Extracellular matrix
Extracelullar matrix (ECM)[edit | edit source]
ECM is produced by cells of a given type of tissue. It consists of macromolecules that form a complexly organised network. Cells have binding sites, receptors for the ECM, which serve to connect cells to the ECM and to regulate their activity. The ECM is a colourless, transparent, gel-like substance in which cells and fibres are embedded. The matrix is divided into a fibrous component and an amorphous component.
The fibrous component of the ECM[edit | edit source]
Collagen fibres[edit | edit source]
Collagen fibres are 1-20 μm long. The properties of a collagen fibre include strength and flexibility. They tend to form bundles. They are produced by connective tissue cells, but also by smooth muscle cells, glial cells, adipocytes and epithelia. About 21 types of collagens are known, differing in sequence and type of amino acid in the chain.The matrix is divided into a fibrous component and an amorphous component.
The most well-known types of collagen include:
- collagen I (75 nm fibrils, bundles of fibers, visible to the eye, most abundant in the body, found in connective cartilage)
- collagen II (thin fibrils 20 nm, separate, no bundles, hyaline and elastic cartilage)
- collagen III (45 nm fibrils, reticular fibres)
- collagen IV (in the lamina basalis)
Collagen ligaments are eosinophilic (acidophilic) and can be stained with eosin, light green, aniline blue, saffron or picrofuchsin. The synthesis of collagen takes place on the ribosomes of GER. The protein follows a pathway to the Golgi complex where it is hydroxylated and glycosylated. Here it is also packaged into membraneless vesicles called vesicle and is then expelled from the cell by exocytosis in the form of a protocollagen molecule (3 polypeptide chains with registration peptides at the ends of the chain to prevent polymerization). From this, protocologen-peptidases are formed and cleave off the registration peptides. A tropocollagen molecule is formed, which can now polymerize to form collagen myofibrils. Then collagen fibrils can form and these polymerize into the final collagen fibre.
Reticular fibres[edit | edit source]
It is made up of type III collagen. The fibres are 0.2-2 μm long and are made up of 45 nm fibrils. They form networks in organs. They are stained by silver impregnation (argyrophilic), PAS reaction (they have a lot of glycoproteins and proteoglycans), Gomori impregnation.
Elastic fibres[edit | edit source]
Elastic fibres are thinner than collagen fibres. They are 0.5-4 μm long. The central protein consists of elastin and surrounding microfibrils. They are elastic, form networks, anastomose and branch. Elasticity is provided by hydrophobic properties. They are produced by fibroblasts. They are stained with special staining methods such as orcein, resorcin-fuchsin and aldehyde-fuchsin.
Amorphous component of the ECM[edit | edit source]
Consists mainly of water and ions. There are also glycosaminoglycans (GAGs), which form large unbranched polysaccharide chains. These chains are composed of disaccharide units and have a negative charge. These are the amino sugars N-acetylglucosamine, N-acetylgalactosamine, which are often sulfonated. In many GAGs, a second sugar, uronic acid with a carboxyl group, is part of the molecule. GAGs are highly hydrophilic and maintain the architecture of the ECM. They prevent its deformation by compression forces due to: hyaluronic acid, chondroitin sulfate, dermatan sulfate, heparan sulfate and heparin, keratan sulfate and proteoglycans. With the exception of hyaluronic acid, GAGs bind covalently to protein to form proteoglycans - large molecules capable of maintaining high hydration of the ECM. Another part of the amorphous component consists of adhesion proteins. Examples include fibronectin, which is a multifunctional glycoprotein that binds to receptors (integrins) on the cell surface, or laminin, which is a sulfonated glycoprotein in the basal lamina.
References[edit | edit source]
- BRICHOVÁ, Hana. Tissues- distribution, basic structure and function Binders-general characteristics Intercellular matter, its formation and composition Cartilage structure [lecture for subject General histology and general embryology, specialization General medicine, 1st Faculty of Medicine Charles University]. Prague. 18.10.2013. Avaliable from <http://uhiem.raycz.cz/uploads/1715_tkane,%20pojiva,mezibunecna%20hmota,%20chrupavka%20www.pdf>.
