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Tendon
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== Structure == A tendon is made of [[dense regular connective tissue]], whose main cellular components are special [[fibroblast]]s called [[tendon cell]]s (tenocytes).<ref name="pmid31768046">{{cite journal | vauthors = Harvey T, Flamenco S, Fan CM | title = A Tppp3+Pdgfra+ tendon stem cell population contributes to regeneration and reveals a shared role for PDGF signalling in regeneration and fibrosis | journal = Nature Cell Biology | volume = 21 | issue = 12 | pages = 1490β1503 | date = December 2019 | pmid = 31768046 | pmc = 6895435 | doi = 10.1038/s41556-019-0417-z }}</ref> Tendon cells synthesize the tendon's [[extracellular matrix]], which abounds with densely-packed [[collagen fibers]]. The collagen fibers run parallel to each other and are grouped into fascicles. Each fascicle is bound by an [[endotendineum]], which is a delicate loose connective tissue containing thin collagen fibrils<ref>Dorlands Medical Dictionary, page 602</ref><ref>{{cite journal | vauthors = Caldini EG, Caldini N, De-Pasquale V, Strocchi R, Guizzardi S, Ruggeri A, Montes GS | title = Distribution of elastic system fibres in the rat tail tendon and its associated sheaths | journal = Acta Anatomica | volume = 139 | issue = 4 | pages = 341β348 | year = 1990 | pmid = 1706129 | doi = 10.1159/000147022 }}</ref> and elastic fibers.<ref>{{cite journal | vauthors = Grant TM, Thompson MS, Urban J, Yu J | title = Elastic fibres are broadly distributed in tendon and highly localized around tenocytes | journal = Journal of Anatomy | volume = 222 | issue = 6 | pages = 573β579 | date = June 2013 | pmid = 23587025 | pmc = 3666236 | doi = 10.1111/joa.12048 }}</ref> A set of fascicles is bound by an [[epitenon]], which is a sheath of [[dense irregular connective tissue]]. The whole tendon is enclosed by a [[fascia]]. The space between the fascia and the tendon tissue is filled with the [[paratenon]], a fatty [[Loose connective tissue|areolar tissue]].<ref>Dorlands Medical Dictionary 2012.Page 1382</ref> Normal healthy tendons are anchored to bone by [[Sharpey's fibres]]. === Extracellular matrix === The dry mass of normal tendons, which is 30β45% of their total mass, is made of: * 60β85% collagen ** 60β80% collagen I ** 0β10% collagen III ** 2% collagen IV ** small amounts of collagens V, VI, and others * 15β40% non-collagenous extracellular matrix components, including: ** 3% [[cartilage oligomeric matrix protein]], ** 1β2% [[elastin]], ** 1β5% [[proteoglycans]], ** 0.2% inorganic components such as [[copper]], [[manganese]], and [[calcium]].<ref name="Jozsa, L. 1997">{{cite book | vauthors = Jozsa L, Kannus P | title = Human Tendons: Anatomy, Physiology, and Pathology. | publisher = Human Kinetics | location = Champaign, IL | date = 1997 }}</ref><ref>{{cite journal | vauthors = Lin TW, Cardenas L, Soslowsky LJ | title = Biomechanics of tendon injury and repair | journal = Journal of Biomechanics | volume = 37 | issue = 6 | pages = 865β877 | date = June 2004 | pmid = 15111074 | doi = 10.1016/j.jbiomech.2003.11.005 }}</ref><ref>{{cite journal | vauthors = Kjaer M | title = Role of extracellular matrix in adaptation of tendon and skeletal muscle to mechanical loading | journal = Physiological Reviews | volume = 84 | issue = 2 | pages = 649β698 | date = April 2004 | pmid = 15044685 | doi = 10.1152/physrev.00031.2003 }}</ref><ref>{{cite journal | vauthors = Taye N, Karoulias SZ, Hubmacher D | title = The "other" 15-40%: The Role of Non-Collagenous Extracellular Matrix Proteins and Minor Collagens in Tendon | journal = Journal of Orthopaedic Research | volume = 38 | issue = 1 | pages = 23β35 | date = January 2020 | pmid = 31410892 | pmc = 6917864 | doi = 10.1002/jor.24440 }}</ref> Although most of a tendon's collagen is [[type I collagen]], many minor collagens are present that play vital roles in tendon development and function. These include type II collagen in the [[Cartilage|cartilaginous]] zones, type III collagen in the [[reticulin]] fibres of the vascular walls, type IX collagen, type IV collagen in the basement membranes of the [[capillaries]], type V collagen in the vascular walls, and type X collagen in the mineralized fibrocartilage near the interface with the bone.<ref name="Jozsa, L. 1997" /><ref>{{cite journal | vauthors = Fukuta S, Oyama M, Kavalkovich K, Fu FH, Niyibizi C | title = Identification of types II, IX and X collagens at the insertion site of the bovine achilles tendon | journal = Matrix Biology | volume = 17 | issue = 1 | pages = 65β73 | date = April 1998 | pmid = 9628253 | doi = 10.1016/S0945-053X(98)90125-1 }}</ref> ==== Ultrastructure and collagen synthesis ==== Collagen fibres coalesce into [[macroaggregate]]s. After secretion from the cell, cleaved by [[procollagen]] N- and C-[[protease]]s, the tropocollagen molecules spontaneously assemble into insoluble fibrils. A collagen molecule is about 300 nm long and 1β2 nm wide, and the diameter of the fibrils that are formed can range from 50β500 nm. In tendons, the fibrils then assemble further to form fascicles, which are about 10 mm in length with a diameter of 50β300 ΞΌm, and finally into a tendon fibre with a diameter of 100β500 ΞΌm.<ref>{{cite journal | vauthors = Fratzl P | title = Cellulose and collagen: from fibres to tissues. | journal = Current Opinion in Colloid & Interface Science | year = 2009 | volume = 8 | issue = 1 | pages = 32β39 | doi = 10.1016/S1359-0294(03)00011-6}}</ref> The collagen in tendons are held together with [[proteoglycan]] (a compound consisting of a protein bonded to glycosaminoglycan groups, present especially in connective tissue) components including [[decorin]] and, in compressed regions of tendon, [[aggrecan]], which are capable of binding to the collagen fibrils at specific locations.<ref>{{cite journal | vauthors = Zhang G, Ezura Y, Chervoneva I, Robinson PS, Beason DP, Carine ET, Soslowsky LJ, Iozzo RV, Birk DE | title = Decorin regulates assembly of collagen fibrils and acquisition of biomechanical properties during tendon development | journal = Journal of Cellular Biochemistry | volume = 98 | issue = 6 | pages = 1436β1449 | date = August 2006 | pmid = 16518859 | doi = 10.1002/jcb.20776 | s2cid = 39384363 }}</ref> The proteoglycans are interwoven with the collagen fibrils{{spaced ndash}} their [[glycosaminoglycan]] (GAG) side chains have multiple interactions with the surface of the fibrils{{spaced ndash}} showing that the proteoglycans are important structurally in the interconnection of the fibrils.<ref>{{cite journal | vauthors = Raspanti M, Congiu T, Guizzardi S | title = Structural aspects of the extracellular matrix of the tendon: an atomic force and scanning electron microscopy study | journal = Archives of Histology and Cytology | volume = 65 | issue = 1 | pages = 37β43 | date = March 2002 | pmid = 12002609 | doi = 10.1679/aohc.65.37 | doi-access = free }}</ref> The major GAG components of the tendon are [[dermatan sulfate]] and [[chondroitin sulfate]], which associate with collagen and are involved in the fibril assembly process during tendon development. Dermatan sulfate is thought to be responsible for forming associations between fibrils, while chondroitin sulfate is thought to be more involved with occupying volume between the fibrils to keep them separated and help withstand deformation.<ref>{{cite journal | vauthors = Scott JE, Orford CR, Hughes EW | title = Proteoglycan-collagen arrangements in developing rat tail tendon. An electron microscopical and biochemical investigation | journal = The Biochemical Journal | volume = 195 | issue = 3 | pages = 573β581 | date = June 1981 | pmid = 6459082 | pmc = 1162928 | doi = 10.1042/bj1950573 }}</ref> The dermatan sulfate side chains of decorin aggregate in solution, and this behavior can assist with the assembly of the collagen fibrils. When decorin molecules are bound to a collagen fibril, their dermatan sulfate chains may extend and associate with other dermatan sulfate chains on decorin that is bound to separate fibrils, therefore creating interfibrillar bridges and eventually causing parallel alignment of the fibrils.<ref>{{cite journal | vauthors = Scott JE | title = Elasticity in extracellular matrix 'shape modules' of tendon, cartilage, etc. A sliding proteoglycan-filament model | journal = The Journal of Physiology | volume = 553 | issue = Pt 2 | pages = 335β343 | date = December 2003 | pmid = 12923209 | pmc = 2343561 | doi = 10.1113/jphysiol.2003.050179 }}</ref> === Tenocytes === The [[tenocytes]] produce the collagen molecules, which aggregate end-to-end and side-to-side to produce collagen fibrils. Fibril bundles are organized to form fibres with the elongated tenocytes closely packed between them. There is a three-dimensional network of cell processes associated with collagen in the tendon. The cells communicate with each other through [[gap junctions]], and this signalling gives them the ability to detect and respond to mechanical loading.<ref>{{cite journal | vauthors = McNeilly CM, Banes AJ, Benjamin M, Ralphs JR | title = Tendon cells in vivo form a three dimensional network of cell processes linked by gap junctions | journal = Journal of Anatomy | volume = 189 | issue = Pt 3 | pages = 593β600 | date = December 1996 | pmid = 8982835 | pmc = 1167702 }}</ref> These communications happen by two proteins essentially: [[GJA1|connexin 43]], present where the cells processes meet and in cell bodies [[GJB1|connexin 32]], present only where the processes meet.<ref name="Benjamin_1997">{{cite journal | vauthors = Benjamin M, Ralphs JR | title = Tendons and ligaments--an overview | journal = Histology and Histopathology | volume = 12 | issue = 4 | pages = 1135β1144 | date = October 1997 | pmid = 9302572 | doi = 10.14670/HH-12.1135 | doi-broken-date = 1 November 2024 | url = https://www.hh.um.es/pdf/Vol_12/12_4/Tendons%20and%20ligaments%20-%20an%20overview.pdf }}</ref> Blood vessels may be visualized within the endotendon running parallel to collagen fibres, with occasional branching transverse [[anastomosis|anastomoses]]. The internal tendon bulk is thought to contain no nerve fibres, but the epitenon and paratenon contain nerve endings, while [[Golgi tendon organs]] are present at the [[myotendinous junction]] between tendon and muscle. Tendon length varies in all major groups and from person to person. Tendon length is, in practice, the deciding factor regarding actual and potential muscle size. For example, all other relevant biological factors being equal, a man with a shorter tendons and a longer biceps muscle will have greater potential for muscle mass than a man with a longer tendon and a shorter muscle. Successful [[Bodybuilding|bodybuilders]] will generally have shorter tendons. Conversely, in sports requiring athletes to excel in actions such as running or jumping, it is beneficial to have longer than average [[Achilles tendon]] and a shorter [[calf muscle]].<ref>{{cite web|title=Having a short Achilles tendon may be an athlete's Achilles heel | work = Sports Injury Bulletin |url=http://www.sportsinjurybulletin.com/archive/achilles-tendon.html|access-date=2007-10-26|archive-date=2007-10-21|archive-url=https://web.archive.org/web/20071021223203/http://www.sportsinjurybulletin.com/archive/achilles-tendon.html|url-status=dead}}</ref> Tendon length is determined by genetic predisposition, and has not been shown to either increase or decrease in response to environment, unlike muscles, which can be shortened by trauma, use imbalances and a lack of recovery and stretching.<ref>{{cite journal | vauthors = Young M | title = A review on postural realignment and its muscular and neural components. | journal = British Journal of Sports Medicine | date = 2002 | volume = 9 | issue = 12 | pages = 51β76 |url=http://www.elitetrack.com/article_files/posture.pdf |access-date=2010-06-23 |archive-date=2019-04-06 |archive-url=https://web.archive.org/web/20190406231332/http://www.elitetrack.com/article_files/posture.pdf |url-status=dead }}</ref> In addition tendons allow muscles to be at an optimal distance from the site where they actively engage in movement, passing through regions where space is premium, like the [[carpal tunnel]].<ref name="Benjamin_1997" />
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