Collagens, network-forming fibers

Collagens (see p. 344), of which there are at least 19 different varieties, form fibers, fibrils, networks, and ligaments. Their characteristic properties are tensile strength and flexibility. Elastin is a fiber protein with a high degree of elasticity. 

Collagens differ in their ability to form fibers and to organize the fibers into networks. In tendons, for instance, long type I collagen fibrils are packed side by side in parallel bundles, forming thick collagen fibers. Tendons connect muscles to bones and must withstand enormous forces. Because type... 

In fat tissues, capillaries and collagen fibers form a supporting network around spherical adipocytes. Almost the entire volume of these metabolically active cells is taken up by fat droplets. 

Rehcular connective tissue this is a network of reticular fibers made from fine collagen, type 111. These fibers form a soft skeleton to support the lymphoid organs such as lymph nodes, bone marrow and spleen. 

Because it performs such a wide variety of functions, collagen is the most abundant single protein in most vertebrates. In large animals, it may make up a third of the total protein mass. Collagen fibers form the matrix, or cement, material in bone, on which the mineral constituents precipitate. These fibers constitute the major portion of tendons. A network of collagen fibers is an important constituent of skin. Basically, collagen holds most animals together. 

Elastin - Elastin is a highly elastic fiber present in ligaments and arterial blood vessels. The polypeptide is rich in glycine, alanine, and valine. Its secondary structure is the most random of the fibrous proteins described here. Like collagen, elastin contains lysine groups involved in cross-links between the chains. In elastin, however, four lysine chains can be combined to form a desmosine cross-link (see here). Thus, fewer cross-links are needed to provide strength for the chains and a more elastic network is created. 

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