Comparative ultrastructure of hypertrophic scars and keloids.

Clinical differences between hypertrophic scars and keloids have long been recognized by plastic surgeons and dermatologists. Yet, translating these differences into morphological or biochemical distinctions has prompted much conflict in the literature. Fine structural analyses of eighty lesions has allowed a detailed comparison in terms of tissue organization, fibroblast cell types, microvascular comparisons and differences in organelle content of endothelial cells. Both lesions consistently contain nodules of collagen, which is annealed into broad sheets of fibrils. Both lesions have a predominant active fibroblast cell type but keloids tend to have more quiescent forms. Both lesions show high levels of occluded microvessels. Comparison of numbers and magnitude of several organelles of endothelial cells suggest keloids may be more similar to mature scars than to hypertrophic scars. Thick plastic sections of keloid tissues stained with Toluidine Blue O show a glazing of the collagen bundles whereas they are crisp in hypertrophic scars. When examined by transmission electron microscopy the collagen fibrils of keloids in the glazed areas were larger, more irregular and the interfibrillar distance was less than in hypertrophic scars. The occurrence of the irregular fibrils in keloids may reflect a significant difference in terms of collagen synthesis, fusion or breakdown. It is also suggested that the essential difference between keloids and hypertrophic scars may be in the volume of microvessels injured, and, hence, the amount regenerated, the number of pericytes, fibroblasts and, consequently, the amount of collagen synthesized.