Wikipedia

Skeleton

This article is about skeletons in general. For the human skeleton, seeHuman skeleton.For other uses, seeSkeleton (disambiguation).

Askeletonis thestructural framethat supports the body of mostanimals.There are several types of skeletons, including theexoskeleton,which is a rigid outer shell that holds up an organism's shape; theendoskeleton,a rigid internal frame to which theorgansandsoft tissuesattach; and thehydroskeleton,a flexible internal structure supported by thehydrostatic pressureofbody fluids.

Skeleton
Ahorseandhuman skeletonplaced in a display atAustralian Museumin Sydney
Details
Identifiers
Greekσκελετός
MeSHD012863
Anatomical terminology

Vertebratesare animals with an endoskeleton centered around anaxialvertebral column,and their skeletons are typically composed ofbonesandcartilages.Invertebratesare other animals that lack a vertebral column, and their skeletons vary, including hard-shelled exoskeleton (arthropodsand mostmolluscs), plated internal shells (e.g.cuttlebonesin somecephalopods) or rods (e.g.ossiclesinechinoderms), hydrostatically supported bodycavities(most), andspicules(sponges). Cartilage is a rigidconnective tissuethat is found in the skeletal systems of vertebrates and invertebrates.

Etymology

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The termskeletoncomes fromAncient Greekσκελετός(skeletós)'dried up'.[1]Sceletonis an archaic form of the word.[2]

Classification

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Skeletons can be defined by several attributes. Solid skeletons consist of hard substances, such asbone,cartilage,orcuticle.These can be further divided by location; internal skeletons are endoskeletons, and external skeletons are exoskeletons. Skeletons may also be defined by rigidity, where pliant skeletons are more elastic than rigid skeletons.[3]Fluid orhydrostatic skeletonsdo not have hard structures like solid skeletons, instead functioning via pressurized fluids. Hydrostatic skeletons are always internal.[4]

Exoskeletons

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Main article:Exoskeleton
Exoskeletonof anant

An exoskeleton is an external skeleton that covers the body of an animal, serving as armor to protect an animal from predators. Arthropods have exoskeletons that encase their bodies, and have to undergo periodicmoultingorecdysisas the animals grow. Theshellsofmolluscsare another form of exoskeleton.[4]Exoskeletons provide surfaces for the attachment of muscles, and specialized appendanges of the exoskeleton can assist with movement and defense. In arthropods, the exoskeleton also assists withsensory perception.[5]

An external skeleton can be quite heavy in relation to the overall mass of an animal, so on land, organisms that have an exoskeleton are mostly relatively small. Somewhat larger aquatic animals can support an exoskeleton because weight is less of a consideration underwater. Thesouthern giant clam,a species of extremely large saltwater clam in thePacific Ocean,has a shell that is massive in both size and weight.Syrinx aruanusis a species of sea snail with a very large shell.

Endoskeletons

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Main article:Endoskeleton
Endoskeletonof abat

Endoskeletons are the internal support structure of an animal, composed ofmineralized tissues,such as the bone skeletons found in most vertebrates.[6]Endoskeletons are highly specialized and vary significantly between animals.[4]They vary in complexity from functioning purely for support (as in the case ofsponges), to serving as an attachment site for muscles and a mechanism for transmitting muscular forces. A true endoskeleton is derived frommesodermaltissue. Endoskeletons occur inchordates,echinoderms, and sponges.

Rigidity

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Pliant skeletons are capable of movement; thus, whenstressis applied to the skeletal structure, it deforms and then regains its original shape. This skeletal structure is used in some invertebrates, for instance in the hinge ofbivalve shellsor themesogleaofcnidarianssuch asjellyfish.Pliant skeletons are beneficial because onlymusclecontractions are needed to bend the skeleton; upon muscle relaxation, the skeleton will return to its original shape.Cartilageis one material that a pliant skeleton may be composed of, but most pliant skeletons are formed from a mixture ofproteins,polysaccharides,and water.[3]For additional structure or protection, pliant skeletons may be supported by rigid skeletons. Organisms that have pliant skeletons typically live in water, which supports body structure in the absence of a rigid skeleton.[7]

Rigid skeletons are not capable of movement when stressed, creating a strong support system most common interrestrial animals.Such a skeleton type used by animals that live in water are more for protection (such asbarnacleandsnailshells) or for fast-moving animals that require additional support of musculature needed for swimming through water. Rigid skeletons are formed from materials includingchitin(in arthropods),calciumcompounds such ascalcium carbonate(instony coralsandmollusks) andsilicate(fordiatomsandradiolarians).

Hydrostatic skeletons

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Main article:Hydrostatic skeleton

Hydrostatic skeletons are flexible cavities within an animal that provide structure through fluid pressure, occurring in some types ofsoft-bodied organisms,including jellyfish,flatworms,nematodes,and earthworms. The walls of these cavities are made of muscle and connective tissue.[4]In addition to providing structure for an animal's body, hydrostatic skeletons transmit the forces of muscle contraction, allowing an animal to move by alternating contractions and expansions of muscles along the animal's length.[8]

Cytoskeleton

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Main article:Cytoskeleton

The cytoskeleton (cyto-meaning 'cell'[9]) is used to stabilize and preserve the form of the cells. It is a dynamic structure that maintains cell shape, protects the cell, enables cellular motion using structures such asflagella,ciliaandlamellipodia,and transport within cells such as the movement ofvesiclesandorganelles,and plays a role in cellular division. The cytoskeleton is not a skeleton in the sense that it provides the structural system for the body of an animal; rather, it serves a similar function at the cellular level.[10]

Vertebrate skeletons

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Pithecometra:FromThomas Huxley's 1863Evidence as to Man's Place in Nature,the compared skeletons of apes to humans.

Vertebrate skeletons are endoskeletons, and the main skeletal component is bone.[6]Bones compose a unique skeletal system for each type of animal. Another important component is cartilage which inmammalsis found mainly in the joint areas. In other animals, such as thecartilaginous fishes,which include thesharks,the skeleton is composed entirely ofcartilage.Thesegmentalpattern of the skeleton is present in all vertebrates, with basic units being repeated, such as in the vertebral column and the ribcage.[11][12]

Bones are rigidorgansproviding structural support for the body, assistance in movement by opposingmuscle contraction,and the forming of a protective wall around internal organs. Bones are primarily made of inorganic minerals, such ashydroxyapatite,while the remainder is made of an organic matrix and water. The hollow tubular structure of bones provide considerable resistance against compression while staying lightweight. Most cells in bones areosteoblasts,osteoclasts,orosteocytes.[13]

Bone tissue is a type of denseconnective tissue,a type ofmineralized tissuethat gives rigidity and ahoneycomb-likethree-dimensional internal structure. Bones also produceredandwhite blood cellsand serve as calcium and phosphate storage at the cellular level. Other types of tissue found in bones includebone marrow,endosteumandperiosteum,nerves,blood vesselsand cartilage.

Duringembryonic development,bones are developed individually from skeletogenic cells in the ectoderm and mesoderm. Most of these cells develop into separate bone, cartilage, and joint cells, and they are then articulated with one another. Specialized skeletal tissues are unique to vertebrates. Cartilage grows more quickly than bone, causing it to be more prominent earlier in an animal's life before it is overtaken by bone.[14]Cartilage is also used in vertebrates to resist stress at points of articulation in the skeleton. Cartilage in vertebrates is usually encased inperichondriumtissue.[15]Ligamentsare elastic tissues that connect bones to other bones, andtendonsare elastic tissues that connect muscles to bones.[16]

Amphibians and reptiles

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The skeletons of turtles have evolved to develop ashellfrom the ribcage, forming an exoskeleton.[17]The skeletons ofsnakesandcaecilianshave significantly more vertebrae than other animals. Snakes often have over 300, compared to the 65 that is typical in lizards.[18]

Birds

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Further information:Bird anatomy § Skeletal system

The skeletons of birds are adapted forflight.The bones in bird skeletons are hollow and lightweight to reduce the metabolic cost of flight. Several attributes of the shape and structure of the bones are optimized to endure the physical stress associated with flight, including a round and thinhumeral shaftand the fusion of skeletal elements into singleossifications.[19]Because of this, birds usually have a smaller number of bones than other terrestrial vertebrates. Birds also lack teeth or even a truejaw,instead having evolved abeak,which is far more lightweight. The beaks of many baby birds have a projection called anegg tooth,which facilitates their exit from the amniotic egg.

Fish

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Further information:Fish anatomy § Skeleton,andFish bone

The skeleton, which forms the support structure inside the fish is either made of cartilage as in theChondrichthyes,or bones as in theOsteichthyes.The main skeletal element is the vertebral column, composed of articulating vertebrae which are lightweight yet strong. The ribs attach to the spine and there are no limbs or limb girdles. They are supported only by the muscles. The main external features of the fish, thefins,are composed of either bony or soft spines called rays which, with the exception of the caudal fin (tail fin), have no direct connection with the spine. They are supported by the muscles which compose the main part of the trunk.

Cartilaginous fish, such as sharks, rays, skates, and chimeras, have skeletons made entirely of cartilage. The lighter weight of cartilage allows these fish to expend less energy when swimming.[4]

Mammals

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Marine mammals

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Californian sea lion

To facilitate the movement ofmarine mammalsin water, the hind legs were either lost altogether, as in the whales andmanatees,or united in a singletail finas in thepinnipeds(seals). In the whale, thecervical vertebraeare typically fused, an adaptation trading flexibility for stability during swimming.[20]

Humans

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Main article:Human skeleton
Study of Skeletons,c. 1510,byLeonardo da Vinci

The skeleton consists of both fused and individual bones supported and supplemented by ligaments, tendons,musclesand cartilage. It serves as a scaffold which supports organs, anchors muscles, and protects organs such as the brain,lungs,heartandspinal cord.[21]The biggest bone in the body is thefemurin the upper leg, and the smallest is thestapesbone in themiddle ear.In an adult, the skeleton comprises around 13.1% of the total body weight,[22]and half of this weight is water.

Fused bones include those of thepelvisand thecranium.Not all bones are interconnected directly: There are three bones in eachmiddle earcalled theossiclesthat articulate only with each other. Thehyoid bone,which is located in the neck and serves as the point of attachment for thetongue,does not articulate with any other bones in the body, being supported by muscles and ligaments.

There are 206 bones in the adult human skeleton, although this number depends on whether the pelvic bones (thehip boneson each side) are counted as one or three bones on each side (ilium, ischium, and pubis), whether the coccyx or tail bone is counted as one or four separate bones, and does not count the variablewormian bonesbetween skull sutures. Similarly, the sacrum is usually counted as a single bone, rather than five fused vertebrae. There is also a variable number of small sesamoid bones, commonly found in tendons. The patella or kneecap on each side is an example of a larger sesamoid bone. The patellae are counted in the total, as they are constant. The number of bones varies between individuals and with age – newborn babies have over 270 bones some of which fuse together.[citation needed]These bones are organized into a longitudinal axis, theaxial skeleton,to which theappendicular skeletonis attached.[23]

The human skeleton takes 20 years before it is fully developed, and the bones containmarrow,which produces blood cells.

There exist several general differences between the male and female skeletons. The male skeleton, for example, is generally larger and heavier than the female skeleton. In the female skeleton, the bones of the skull are generally less angular. The female skeleton also has wider and shorter breastbone and slimmer wrists. There exist significant differences between the male and female pelvis which are related to the female's pregnancy and childbirth capabilities. The female pelvis is wider and shallower than the male pelvis. Female pelvises also have an enlarged pelvic outlet and a wider and more circular pelvic inlet. The angle between the pubic bones is known to be sharper in males, which results in a more circular, narrower, and near heart-shaped pelvis.[24][25]

Invertebrate skeletons

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Invertebrates are defined by a lack of vertebral column, and they do not have bone skeletons. Arthropods have exoskeletons and echinoderms have endoskeletons. Some soft-bodied organisms, such asjellyfishandearthworms,have hydrostatic skeletons.[26]

Arthropods

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Main article:Arthropod exoskeleton

The skeletons ofarthropods,includinginsects,crustaceans,andarachnids,are cuticle exoskeletons. They are composed ofchitinsecreted by theepidermis.[27]The cuticle covers the animal's body and lines several internal organs, including parts of the digestive system. Arthropods molt as they grow through a process ofecdysis,developing a new exoskeleton, digesting part of the previous skeleton, and leaving theremainderbehind. An arthropod's skeleton serves many functions, working as anintegumentto provide a barrier and support the body, providing appendages for movement and defense, and assisting in sensory perception. Some arthropods, such as crustaceans, absorb biominerals like calcium carbonate from the environment to strengthen the cuticle.[5]

Echinoderms

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The skeletons ofechinoderms,such asstarfishandsea urchins,are endoskeletons that consist of large, well-developedscleriteplates that adjoin or overlap to cover the animal's body. The skeletons ofsea cucumbersare an exception, having a reduced size to assist in feeding and movement. Echinoderm skeletons are composed ofstereom,made up ofcalcitewith amonocrystalstructure. They also have a significantmagnesiumcontent, forming up to 15% of the skeleton's composition. The stereome structure is porous, and the pores fill with connectivestromaltissue as the animal ages. Sea urchins have as many as ten variants of stereome structure. Among extant animals, such skeletons are unique to echinoderms, though similar skeletons were used by somePaleozoicanimals.[28]The skeletons of echinoderms aremesodermal,as they are mostly encased by soft tissue. Plates of the skeleton may be interlocked or connected through muscles and ligaments. Skeletal elements in echinoderms are highly specialized and take many forms, though they usually retain some form of symmetry. The spines of sea urchins are the largest type of echinoderm skeletal structure.[29]

Molluscs

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Some molluscs, such as conchs, scallops, and snails, have shells that serve as exoskeletons. They are produced by proteins and minerals secreted from the animal'smantle.[4]

Sponges

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The skeleton ofspongesconsists of microscopiccalcareousorsiliceousspicules.Thedemospongesinclude 90% of all species of sponges. Their "skeletons" are made of spicules consisting of fibers of the proteinspongin,the mineralsilica,or both. Where spicules of silica are present, they have a different shape from those in the otherwise similarglass sponges.[30]

Cartilage

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Main article:Cartilage

Cartilage is a connective skeletal tissue composed of specialized cells calledchondrocytesthat in anextracellular matrix.This matrix is typically composed of Type IIcollagenfibers,proteoglycans,and water. There are many types of cartilage, includingelastic cartilage,hyaline cartilage,fibrocartilage,and lipohyaline cartilage.[15]Unlike other connective tissues, cartilage does not contain blood vessels. The chondrocytes are supplied by diffusion, helped by the pumping action generated by compression of the articular cartilage or flexion of the elastic cartilage. Thus, compared to other connective tissues, cartilage grows and repairs more slowly.

See also

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References

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  1. ^"skeleton".Mish 2003,p. 1167.
  2. ^"Definition of SCELETON".merriam-webster.Retrieved31 July2022.
  3. ^abRuppert, Fox & Barnes 2003,p. 102.
  4. ^abcdef"Why animals developed four types of skeletons".National Geographic.19 October 2021. Archived fromthe originalon 19 October 2021.Retrieved31 July2022.
  5. ^abPoliti, Yael; Bar-On, Benny; Fabritius, Helge-Otto (2019), Estrin, Yuri; Bréchet, Yves; Dunlop, John; Fratzl, Peter (eds.),"Mechanics of Arthropod Cuticle-Versatility by Structural and Compositional Variation",Architectured Materials in Nature and Engineering: Archimats,Springer Series in Materials Science, vol. 282, Cham: Springer International Publishing, pp.287–327,doi:10.1007/978-3-030-11942-3_10,ISBN978-3-030-11942-3,S2CID109418804
  6. ^abde Buffrénil, Vivian; de Ricqlès, Armand J; Zylberberg, Louise; Padian, Kevin; Laurin, Michel; Quilhac, Alexandra (2021).Vertebrate skeletal histology and paleohistology.Boca Raton, FL: CRC Press. pp. xii + 825.ISBN978-1351189576.
  7. ^Pechenik 2015.[page needed]
  8. ^Kier, William M. (15 April 2012)."The diversity of hydrostatic skeletons".Journal of Experimental Biology.215(8):1247–1257.doi:10.1242/jeb.056549.ISSN0022-0949.PMID22442361.S2CID1177498.
  9. ^"cyt-orcyto- ".Mish 2003,p. 312.
  10. ^Fletcher, Daniel A.; Mullins, R. Dyche (2010)."Cell mechanics and the cytoskeleton".Nature.463(7280):485–492.Bibcode:2010Natur.463..485F.doi:10.1038/nature08908.ISSN1476-4687.PMC2851742.PMID20110992.
  11. ^Billet, Guillaume; Bardin, Jérémie (13 October 2021)."Segmental Series and Size: Clade-Wide Investigation of Molar Proportions Reveals a Major Evolutionary Allometry in the Dentition of Placental Mammals".Systematic Biology.70(6):1101–1109.doi:10.1093/sysbio/syab007.PMID33560370.
  12. ^Buffrénil, Vivian de; Quilhac, Alexandra (2021)."An Overview of the Embryonic Development of the Bony Skeleton".Vertebrate Skeletal Histology and Paleohistology.CRC Press:29–38.doi:10.1201/9781351189590-2.ISBN978-1351189590.S2CID236422314.
  13. ^Sommerfeldt, D.; Rubin, C. (1 October 2001)."Biology of bone and how it orchestrates the form and function of the skeleton".European Spine Journal.10(2):S86 –S95.doi:10.1007/s005860100283.ISSN1432-0932.PMC3611544.PMID11716022.
  14. ^Lefebvre, Véronique; Bhattaram, Pallavi (1 January 2010), Koopman, Peter (ed.), "Chapter Eight - Vertebrate Skeletogenesis",Current Topics in Developmental Biology,Organogenesis in Development,90,Academic Press:291–317,doi:10.1016/S0070-2153(10)90008-2,PMC3077680,PMID20691853
  15. ^abGillis, J. Andrew (2019),"The Development and Evolution of Cartilage",Reference Module in Life Sciences,Elsevier,ISBN978-0-12-809633-8
  16. ^Vorvick, Linda J. (13 August 2020)."Tendon vs. Ligament".A.D.A.M. Medical Encyclopedia.Ebix.Retrieved6 August2021– via MedLinePlus.
  17. ^Nagashima, Hiroshi; Kuraku, Shigehiro; Uchida, Katsuhisa; Kawashima-Ohya, Yoshie; Narita, Yuichi; Kuratani, Shigeru (1 March 2012)."Body plan of turtles: an anatomical, developmental and evolutionary perspective".Anatomical Science International.87(1):1–13.doi:10.1007/s12565-011-0121-y.ISSN1447-073X.PMID22131042.S2CID41803725.
  18. ^M. Woltering, Joost (1 June 2012)."From Lizard to Snake; Behind the Evolution of an Extreme Body Plan".Current Genomics.13(4):289–299.doi:10.2174/138920212800793302.PMC3394116.PMID23204918.
  19. ^Dumont, Elizabeth R. (22 July 2010)."Bone density and the lightweight skeletons of birds".Proceedings of the Royal Society B: Biological Sciences.277(1691):2193–2198.doi:10.1098/rspb.2010.0117.PMC2880151.PMID20236981.
  20. ^Bebej, Ryan M; Smith, Kathlyn M (17 March 2018)."Lumbar mobility in archaeocetes (Mammalia: Cetacea) and the evolution of aquatic locomotion in the earliest whales".Zoological Journal of the Linnean Society.182(3):695–721.doi:10.1093/zoolinnean/zlx058.ISSN0024-4082.Retrieved7 March2022.
  21. ^"Skeletal System: Facts, Function & Diseases".Live Science.Archivedfrom the original on 7 March 2017.Retrieved7 March2017.
  22. ^Reynolds & Karlotski 1977,p. 161
  23. ^Tözeren 2000,pp. 6–10.
  24. ^Balaban 2008,p. 61
  25. ^Stein 2007,p. 73.
  26. ^Langley, Liz (19 October 2021)."Why animals developed four types of skeletons".National Geographic. Archived fromthe originalon 19 October 2021.Retrieved1 November2022.
  27. ^Vincent, Julian F. V. (1 October 2002)."Arthropod cuticle: a natural composite shell system".Composites Part A: Applied Science and Manufacturing.33(10):1311–1315.doi:10.1016/S1359-835X(02)00167-7.ISSN1359-835X.
  28. ^Kokorin, A. I.; Mirantsev, G. V.; Rozhnov, S. V. (1 December 2014)."General features of echinoderm skeleton formation".Paleontological Journal.48(14):1532–1539.doi:10.1134/S0031030114140056.ISSN1555-6174.S2CID84336543.
  29. ^Nebelsick, James H.; Dynowski, Janina F.; Grossmann, Jan Nils; Tötzke, Christian (2015), Hamm, Christian (ed.),"Echinoderms: Hierarchically Organized Light Weight Skeletons",Evolution of Lightweight Structures: Analyses and Technical Applications,Biologically-Inspired Systems, vol. 6, Dordrecht: Springer Netherlands, pp.141–155,doi:10.1007/978-94-017-9398-8_8,ISBN978-94-017-9398-8,retrieved31 July2022
  30. ^Barnes, Fox & Barnes 2003,pp. 105–106.

Bibliography

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Media related toSkeletonsat Wikimedia Commons

The WikibookAnatomy and Physiology of Animalshas a page on the topic of:The Skeleton
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