Editing Tendon (section)

==== 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>