Mark left on the skin after a wound heals. Cells called fibroblasts produce collagen fibres, which form bundles that make up the bulk of scar tissue. Scars have a blood supply but no oil glands or elastic tissue, so they can be slightly painful or itchy. Hypertrophic scars grow overly thick and fibrous but remain within the original wound site. Scars can also develop into tumourlike growths called keloids, which extend beyond the wound's limits. Both can inhibit movement when they result from serious burns over large areas, especially around a joint. Scars, especially those from unaided healing of third-degree burns, can become malignant. Treatment of serious scars is one of the most important problems in plastic surgery.