Extracellular matrix Totally Explained

Everything about Extracellular Matrix totally explained

In biology, the extracellular matrix (ECM) is the extracellular part of animal tissue that usually provides structural support to the cells in addition to performing various other important functions. The extracellular matrix is the defining feature of connective tissue in animals.
Extracellular matrix includes the interstitial matrix and the basement membrane. Interstitial matrix is present between various cells (for example, in the intercellular spaces). Gels of polysaccharides and fibrous proteins fill the interstitial space and act as a compression buffer against the stress placed on the ECM. Basement membranes are sheet-like depositions of ECM on which various epithelial cells rest.

Role and importance

Due to its diverse nature and composition, the ECM can serve many functions, such as providing support and anchorage for cells, segregating tissues from one another, and regulating intercellular communication. The ECM regulates a cell's dynamic behavior. In addition, it sequesters a wide range of cellular growth factors, and acts as a local depot for them. by enzymes such as serine and Threonine proteases and Matrix metalloproteinase. Once secreted they then aggregate with the existing matrix. The ECM is composed of an interlocking mesh of fibrous proteins and glycosaminoglycans (GAGs).

Proteoglycan matrix components

GAGs are carbohydrate polymers and are usually attached to extracellular matrix proteins to form proteoglycans (hyaluronic acid is a notable exception, see below). Proteoglycans have a net negative charge that attracts water molecules, keeping the ECM and resident cells hydrated. Proteoglycans may also help to trap and store growth factors within the ECM.
Described below are the different types of proteoglycan found within the extracellular matrix.

Heparan sulfate proteoglycans

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 extracellular matrix proteins. It is in this form that HS binds to a variety of protein ligands and regulates a wide variety of biological activities, including developmental processes, angiogenesis, blood coagulation and tumour metastasis.
In the extracellular matrix, especially basement membranes, the multi-domain proteins perlecan, agrin and collagen XVIII are the main proteins to which heparan sulfate is attached.

Chondroitin sulfate proteoglycans

Chondroitin sulfates contribute to the tensile strength of cartilage, tendons, ligaments and walls of the aorta. They have also been known to affect neuroplasticity.

Keratan sulfate proteoglycans

Keratan sulfates have a variable sulfate content and unlike many other GAGs, doesn't contain uronic acid. It is present in the cornea, cartilage, bones and the horns of animals.

Non-proteoglycan matrix components

Hyaluronic acid

Hyaluronic acid (or "hyaluronan") is a polysaccharide consisting of alternative residues of D-glucuronic acid and N-acetylglucosamine, and unlike other GAGs isn't 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 a lot 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.
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.

Collagen

Collagens are, in most animals, the most abundant glycoproteins 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 proteinases to allow extracellular assembly. Diseases such as osteogenesis imperfecta and epidermolysis bullosa are linked with genetic defects in collagen-encoding genes.

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