5.1 Extracellular matrix
The extracellular matrix is composed of proteins and polysaccharides that are secreted by cells and assembled into an organised meshwork external to the cells. The amount and type of extracellular matrix varies from tissue to tissue, and different types of extracellular matrix perform different functions. In many tissues, it acts rather like cement between cells; in others, it forms a support or scaffold. The extracellular matrix is not merely a scaffold, however, but can have an active role in regulating cell behaviour, and the communication between cells. Connective tissues have a relatively large proportion of extracellular material which determines their properties: for example, the rigidity of bone is due to an extracellular matrix composed of tough collagen fibres that is hardened by mineral salts (see below).
In most animal tissues, the extracellular matrix is secreted mainly by cells called fibroblasts, and the major component is a gel-like substance in which proteins are embedded. The gel is composed of polysaccharides called glycosaminoglycans (GAGs), which are often attached to proteins to form heavily glycosylated proteins called proteoglycans. The GAGs are negatively charged, and attract positively charged ions, particularly sodium ions. The high concentration of positive ions leads to the movement of water by diffusion into the matrix, giving it its gel-like properties, including the ability to resist compression.
Fibrous proteins and other substances within the extracellular matrix provide additional strength or elasticity. The most abundant extracellular protein in animal tissues is collagen, of which there are several types. Collagen is a very tough fibrous protein that can, in its different forms, provide great strength and/or flexibility (Figure 24). Elastin, as its name suggests, is a flexible protein that is found in the extracellular matrix of tissues such as blood vessels. In bone, mineral salts are laid down in connective tissue to provide support.
Some cells are linked to particular proteins embedded in the extracellular matrix. For example, an extracellular matrix protein known as fibronectin binds to cell membrane glycoproteins known as integrins. On the inside of the cell, the integrins are linked to actin filaments of the cytoskeleton, as shown in Figure 25. Integrins can transduce information from the extracellular matrix to the cytoskeleton, which may then stimulate the cell to change shape, move or respond in other ways, to certain changes in its external environment.
Interactions between cells and the extracellular matrix, then, play important roles in maintaining tissue structure, and also in cell properties such as adhesion and migration.