Wound healing is the process of the body regenerating and repairing injured tissue. Healing involves the removal of dead (necrotic) tissue and the replacement of the damaged tissue with new tissue. The process of wound healing occurs in phases, with each of the phases overlapping.
The inflammatory phase of wound healing begins immediately after injury. Injury causes disruption of the small vessels in and around the skin resulting in bleeding. The immediate response is for the vessels to constrict, limiting hemorrhage. Skin cells expose the collagen on their surface to attract platelets. Platelets are the body's clotting cells and they accumulate in groups (aggregation) on the skin surface, forming a platelet plug. After the platelets have attached to the skin surface, they become activated, releasing growth factors, enzymes and other compounds important in the immune response.
The coagulation cascade is activated and results in the activation of thrombin and conversion of fibrinogen to fibrin. Inflammatory cells migrate to the site of injury and in combination with platelet aggregation, a clot is formed.
After a clot is formed, it soon dissipates and the blood vessels open up (vasodilation) to increase blood flow to the injury site. This occurs about 10 minutes after the injury. This increased blood flow bringing inflammatory cells to fight infection and other cells to clean up the damaged tissue. The result is redness and swelling around the injured site for 2-3 days. In the image to the right, the redness surrounding the wound can be seen.
The proliferative phase overlaps the inflammatory phase and occurs 3-5 days following injury and lasts for 14-21 days. The three important segments of the proliferative phase of wound healing are epithelialization, fibroplasia, angiogenesis.
Epithelialization is the process of new skin formation over the injured area. The process begins almost immediately after the injury as the skin cells (epidermal cells), undergo a change allowing them to detach and migrate across the wound bed. The skin cells have enzymes designed to break up the scab (eschar) which forms on the wound surface. A moist environment promotes wound healing by encouraging epithelialization.
Fibroplasia is the process of fibroblasts forming and laying down new collagen. Collagen is the main component of skin and critical for structure, strength and support. Collagen is formed approximately 3 days after injury and collagen formation continues for about 6 weeks. Collagen is laid down in a random, cross-linked fashion to increase strength over the injured surface. Elastin is another important structural protein which is present in small amounts and provides elasticity to skin.
Angiogenesis is the process of forming new blood vessels. An ample blood supply is necessary to sustain new skin growth. The small blood vessels which form cause a redness to form around the scar tissue. Macrophages, one of the key cells designed to clean up the tissue debris also secretes growth factors to stimulate the growth of new blood vessels.
The wound starts to contract after collagen deposition begins and maximum contraction occurs between 5 days and 2 weeks. The rate of wound contraction depends on the size and location of the wound and how much laxity the skin has in the area.The tensile strength of the wound is only 35% of it's original strength after two weeks. The images below are of a toe ulceration and although this is a chronic wound and may not be completely representative of new wound healing, epithelialization starts from the wound margins and migration occurs towards the center of the wound. This occurs in combination with wound contraction.
The maturation phase of wound healing is characterized by collagen remodeling. During the proliferation phase, type III collagen is laid down in a random, disorganized fashion. Type I collagen, which is a stronger type of collagen, replaces type III and the collagen becomes more organized. Remodeling usually begins about 21 days after the injury and continues for about a year. The tensile strength of a wound reaches it's peak in 60 days, but a healed wound will never reach full strength and may only have 80% of it's original tensile strength.
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