Collagen is a fibrous protein that forms the bulk of the skin, collagen. fibers of connective tissue and the basic substance of the cartilage and bone. Produced by fibroblasts from water-soluble precursor - prokollagena. Native collagen not exposed proteases, but is destroyed by collagenase, found a number of microbes. Age changes of connective tissue increased content of coarse-fibered collagen structures. Cm. also, Collagen diseases, connective tissue.