Editing Tendon (section)

=== Extracellular matrix ===
The dry mass of normal tendons, which is 30–45% of their total mass, is made of:
* 60–85% collagen
** 60–80% collagen I
** 0–10% collagen III
** 2% collagen IV
** small amounts of collagens V, VI, and others
* 15–40% non-collagenous extracellular matrix components, including:
** 3% [[cartilage oligomeric matrix protein]],
** 1–2% [[elastin]],
** 1–5% [[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 = 865–877 | 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 = 649–698 | 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 = 23–35 | 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 = 65–73 | 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&nbsp;nm long and 1–2&nbsp;nm wide, and the diameter of the fibrils that are formed can range from 50–500&nbsp;nm. In tendons, the fibrils then assemble further to form fascicles, which are about 10&nbsp;mm in length with a diameter of 50–300 μm, and finally into a tendon fibre with a diameter of 100–500 μm.<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 = 32–39 | 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 = 1436–1449 | 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 = 37–43 | 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 = 573–581 | 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 = 335–343 | date = December 2003 | pmid = 12923209 | pmc = 2343561 | doi = 10.1113/jphysiol.2003.050179 }}</ref>