ECM

ExtraCELLULAR MATRIX... (ECM)
    
ECM… is extracellular space filled with a network of macromolecules (polysaccharides & proteins) secreted by a cell. The ECM molecules are secreted by cells called fibroblasts. The ECM occurs in all tissues, even blood (plasma is the matrix). It serves as a scaffold or support for cells and some cellular functions. The ECM is made of polysaccharides (glycosaminoglycans) linked to proteins (proteoglycans) and fibrous proteins including collagen & elastin and adhesive proteins, like fibronectin (a dimer composed of two lagre subunits joined by a pair of –S-S- bonds) & laminin. A major component of ECM's is hyaluronic acid, a large polysaccharide made of glucuronic acid & glucosamine that attracts water can serve as a lubricant in joint fluid.

     Proteoglycans retain water forming a hydrated-gel substance with embedded fibers through which ions, hormones & nutrients can freely move. Collagen (with long, stiff, helical structures) gives the ECM strength; elastins give it resilience; & fibronectins help fibroblast & other cells attach to the ECM (fibronectin domains binds to collagen & heparin and to specific integrins on the surfaces of various types of cells). Laminin promotes attachment of epithelial cells to the basal lamina and integrins.
     
Beneath epithelia the ECM is organized as a thin sheet called the basal lamina (40-120 nm thick). This lamina contains a specialized form of collagen (type IV collagen) which has a more flexible structure then regular fibrilar collagen. It forms a flexible, sheetlike multi-layered network.
     T
he ECM of connective tissue is the collective term for the fibers & ground substances which house the cells of that tissue. This ECM can exist in 3 states: solid, semi-solid, & liquid. Bone is a connective tissue with a solid extracellular matrix. Loose and dense connective tissues as well as cartilage have semi-solid extracellular matrices. Blood and lymph are connective tissues with liquid extracellular matrices..