Collagen is the major insoluble fibrous protein in the extracellular matrix and in connective tissue. In fact, it is the single most abundant protein in the animal kingdom. There are at least 16 types of collagen, but 80 – 90 percent of the collagen in the body consists of types I, II, and III. These collagen molecules pack together to form long thin fibrils of similar structure. Type IV, in contrast, forms a two-dimensional reticulum; several other types associate with fibril-type collagens, linking them to each other or to other matrix components. At one time it was thought that all collagens were secreted by fibroblasts in connective tissue, but we now know that numerous epithelial cells make certain types of collagens. The various collagens and the structures they form all serve the same purpose, to help tissues withstand stretching.