Wound healing is an intricate process where the skin (or another organ-tissue) repairs itself after injury. In normal skin, the epidermis (outermost layer) and dermis (inner or deeper layer) exist in a steady-state equilibrium, forming a protective barrier against the external environment. Once the protective barrier is broken, the normal (physiologic) process of wound healing is immediately set in motion. The classic model of wound healing is divided into three or four sequential, yet overlapping, phases: (1) hemostasis (not considered a phase by some authors), (2) inflammation, (3) proliferation and (4) remodeling. Upon injury to the skin, a set of complex biochemical events takes place in a closely orchestrated cascade to repair the damage. Within the first few minutes after the injury, platelets adhere to the site of injury, become activated, and aggregate (join together); followed by activation of the coagulation cascade which forms a clot of aggregated platelets in a mesh of cross-linked fibrin protein. This clot stops active bleeding (hemostasis). During the inflammation phase, bacteria and cell debris are phagocytosed and removed from the wound by white blood cells. Platelet-derived growth factors (stored in the alpha granules of the platelets) are released into the wound that cause the migration and division of cells during the proliferative phase. The proliferation phase is characterized by angiogenesis, collagen deposition, granulation tissue formation, epithelialization, and wound contraction. In angiogenesis, vascular endothelial cells form new blood vessels. In fibroplasia and granulation tissue formation, fibroblasts grow and form a new, provisional extracellular matrix (ECM) by excreting collagen and fibronectin. Concurrently, re-epithelialization of the epidermis occurs, in which epithelial cells proliferate and ‘crawl’ atop the wound bed, providing cover for the new tissue. During wound contraction, myofibroblasts decrease the size of the wound by gripping the wound edges and contracting using a mechanism that resembles that in smooth muscle cells. When the cells’ roles are close to complete, unneeded cells undergo apoptosis. During maturation and remodeling, collagen is remodeled and realigned along tension lines, and cells that are no longer needed are removed by apoptosis. However, this process is not only complex but fragile, and is susceptible to interruption or failure leading to the formation of non-healing chronic wounds. Factors that contribute to non-healing chronic wounds are diabetes, venous or arterial disease, infection, and metabolic deficiencies of old age.
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