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Protection from mechanical injury, [[chemical hazard]]s, and bacterial invasion is provided by the [[Human skin|skin]] because the [[Epidermis (skin)|epidermis]] is relatively thick and covered with [[keratin]]. [[Secretion]]s 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 [[wound healing]]. After [[hemostasis]], [[inflammation]] [[white blood cells]], including [[phagocytic]] [[macrophages]] arrive at the injury site. Once the invading [[microorganisms]] have been brought under control, the skin proceeds to [[Healing|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>Martin, P. Wound Healing-aiming for perfect skin regeneration. Science (1997), 276, 75-81</ref> In a major injury, if [[Epithelium|epithelial]] [[cell migration]] and tissue contraction cannot cover the wound, [[surgical 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]]. Active cells, called [[fibroblasts]], produce [[collagen]]ous 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 major injuries, 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]]s, [[muscle]] cells, and [[nerve]]s 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.

Human skin cells are capable of repairing [[ultraviolet|UV]]-induced [[DNA damage (naturally occurring)|DNA damage]]s by the process of [[nucleotide excision repair]].<ref name = Lee2020>Lee JW, Ratnakumar K, Hung KF, Rokunohe D, Kawasumi M. Deciphering UV-induced DNA Damage Responses to Prevent and Treat Skin Cancer. Photochem Photobiol. 2020 May;96(3):478-499. doi: 10.1111/php.13245. Epub 2020 May 4. PMID 32119110; PMCID: PMC7651136</ref> This repair process protects against skin cancer.<ref name = Lee2020/>

==See also==
*[[Wound healing]]

==References==
{{Reflist}}

{{Integumentary physiology}}

[[Category:Skin physiology]]