Skin repair
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2008-06-24T09:56:56Z
84.29.140.190
{{Unreferenced|date=November 2007}}
Protection from mechanical injury, [[chemical hazard|chemical hazards]], and bacterial invasion is provided by the [[skin]] because the [[Epidermis (skin)|epidermis]] is relatively thick and covered with [[keratin]]. [[Secretion|Secretions]] from [[sebaceous glands]] and [[sweat glands]] also benefit this protective barrier. In the event of an injury that damages the skin's protective barrier, the body triggers a response called [[inflammation]], which sends fluids carrying [[phagocytic]] [[white blood cells]] to the injury site. Once the invading [[microorganisms]] have been brought under control, the skin proceeds to heal itself. The ability of the skin to heal even after considerable damage has occurred is due to the presence of [[stem cells]] in the [[dermis]] and [[cell (biology)|cells]] in the [[stratum basale]] of the epidermis, all of which can generate new [[Biological tissue|tissue]].
When an injury extends through the epidermis into the dermis, bleeding occurs and the inflammatory response begins. Clotting mechanisms in the blood are soon activated, and a clot of scab is formed within several hours. The scab temporarily restores the integrity of the epidermis and restricts the entry of microorganisms. After the scab is formed, cells of the stratum basale begin to divide by [[mitosis]] and migrate to the edges of the scab. A week after the injury, the edges of the wound are pulled together by contraction. Contraction is an important part of the healing process when damage has been extensive, and involves shrinking in size of underlying contractile connective tissue, which brings the wound margins toward one another.<ref>1</ref> In a major injury, if [[Epithelium|epithelial]] [[cell migration]] and tissue contraction cannot cover the wound, [[Suture|suturing]] the edges of the injured skin together, or even replacement of lost skin with [[skin grafting|skin grafts]], may be required to restore the skin.
As epithelial cells continue to migrate around the scab, the [[dermis]] is repaired by the activity of [[stem cells]]. These active cells produce [[collagen|collagenous]] fibers and [[ground substance]]. [[Blood vessels]] soon grow into the dermis, restoring circulation. If the injury is very minor, the epithelial cells eventually restore the epidermis once the dermis has been regenerated.
In injuries that are not minor, the repair mechanisms are unable to restore the skin to its original condition. The repaired region contains an abnormally large number of collagenous fibers, and relatively few blood vessels. Damaged [[sweat gland|sweat]] and [[sebaceous]] glands, [[hair follicle|hair follicles]], [[muscle]] cells, and [[nerve|nerves]] are seldom repaired. They are usually replaced by the [[Fibrous connective tissue|fibrous tissue]]. The result is the formation of an inflexible, fibrous [[scar]] tissue.
[[Category:Integumentary system]]
== References ==
1. Martin, P. Wound Healing-aiming for perfect skin regeneration. Science (1997), 276, 75-81