Characteristics of granulation tissue which promote hypertrophic scarring

Hypertrophic scars and keloids are characterized by nodules of collagen that originate in granulation tissue arising from full thickness or deep 2 degrees injuries to the skin. Fifty-six granulation tissues of varying ages post-injury were examined morphologically for evidences of how the nodules and, thus, the scar form. New microvessels grow in ascension towards the free surface in a milieu of inflammatory cells and fibroblasts. Collagen deposition increases with time from the base of the wound to the free surface and begins to concentrate between lateral branching of the new microvessels. Computer derived serial reconstructions of hypertrophic scar nodules indicate they are of varying shape and size probably due to fusion of adjacent microvascular collagen masses between lateral branches. This is accomplished by the gradual but persistent degeneration of microvascular endothelia and pericytes. Fifty-six pieces of granulation tissue taken from 5 cases of varying age post-injury were implanted into nude mice. Several proceeded to develop scar and some of those developed nodules. The latter developed only when the zero-time implant contained lateral microvascular branches. Hypertrophic scars and keloids are a product of granulation tissue elements, the most important of which are primed active fibroblasts and excessive microvascular regeneration, including lateral branching, which subsequently degenerates, in part, promoting nodule formation and remodeling.