New Insights into the Extracellular Matrix

This chapter introduces the biochemical components of the extracellular matrix (ECM) and also discusses current topics of extracellular matrix research, focusing specifically on those that involve the ECM present in the liver. ECM forms both the classical "ground substance" produced by mesenchymal tissues and is also the major component of basement membranes. However, it also possesses several physiological and pathological functions. Most components of the ECM are large macromolecules with domains for various functions affecting differentiation, cell motility, adhesion, and other activities of cells. The ECM refers to the insoluble protein complex between cells that is composed of families of macromolecules consisting of collagens, elastin, glycoproteins, and proteoglycans. In addition, many other macromolecules are functionally related to the ECM, such as cell adhesion molecules and their receptors, members of the transforming growth factor-β superfamily, and degradative enzymes such as metalloproteinases. Collagens are characterized by at least one triple helical domain, whereas laminins, fibronectins, and tenascins are all glycoproteins. Proteoglycans such as biglycan, decorin, or aggrecan, in addition to their glycosaminoglycan chains, are often ligands for cytokines as well as binding strongly to many other ECM macromolecules. The ECMs of the integrin family are the major receptors for matrix macromolecules and often act as mechanoreceptors, transmitting mechanical signals to the cytoskeleton, microtubules, intermediate filaments, and microfilaments.