Extracellular matrix - a substance secreted by cells - is a main component of the human body. As an example, collagen fiber - a main component of extracellular matrix material - represents 25 to 30% of the total protein mass of our bodies. It serves to reinforce the body structure and plasma membranes so that we can withstand gravity and tension, and plays a role as an adhesive substance and a signaling molecule for cells. The material has many constituents such as fibrous components, glycosaminoglycans, proteoglycans and glycoproteins (Fig. 11-1).
Collagen fiber is a typical fibrous extracellular matrix component. Such fibers are distributed all over the human body, and are essential for the maintenance of the body’s structure. As an example, collagen fiber is the main component of skin and bones. The unit protein that makes up this fiber consists of a repetition of three amino acids (glycine-proline-X, where X is any amino acid). Collagen proteins are polymerized to form thick collagen fibers (Fig. 11-4). Among the many types of collagen fiber that exist, one with a long, fibrous structure and another with a sheet structure (i.e., a two-dimensional network) are dominant. The former is mainly found in parts to which pressure is applied (such as bones and the dermis layer of skin), and plays a reinforcing role. The latter exists as a main component of basal lamina (located on the basal side of epithelial tissues), around muscle cells, in glomeruli (which filter urine in the kidneys), and plays an important role in each place.
Fig. 11-4. Nucleotide types
Collagen proteins as a basic unit are bundled in a fibrous form and constitute collagen fibers. Three collagen proteins, after glycosylation to amino acids, are linked with disulfide bonds to form a triple-helical structure. Prolines of collagens are also modified with addition of the hydroxyl group and become hydroxyprolines. Hydrogen bonds are formed between the hydroxyl group of the hydroxyproline and other amino acids, thereby making the fibers strong and stable. The triple-helical structures are chemically bound to form thick collagen fibers. The bar in the photo is 50 nm long.
Glycosaminoglycans and Proteoglycans
Glycosaminoglycans are macromolecules consisting of a repetition of two saccharide types. They are also called mucopolysaccharides because they exhibit viscosity in an aqueous solution due to their polymer structure. Their molecular weight ranges from 100,000 to as much as 10,000,000. Many glycosaminoglycans form complexes known as proteoglycans with proteins (Fig. 11-5). Many proteoglycans, due to the addition of a sulfate group to their carbohydrates, have a negative charge. Since the aggregates of glycosaminoglycans - hydrophilic molecules - retain many water molecules, they play a role in expanding intercellular space and also as a shock absorber against external pressure. Their polymer structure also gives viscosity to extracellular matrix material.
Proteins that contain oligosaccharides attached to the amino acids located at particular sites of proteins are collectively known as Glycoproteins. Approximately 60% of proteins have oligosaccharides attached to them, and are therefore glycoproteins. As an example, most of the proteins and secretory proteins incorporated in the plasma membrane are glycoproteins. Many of the glycoproteins secreted as extracellular matrix components act as cell adhesive molecules. These molecules have a great impact on cell division, movement and differentiation, through cell adhesion.
The basal lamina is a layer containing collagen fibers, glycosaminoglycans, proteoglycans, and glycoproteins (Column Fig. 11-1), and plays a number of important roles. The basal laminae located on the basal side of epithelial tissues contribute to tissue stabilization by adhering to epithelial cells. The basal laminae located around muscle cells contribute to plasma membrane reinforcement, and those located in glomeruli - urine filtration structures - play a role in filtering urine. During the developmental process of animals, molecules in basal laminae also have essential roles on cell migration, differentiation and growth.