Editing Synovial fluid (section)

==Structure==
The inner membrane of synovial joints is called the [[synovial membrane]] and secretes synovial fluid into the joints.<ref>{{Citation|last1=Bay-Jensen|first1=A. C.|title=Chapter 31 - Structural Biomarkers|date=2016-01-01|url=http://www.sciencedirect.com/science/article/pii/B9780128098479000313|work=Biochemistry of Collagens, Laminins and Elastin|pages=203–233|editor-last=Karsdal|editor-first=Morten A.|publisher=Academic Press|language=en|doi=10.1016/b978-0-12-809847-9.00031-3|isbn=978-0-12-809847-9|access-date=2020-10-18|last2=Sand|first2=J. M. B.|last3=Genovese|first3=F.|last4=Siebuhr|first4=A. S.|last5=Nielsen|first5=M. J.|last6=Leeming|first6=D. J.|last7=Manon-Jensen|first7=T.|last8=Karsdal|first8=M. A.|url-access=subscription}}</ref> Synovial fluid is an ultrafiltrate from blood, and contains proteins derived from the [[blood plasma]] and proteins that are
produced by cells within the joint tissues.<ref>{{Cite journal|title = A Normative Study of the Synovial Fluid Proteome from Healthy Porcine Knee Joints|journal = Journal of Proteome Research|date = 2014-09-03|pmc = 4184458|pmid = 25160569|pages = 4377–4387|volume = 13|issue = 10|doi = 10.1021/pr500587x|language = EN|first1 = Tue|last1 = Bennike|first2 = Ugur|last2 = Ayturk|first3 = Carla M.|last3 = Haslauer|first4 = John W.|last4 = Froehlich|first5 = Benedikt L.|last5 = Proffen|first6 = Omar|last6 = Barnaby|first7 = Svend|last7 = Birkelund|first8 = Martha M.|last8 = Murray|first9 = Matthew L.|last9 = Warman}}</ref> The fluid contains [[hyaluronan]] secreted by fibroblast-like cells in the synovial membrane, lubricin (proteoglycan 4; [[PRG4]]) secreted by the surface [[chondrocytes]] of the [[Wiktionary:articular|articular]] cartilage and interstitial fluid filtered from the [[blood plasma]].<ref>{{cite journal |vauthors = Jay GD, Waller KA|title = The biology of lubricin: near frictionless joint motion|journal = [[Matrix Biol.|Matrix Biology]]|volume = 39|pages = 17–24|year = 2014|pmid = 25172828|doi = 10.1016/j.matbio.2014.08.008 |doi-access=free}}</ref>  This fluid forms a thin layer (roughly 50 [[micrometre|μm]]) at the surface of cartilage and also seeps into microcavities and irregularities in the articular cartilage surface, filling all empty space.<ref name="edwards2000">{{cite web
 |url=http://www.ucl.ac.uk/~regfjxe/NORMALJOINT.htm 
 |title=Normal Joint Structure 
 |editor=Edwards, Jo 
 |year=2000 
 |work=Notes on Rheumatology 
 |publisher=[[University College London]] 
 |access-date=5 April 2013 
 |archive-url=https://web.archive.org/web/20121119080937/http://www.ucl.ac.uk/~regfjxe/NORMALJOINT.htm 
 |archive-date=19 November 2012 
 |url-status=dead 
}}</ref>  The fluid in [[articular]] cartilage effectively serves as a synovial fluid reserve. During movement, the synovial fluid held in the cartilage is squeezed out [[mechanics|mechanically]] to maintain a layer of fluid on the cartilage surface (so-called ''weeping lubrication'').
The functions of the synovial fluid include:

*reduction of friction — synovial fluid lubricates the articulating joints<ref>{{Cite book|title = New Atlas of Human Anatomy|publisher = MetroBooks|year = 2000|isbn = 9781586630973|location = China|oclc = 850877694|editor-last = McCracken|editor-first = Thomas}}</ref>{{Page needed|date=April 2013}}
*shock absorption — as a [[dilatant]] fluid, that possesses [[rheopectic]] properties,<ref>{{cite journal |author=Christorpher, GF |title=The role of protein content on the steady and oscillatory shear rheology of model synovial fluids |journal=  Soft Matter|volume=2014 |issue=10 |pages=5965–5973 |pmid=  24989639|doi=10.1039/C4SM00716F|date=2014-07-24 |bibcode=2014SMat...10.5965Z }}</ref> becoming more viscous under applied pressure; the synovial fluid in [[Wiktionary:diarthrosis|diarthrotic]] joints becomes thick the moment shear is applied in order to protect the joint and subsequently, thins to normal viscosity instantaneously to resume its lubricating function between shocks.<ref>{{cite web|url=http://www.orthopaedicsone.com/display/Main/Synovial%20fluid|title=Synovial fluid - OrthopaedicsOne Articles - OrthopaedicsOne|access-date=20 September 2016}}</ref> {{Dubious|date=December 2015}}
*nutrient and waste transportation — the fluid supplies oxygen and nutrients and removes carbon dioxide and metabolic wastes from the [[chondrocytes]] in the surrounding [[cartilage]]
*molecular sieving - pressure within the joint forces [[hyaluronan]] in the fluid against the synovial membrane forming a barrier against cells migrating into, or fluid migrating out of, the joint space. This function is dependent on the [[molecular weight]] of the [[hyaluronan]].<ref>{{cite journal |vauthors = Sabaratnam S, Arunan V, Coleman PJ, Mason RM, Levick JR|title = Size selectivity of hyaluronan molecular sieving by extracellular matrix in rabbit synovial joints|journal = [[J. Physiol.|The Journal of Physiology]]|volume = 567|issue = Pt 2|pages = 569–81|year = 2005|pmid = 15961430|doi = 10.1113/jphysiol.2005.088906 |doi-access=free |pmc=1474196}}</ref>

===Composition===
Synovial tissue is sterile and composed of vascularized connective tissue that lacks a basement membrane. Two cell types (type A and type B) are present: Type A is derived from blood monocytes, and it removes the wear-and-tear debris from the synovial fluid. Type B produces [[hyaluronan]]. Synovial fluid is made of [[hyaluronic acid]] and lubricin, proteinases, and collagenases. Synovial fluid exhibits [[non-Newtonian fluid|non-Newtonian flow]] characteristics; the viscosity coefficient is not a constant and the fluid is not linearly viscous.  Synovial fluid has anti-[[thixotropic]] characteristics; under conditions of sudden increase in pressure as in some kind of mechanical shock, its viscosity suddenly increases.<ref>< {{cite journal
 |title=Rheopexy of synovial fluid and protein aggregation |journal=Journal of the Royal Society Interface |author=Oates, Katherine |pmid=16849228 |doi=10.1098/rsif.2005.0086 |volume=3 |issue=6 |pmc=1618490 |year=2006 |pages=167–74}}</ref> Normal synovial fluid contains 3–4&nbsp;mg/ml [[hyaluronan]] (hyaluronic acid),<ref name=hui2012>{{cite journal
 |last1=Hui |first1=Alexander Y. |last2=McCart |first2=William J. |last3=Masuda |first3=Koichi |last4=Firestein |first4=Gary S. |last5=Sah |first5=Robert L.|date=Jan–Feb 2012|title=A Systems Biology Approach to Synovial Joint Lubrication in Health, Injury, and Disease |journal=Systems Biology and Medicine |series=Wiley Interdisciplinary Reviews |volume=4 |issue=1 |pages=15–7 |doi=10.1002/wsbm.157 |pmc=3593048 |pmid=21826801}}</ref> a polymer of [[disaccharide]]s composed of D-glucuronic acid and D-N-acetyl[[glucosamine]] joined by alternating beta-1,4 and beta-1,3 [[glycosidic bond]]s.<ref>{{cite journal |url=http://www.glycoforum.gr.jp/science/hyaluronan/HA01/HA01E.html |title=GlycoForum / Science of Hyaluronan |journal=Glycoforum |date=15 December 1997 |volume=1 |pages=A2 |last1=Hascall |first1=Vincent C. |last2=Laurent |first2=Torvard C. }}</ref>{{MEDRS|date=February 2013}} Hyaluronan is synthesized by the synovial membrane and secreted into the joint cavity to increase the viscosity and elasticity of articular cartilages and to lubricate the surfaces between [[synovium]] and cartilage.<ref>{{cite web |url=http://www.orthop.washington.edu/?q=patient-care/articles/arthritis/joints.html |publisher=University of Washington Medicine - Department of Orthopaedics and Sports Medicine |title=Joints |access-date=2013-02-04 |archive-date=2013-08-07 |archive-url=https://web.archive.org/web/20130807171425/http://www.orthop.washington.edu/?q=patient-care%2Farticles%2Farthritis%2Fjoints.html |url-status=dead }}</ref>{{MEDRS|date=February 2013}}

Synovial fluid contains [[PRG4|lubricin (also known as PRG4)]] as a second lubricating component, secreted by synovial [[fibroblast]]s.<ref name=jay2000>{{cite journal
 |last1=Jay |first1=GD |last2=Britt |first2=DE |last3=Cha |first3=CJ |date=March 2000 |title=Lubricin is a product of megakaryocyte stimulating factor gene expression by human synovial fibroblasts |journal=[[J Rheumatol]] |volume=27 |issue=3 |pages=594&ndash;600 |type=abstract |pmid=10743795 }}</ref>  Chiefly, it is responsible for so-called boundary-layer lubrication, which reduces friction between opposing surfaces of cartilage. There also is some evidence that it helps regulate synovial cell growth.<ref>{{cite journal |author=Warman M |journal=[[Arthritis Research & Therapy]] |year=2003 |volume=5 |issue=Suppl 3 |pages=5–7 |title=Delineating biologic pathways involved in skeletal growth and homeostasis through the study of rare Mendelian diseases that affect bones and joints |doi=10.1186/ar804 |doi-access=free |pmc=2833794 }}</ref>

It also contains [[Phagocyte|phagocytic cells]] that remove microbes and the debris that results from normal wear and tear in the joint.