Editing Extracellular matrix (section)

===Proteoglycans===
[[Glycosaminoglycan]]s (GAGs) are [[carbohydrate]] [[polymer]]s and mostly attached to extracellular matrix proteins to form [[proteoglycan]]s (hyaluronic acid is a notable exception; see below). Proteoglycans have a net negative charge that attracts positively charged sodium ions (Na<sup>+</sup>), which attracts water molecules via osmosis, keeping the ECM and resident cells hydrated. Proteoglycans may also help to trap and store [[growth factors]] within the ECM.{{cn|date=April 2025}}

Described below are the different types of proteoglycan found within the extracellular matrix.{{cn|date=April 2025}}

====Heparan sulfate====
[[Heparan sulfate]] (HS) is a linear [[polysaccharide]] found in all animal tissues. It occurs as a [[proteoglycan]] (PG) in which two or three HS chains are attached in close proximity to cell surface or ECM proteins.<ref>{{cite book | title=Proteoglycans: structure, biology and molecular interactions | url=https://archive.org/details/proteoglycansstr00iozz | url-access=limited | vauthors = Gallagher JT, Lyon M | chapter=Molecular structure of Heparan Sulfate and interactions with growth factors and morphogens | veditors = Iozzo RV | year=2000 | publisher=Marcel Dekker Inc. New York, New York | pages=[https://archive.org/details/proteoglycansstr00iozz/page/n41 27]–59 |isbn=9780824703349 }}</ref><ref>{{cite journal | vauthors = Iozzo RV | s2cid = 14638091 | title = Matrix proteoglycans: from molecular design to cellular function | journal = Annual Review of Biochemistry | volume = 67 | issue = 1 | pages = 609–52 | year = 1998 | pmid = 9759499 | doi = 10.1146/annurev.biochem.67.1.609 | doi-access = free }}{{closed access}}</ref>  It is in this form that HS binds to a variety of protein [[ligand]]s and regulates a wide variety of biological activities, including [[developmental processes]], [[angiogenesis]], [[blood coagulation]], and tumour [[metastasis]].{{cn|date=April 2025}}

In the extracellular matrix, especially [[basement membrane]]s, the [[protein domain|multi-domain]] proteins [[perlecan]], [[agrin]], and [[type XVIII collagen|collagen XVIII]] are the main proteins to which heparan sulfate is attached.{{cn|date=April 2025}}

====Chondroitin sulfate====
[[Chondroitin sulfate]]s contribute to the tensile strength of cartilage, [[tendon]]s, [[ligament]]s, and walls of the [[aorta]]. They have also been known to affect [[neuroplasticity]].<ref>{{cite book |doi=10.1016/S0070-2153(05)69008-4 |pmid=16243601 |chapter=Critical Period Mechanisms in Developing Visual Cortex |title=Neural Development |volume=69 |pages=215–237 |series=Current Topics in Developmental Biology |year=2005 |last1=Hensch |first1=Takao K. |isbn=978-0-12-153169-0 }}</ref>

====Keratan sulfate====
[[Keratan sulfate]]s have a variable sulfate content and, unlike many other GAGs, do not contain [[uronic acid]]. They are present in the [[cornea]], cartilage, [[bone]]s, and the [[Horn (anatomy)|horns]] of [[animal]]s.{{cn|date=April 2025}}