Collagen fibre bundle (connective tissue) in human heart muscle, animated coloured scanning electron micrograph (SEM). Collagen is a type of structural protein that is part of the extracellular matrix (ECM). It is the most abundant protein in mammals. The extracellular matrix is a complex structural tissue component surrounding and supporting cells in mammalian tissues. Collagen forms elongated fibrils and is primarily found in fibrous tissues such as tendons, ligaments and skin. It is also abundant in corneas, cartilage, bones, blood vessels, gastrointestine and muscle tissue. Fibroblasts is the predominant cell that produces collagen.