolated notochordal remnants may escape their lineage-specific destination in the nucleus pulposus and instead attach to the outer surfaces of thevertebral bodies,from which notochordal cells largely regress.[12]

In amphibians and fish

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During development of amphibians and fish, the notochord induces development of thehypochordthrough secretion ofvascular endothelial growth factor.The hypochord is a transient structure ventral to the notochord, and is primarily responsible for correct development of the dorsal aorta.[13]

Notochord flexion,when the notochord bends to form a part of the developing caudal fin, is a hallmark of an early growth stage of some fish.[14][15][better source needed]

In humans

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By the age of 4, all notochord residue is replaced by a population ofchondrocyte-like cells of unclear origin.[16]Persistence of notochordal cells within the vertebra may cause a pathologic condition:persistent notochordal canal.[17]If the notochord and the nasopharynx do not separate properly during embryonic development, a depression (Tornwaldt bursa) orTornwaldt cystmay form.[18]The cells are the likely precursors to a rare cancer calledchordoma.[19]

Neurology

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Research into the notochord has played a key role in understanding the development of thecentral nervous system.By transplanting and expressing a second notochord near thedorsalneural tube, 180degreesopposite of the normal notochord location, one can induce the formation ofmotor neuronsin the dorsal tube. Motor neuron formation generally occurs in the ventral neural tube, while the dorsal tube generally forms sensorycells.[20]

The notochord secretes a protein calledsonic hedgehog(SHH), a keymorphogenregulatingorganogenesisand having a critical role in signaling the development of motor neurons.[21]The secretion of SHH by the notochord establishes the ventral pole of the dorsal-ventral axis in the developing embryo.

Evolution in chordates

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A dissectedspotted African lungfishshowing the notochord

The notochord is the defining feature (synapomorphy) ofchordates,and was present throughout life in many of the earliest chordates. Although thestomochordofhemichordateswas once thought to be homologous or from a common lineal origin, it is now viewed as analogous,convergent,or from a different lineal origin.[22]Pikaiaappears to have a proto-notochord, and notochords are present in severalbasal chordatessuch asHaikouella,Haikouichthys,andMyllokunmingia,all from theCambrian.

TheOrdovicianoceans included many diverse species ofAgnathaand earlyGnathostomatawhich possessed notochords, either with attached bony elements or without, most notably theconodonts,[23]placoderms,[24]andostracoderms.Even after the evolution of the vertebral column inchondrichthyesandosteichthyes,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.

Scenarios for the evolutionary origin of the notochord were comprehensively reviewed by Annona, Holland, and D'Aniello (2015).[25]They point out that, although many of these ideas have not been well supported by advances in molecular phylogenetics and developmental genetics, two of them have actually been revived under the stimulus of modern molecular approaches (the first proposes that the notochord evolvedde novoin chordates, and the second derives it from a homologous structure, the axochord, that was present in annelid-like ancestors of the chordates). Deciding between these two scenarios (or possibly another yet to be proposed) should be facilitated by much more thorough studies of gene regulatory networks in a wide spectrum of animals.

Post-embryonic retention

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In most vertebrates, the notochord develops into secondary structures. In otherchordates,the notochord is retained as an essential anatomical structure. The evolution of the notochord within the phylum Chordata is considered in detail by Holland and Somorjai (2020). Vertebrates now have spines so they do not need a notochord.[26]

The following organisms retain a post-embryonic notochord:

Within lancelets

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The notochord of thelancelet(amphioxus) protrudes beyond the anterior end of the neural tube. This projection serves a second purpose in allowing the animal to burrow within the sediment of shallow waters. There, amphioxus is a filter feeder and spends most of its life partially submerged within the sediment.[7]

Additional images

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  • 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

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  1. ^Schifferl, D., Scholze-Wittler, M., Villaronga Luque, A., Pustet, M., Wittler, L., Veenvliet, J. V., Koch, F., & Herrmann, B. G. (2023). Genome-wide identification of notochord enhancers comprising the regulatory landscape of the brachyury locus in mouse. Development (Cambridge, England), 150(22).https://doi.org/10.1242/dev.202111
  2. ^Wang, F., Zhang, C., Shi, R., Xie, Z.-Y., Chen, L., Wang, K., Wang, Y.-T., Xie, X.-H., & Wu, X.-T. (2018). The embryonic and evolutionary boundaries between notochord and cartilage: A new look at nucleus pulposus-specific markers.Osteoarthritis and Cartilage,26(10), 1274–1282.https://doi.org/10.1016/j.joca.2018.05.022
  3. ^Krämer, Jürgen (2009).Intervertebral Disk Diseases: Causes, Diagnosis, Treatment, and Prophylaxis.Thieme. pp.15–17.ISBN978-3-13-582403-1.
  4. ^Stemple, Derek L. (2005-06-01)."Structure and function of the notochord: an essential organ for chordate".Development.132(11):2503–2512.doi:10.1242/dev.01812.ISSN0950-1991.PMID15890825.
  5. ^Lim, Ye-Wheen; Lo, Harriet P.; Hall, Thomas E.; Parton, Robert G. (2020), "Live Confocal Imaging of Zebrafish Notochord Cells Under Mechanical Stress in Vivo",Caveolae,Methods in Molecular Biology, vol. 2169, New York, NY: Springer US, pp.175–187,doi:10.1007/978-1-0716-0732-9_16,ISBN978-1-0716-0731-2,PMID32548829,S2CID219725868,retrieved2023-01-14
  6. ^M. A. R. Koehl(2000)."Mechanical Design of Fiber-Wound Hydraulic Skeletons: The Stiffening and Straightening of Embryonic Notochords".American Zoologist.40:28–041.doi:10.1093/icb/40.1.28.
  7. ^abHomberger, Dominique G. (2004).Vertebrate dissection.Walker, Warren F. (Warren Franklin), Walker, Warren F. (Warren Franklin). (9th ed.). Belmont, CA: Thomson Brooks/Cole.ISBN0-03-022522-1.OCLC53074665.
  8. ^"The trilaminar germ disk (3rd week)".www.embryology.ch.Archived fromthe originalon 2017-05-31.Retrieved2012-01-09.
  9. ^Hood, Rousseaux, Blakley, Ronald D., Colin G., Patricia M. (29 May 2007). "Embryo and Fetus".Handbook of Toxicologic Pathology (Second Edition).2.Academic Press, Published by Elsevier Inc:895–936.doi:10.1016/b978-0-12-330215-1.50047-8.ISBN9780123302151.{{cite journal}}:CS1 maint: multiple names: authors list (link)
  10. ^Henry Gray (1918).Anatomy of the Human Body.Lea & Febiger. pp.52–54.
  11. ^Gary C. Schoenwolf; Steven B. Bleyl; Philip R. Brauer; Philippa H. Francis-West (1 December 2014).Larsen's Human Embryology E-Book.Elsevier Health Sciences. pp.71–72.ISBN978-1-4557-2791-9.
  12. ^Choi, K.; Cohn, Martin J.; Harfe, Brian D. (2009)."Identification of Nucleus Pulposus Precursor Cells and Notochordal Remnants in the Mouse: Implications for Disk Degeneration and Chordoma Formation".Developmental Dynamics.237(12):3953–3958.doi:10.1002/dvdy.21805.PMC2646501.PMID19035356.
  13. ^Cleaver, Ondine (2000)."Endoderm patterning by the notochord: Development of the hypochord in Xenopus"(PDF).Development.127(4):869–979.doi:10.1242/dev.127.4.869.PMID10648245.
  14. ^ Paxton, John R.; Johnson, G. David; Trnski, Thomas (2001)."Larvae and juveniles of the deepsea" whalefishes "BarbourisiaandRondeletia(Stephanoberyciformes: Barbourisiidae, Rondeletiidae), with comments on family relationships "(PDF).Records of the Australian Museum.53(3):407–425.doi:10.3853/j.0067-1975.53.2001.1352.Archived fromthe original(PDF)on 2003-09-26.
  15. ^ "Brownsnout spookfish"(PDF).Ichthyoplankton Information System. Alaska Fisheries Science Center.National Oceanographic and Atmospheric Administration.July 2008.Retrieved14 March2009.
  16. ^Urban, J. P. G. (2000)."The Nucleus of the Intervertebral Disc from Development to Degeneration".Integrative and Comparative Biology.40:53–061.doi:10.1093/icb/40.1.53.
  17. ^Christopherson, Lr; Rabin, Bm; Hallam, Dk; Russell, Ej (1 January 1999)."Persistence of the notochordal canal: MR and plain film appearance"(Free full text).American Journal of Neuroradiology.20(1):33–6.ISSN0195-6108.PMID9974055.
  18. ^Moody MW, Chi DH, Chi DM, Mason JC, Phillips CD, Gross CW, et al. (2007)."Tornwaldt's cyst: incidence and a case report".Ear Nose Throat J.86(1):45–7, 52.doi:10.1177/014556130708600117.PMID17315835.
  19. ^Pillai S, Govender S (2018)."Sacral chordoma: A review of literature".J Orthop.15(2):679–684.doi:10.1016/j.jor.2018.04.001.PMC5990241.PMID29881220.
  20. ^Wilson, Leigh; Maden, Malcolm (2005)."The mechanisms of dorsoventral patterning in the vertebrate neural tube".Developmental Biology.282(1). Elsevier BV:1–13.doi:10.1016/j.ydbio.2005.02.027.ISSN0012-1606.PMID15936325.
  21. ^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.PMID7916661.S2CID6732599.
  22. ^Kardong, Kenneth V. (1995).Vertebrates: comparative anatomy, function, evolution.McGraw-Hill. pp. 55, 57.ISBN978-0-697-21991-6.
  23. ^"Palaeos Vertebrates 30.000 Conodonta: Overview".Archived fromthe originalon 2006-03-13.Retrieved2007-09-05.
  24. ^"Palaeos Vertebrates 60.000 Placoderm Overview".Archived fromthe originalon 20 December 2010.Retrieved21 November2009.
  25. ^Annona, G.; Holland, N.D.; D'Aniello, S. (2015)."Evolution of the notochord".EvoDevo.6.article 30.doi:10.1186/s13227-015-0025-3.PMC4595329.PMID26446368.
  26. ^Holland, N. D.; Somorjai, I. M. L. (2020)."Serial blockface SEM suggests that stem cells may participate in adult notochord growth in an invertebrate chordate, the Bahamas lancelet".EvoDevo.11.article 22.doi:10.1186/s13227-020-00167-6.PMC7568382.PMID33088474.
  27. ^Joseph J. Luczkovich; Philip J. Motta; Stephen F. Norton; Karel F. Liem (17 April 2013).Ecomorphology of fishes.Springer Science & Business Media. p. 201.ISBN978-94-017-1356-6.
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