Collagen fibers are structural components in vertebrate tissues that transfer stresses, store energy, and dissipate energy. Collagen fibers have a hierarchical structure composed of collagen molecules, microfibrils, fibrils, fibers, and fascicles, and they restrict tendon and other load-bearing tissue deformation

Collagenous fibers are formed of type I, II, or III collagen and are present in all forms of connective tissue. The ratio of collagen fibers to ground material distinguishes two forms of collagenous connective tissue

The most frequent form of collagenous connective tissue is loose connective tissue (areolar connective tissue). It occurs in tiny, elongated bundles separated by ground material areas.

In thick connective tissue with minimal ground material, collagen fibers are plentiful. If the densely packed bundles of fibers are aligned in one direction, it is called regular; if they are oriented in several directions, it is called irregular

The dermis is an example of irregular dense connective tissue, whereas tendons are examples of regular dense connective tissue.