<P> Hyaluronic acid (or "hyaluronan") is a polysaccharide consisting of alternating residues of D - glucuronic acid and N - acetylglucosamine, and unlike other GAGs, is not found as a proteoglycan . Hyaluronic acid in the extracellular space confers upon tissues the ability to resist compression by providing a counteracting turgor (swelling) force by absorbing significant amounts of water . Hyaluronic acid is thus found in abundance in the ECM of load - bearing joints . It is also a chief component of the interstitial gel . Hyaluronic acid is found on the inner surface of the cell membrane and is translocated out of the cell during biosynthesis . </P> <P> Hyaluronic acid acts as an environmental cue that regulates cell behavior during embryonic development, healing processes, inflammation, and tumor development . It interacts with a specific transmembrane receptor, CD44 . </P> <P> Collagens are the most abundant protein in the ECM . In fact, collagen is the most abundant protein in the human body and accounts for 90% of bone matrix protein content . Collagens are present in the ECM as fibrillar proteins and give structural support to resident cells . Collagen is exocytosed in precursor form (procollagen), which is then cleaved by procollagen proteases to allow extracellular assembly . Disorders such as Ehlers Danlos Syndrome, osteogenesis imperfecta, and epidermolysis bullosa are linked with genetic defects in collagen - encoding genes . The collagen can be divided into several families according to the types of structure they form: </P> <Ol> <Li> Fibrillar (Type I, II, III, V, XI) </Li> <Li> Facit (Type IX, XII, XIV) </Li> <Li> Short chain (Type VIII, X) </Li> <Li> Basement membrane (Type IV) </Li> <Li> Other (Type VI, VII, XIII) </Li> </Ol>

What tissue contains the most amount of extracellular matrix (ecm)