wound healing wound healing wound healing refers to a living organism's replacement of destroyed or damaged tissue by newly produced tissue. hand abrasion day of injury 3 days later 17 days later 30 days later in undamaged skin, the epidermis (surface, epithelial layer) and dermis (deeper, connective layer) form a protective barrier against the external environment. when the barrier is broken, a regulated sequence of biochemical events is set into motion to repair the damage. this process is divided into predictable phases: blood clotting (hemostasis), inflammation, tissue growth (cell proliferation), and tissue remodeling (maturation and cell differentiation). blood clotting may be considered to be part of the inflammation stage instead of a separate stage. deep wound on shin with stitches healing over five weeks the wound healing process is not only complex but fragile, and it 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. wound care encourages and speeds wound healing via cleaning and protection from reinjury or infection. depending on each patient's needs, it can range from the simplest first aid to entire nursing specialties such as wound, ostomy, and continence nursing and burn center care. stages hemostasis (blood clotting): within the first few minutes of injury, platelets in the blood begin to stick to the injured site. they change into an amorphous shape, more suitable for clotting, and they release chemical signals to promote clotting. this results in the activation of fibrin, which forms a mesh and acts as "glue" to bind platelets to each other. this makes a clot that serves to plug the break in the blood vessel, slowing/preventing further bleeding. inflammation: during this phase, damaged and dead cells are cleared out, along with bacteria and other pathogens or debris. this happens through the process of phagocytosis, where white blood cells engulf debris and destroy it. platelet-derived growth factors are released into the wound that cause the migration and division of cells during the proliferative phase. proliferation (growth of new tissue): in this phase, angiogenesis, collagen deposition, granulation tissue formation, epithelialization, and wound contraction occur. 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. in 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. maturation (remodeling): during maturation and remodeling, collagen is realigned along tension lines, and cells that are no longer needed are removed by programmed cell death, or apoptosis. approximate times of the different phases of wound healing on a logarithmic scale, with faded intervals marking substantial variation, depending mainly on wound size and healing conditions, but image does not include major impairments that cause chronic wounds. timing and re-epithelialization timing is important to wound healing. critically, the timing of wound re-epithelialization can decide the outcome of the healing. if the epithelization of tissue over a denuded area is slow, a scar will form over many weeks, or months; if the epithelization of a wounded area is fast, the healing will result in regeneration. early vs cellular phase a fluorescence micrograph of cells in drosophila larvae healing after a puncture wound. the arrow points to cells that have fused to form syncytia, and the arrowheads point to cells that are oriented to face the wound. wound healing is classically divided into hemostasis, inflammation, proliferation, and remodeling. although a useful construct, this model employs considerable overlapping among individual phases. a complementary model has recently been described where the many elements of wound healing are more clearly delineated. the importance of this new model becomes more apparent through its utility in the fields of regenerative medicine and tissue engineering (see research and development section below). in this construct, the process of wound healing is divided into two major phases: the early phase and the cellular phase: the early phase, which begins immediately following skin injury, involves cascading molecular and cellular events leading to hemostasis and formation of an early, makeshift extracellular matrix that provides structural staging for cellular attachment and subsequent cellular proliferation. the cellular phase involves several types of cells working together to mount an inflammatory response, synthesize granulation tissue, and restore the epithelial layer. subdivisions of the cellular phase are: macrophages and inflammatory components (within 1–2 days) epithelial-mesenchymal interaction: re-epithelialization (phenotype change within hours, migration begins on day 1 or 2) fibroblasts and myofibroblasts: progressive alignment, collagen production, and matrix contraction (between day 4 and day 14) endothelial cells and angiogenesis (begins on day 4) dermal matrix: elements of fabrication (begins on day 4, lasting 2 weeks) and alteration/remodeling (begins after week 2, lasting weeks to months—depending on wound size). inflammatory phase just before the inflammatory phase is initiated, the clotting cascade occurs in order to achieve hemostasis, or the stopping of blood loss by way of a fibrin clot. thereafter, various soluble factors (including chemokines and cytokines) are released to attract cells that phagocytise debris, bacteria, and damaged tissue, in addition to releasing signaling molecules that initiate the proliferative phase of wound healing. clotting cascade main article: coagulation when tissue is first wounded, blood comes in contact with collagen, triggering blood platelets to begin secreting inflammatory factors. platelets also express sticky glycoproteins on their cell membranes that allow them to aggregate, forming a mass. fibrin and fibronectin cross-link together and form a plug that traps proteins and particles and prevents further blood loss. this fibrin-fibronectin plug is also the main structural support for the wound until collagen is deposited. migratory cells use this plug as a matrix to crawl across, and platelets adhere to it and secrete factors. the clot is eventually lysed and replaced with granulation tissue and then later with collagen. platelets, the cells present in the highest numbers shortly after a wound occurs, release mediators into the blood, including cytokines and growth factors. growth factors stimulate cells to speed their rate of division. platelets release other proinflammatory factors like serotonin, bradykinin, prostaglandins, prostacyclins, thromboxane, and histamine, which serve several purposes, including increasing cell proliferation and migration to the area and causing blood vessels to become dilated and porous. in many ways, extravasated platelets in trauma perform a similar function to tissue macrophages and mast cells exposed to microbial molecular signatures in infection: they become activated, and secrete molecular mediators – vasoactive amines, eicosanoids, and cytokines – that initiate the inflammatory process. vasoconstriction and vasodilation immediately after a blood vessel is breached, ruptured cell membranes release inflammatory factors like thromboxanes and prostaglandins that cause the vessel to spasm to prevent blood loss and to collect inflammatory cells and factors in the area. this vasoconstriction lasts five to ten minutes and is followed by vasodilation, a widening of blood vessels, which peaks at about 20 minutes post-wounding. vasodilation is the result of factors released by platelets and other cells. the main factor involved in causing vasodilation is histamine. histamine also causes blood vessels to become porous, allowing the tissue to become edematous because proteins from the bloodstream leak into the extravascular space, which increases its osmolar load and draws water into the area. increased porosity of blood vessels also facilitates the entry of inflammatory cells like leukocytes into the wound site from the bloodstream. polymorphonuclear neutrophils within an hour of wounding, polymorphonuclear neutrophils (pmns) arrive at the wound site and become the predominant cells in the wound for the first two days after the injury occurs, with especially high numbers on the second day. they are attracted to the site by fibronectin, growth factors, and substances such as kinins. neutrophils phagocytise debris and kill bacteria by releasing free radicals in what is called a respiratory burst. they also cleanse the wound by secreting proteases that break down damaged tissue. functional neutrophils at the wound site only have life-spans of around two days, so they usually undergo apoptosis once they have completed their tasks and are engulfed and degraded by macrophages. other leukocytes to enter the area include helper t cells, which secrete cytokines to cause more t cells to divide and to increase inflammation and enhance vasodilation and vessel permeability. t cells also increase the activity of macrophages. macrophages one of the roles of macrophages is to phagocytize other expended phagocytes, bacteria and damaged tissue, and they also debride damaged tissue by releasing proteases. macrophages function in regeneration and are essential for wound healing. they are stimulated by the low oxygen content of their surroundings to produce factors that induce and speed angiogenesis and they also stimulate cells that reepithelialize the wound, create granulation tissue, and lay down a new extracellular matrix. by secreting these factors, macrophages contribute to pushing the wound healing process into the next phase. they replace pmns as the predominant cells in the wound by two days after injury. the spleen contains half the body's monocytes in reserve ready to be deployed to injured tissue. attracted to the wound site by growth factors released by platelets and other cells, monocytes from the bloodstream enter the area through blood vessel walls. numbers of monocytes in the wound peak one to one and a half days after the injury occurs. once they are in the wound site, monocytes mature into macrophages. macrophages also secrete a number of factors such as growth factors and other cytokines, especially during the third and fourth post-wounding days. these factors attract cells involved in the proliferation stage of healing to the area. in wound healing that result in incomplete repair, scar contraction occurs, bringing varying gradations of structural imperfections, deformities and problems with flexibility. macrophages may restrain the contraction phase. scientists have reported that removing the macrophages from a salamander resulted in failure of a typical regeneration response (limb regeneration), instead bringing on a repair (scarring) response. decline of inflammatory phase as inflammation dies down, fewer inflammatory factors are secreted, existing ones are broken down, and numbers of neutrophils and macrophages are reduced at the wound site. these changes indicate that the inflammatory phase is ending and the proliferative phase is underway. in vitro evidence, obtained using the dermal equivalent model, suggests that the presence of macrophages actually delays wound contraction and thus the disappearance of macrophages from the wound may be essential for subsequent phases to occur. because inflammation plays roles in fighting infection, clearing debris and inducing the proliferation phase, it is a necessary part of healing. however, inflammation can lead to tissue damage if it lasts too long. thus the reduction of inflammation is frequently a goal in therapeutic settings. inflammation lasts as long as there is debris in the wound. thus, if the individual's immune system is compromised and is unable to clear the debris from the wound and/or if excessive detritus, devitalized tissue, or microbial biofilm is present in the wound, these factors may cause a prolonged inflammatory phase and prevent the wound from properly commencing the proliferation phase of healing. this can lead to a chronic wound. proliferative phase about two or three days after the wound occurs, fibroblasts begin to enter the wound site, marking the onset of the proliferative phase even before the inflammatory phase has ended. as in the other phases of wound healing, steps in the proliferative phase do not occur in a series but rather partially overlap in time. angiogenesis further information: angiogenesis also called neovascularization, the process of angiogenesis occurs concurrently with fibroblast proliferation when endothelial cells migrate to the area of the wound. because the activity of fibroblasts and epithelial cells requires oxygen and nutrients, angiogenesis is imperative for other stages in wound healing, like epidermal and fibroblast migration. the tissue in which angiogenesis has occurred typically looks red (is erythematous) due to the presence of capillaries. angiogenesis occurs in overlapping phases in response to inflammation: latent period: during the haemostatic and inflammatory phase of the wound healing process, vasodilation and permeabilisation allow leukocyte extravasation and phagocytic debridement and decontamination of