Arachidonic acid mediates the production of prostaglandins, thromboxane, and leukotrienes. Prostaglindin itself is a powerful vasodilator and works with histamine, bradykinin and complement system to enhance vascular permeability.
Thromboxane plays a role in platelet aggregation and vasodilatation. Complement cascade is activated by platelet and neutral protease is involved in the activation of complement system. In the final common pathway of complement cascade, C5a and C3a are formed, which have an influence on anaphylatoxin that induces mast cell degranulation and histamine release. Furthermore, they trigger vasodilatation as well as intravascular leukocyte aggregation and migration so that vascular endothelium could adhere to the wound. C5a is approximately 100 to 1,000 times more effective than C3a, but is detected in a smaller amount than C3a.
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Neutrophils are involved with bacterial phagocytosis and prevention of infection in wounds. It is known, however, that wound healing process continues even in animal models depleted of neutrophil, suggesting that the role of neutrophil is not very important for wound healing. Nevertheless, the risk of infection increases without neutrophil.
Certain factors work as chemo-attractants of neutrophil, leading to inflammatory response. Activated neutrophil releases lysosomal enzymes, including neutral proteases, elastase and collagenase, which are beneficial against oxygen free radicals and for host defense. When not in use anymore, these neutrophils are removed by tissue macrophage, which migrates to the wound on Day 3 of sustaining the wound and plays an important role as a cell that controls and modifies wound healing.
Macrophages are divided to inflammatory ones and responsive ones. By means of enzyme release and phagocytosis, they control the degradation of extracellular connective tissue and wound matrix remodeling. They also modifies wound healing by producing growth factors, such as FGF, PDGF, TGF, Interleukin (IL) or Tumor Necrosis Factor (TNF).
Extracellular matrix consists of protein-embedded polysaccharide gel, released by fibroblast, and contains structural proteins, such as collagen or elastin, and cell-adhesion proteins, such as fibronectin or laminin (the main components of endothelial basement membrane of blood vessel). Polysaccharide gel is made of a pair of glycosaminoglycan and proteoglycan, which lets nutritions diffused into cells and maintains cellular structure. Adhesion proteins make cells adhered to the matrix.
Fibronectin is an adhesion molecule just like thrombospondin, von Willebrand factor and laminin. As a strong chemo-attractant of circulating monocyte, it stimulates monocyte to differentiate to active macrophage. It also has opsonin (a substance that helps phagocytosis of leukocyte) activity.
Fibroplasia phase, the second phase of wound healing, starts when the number of macrophages and fibroblasts are increased, at the same time when the number of neutrophils are decreased, in the wound. This results in the production of substrate, including collagen synthesis, and simultaneous angiogenesis. At this point, the inflammatory response is finished and inflammatory mediators are not produced anymore. The existing inactivated inflammatory mediators spread out from the wound or are removed by wound macrophage. The number of neutrophils in the wound is decreased compared to the beginning, and fibroplasia starts in 5 days after sustaining the wound and lasts for about 2 weeks.
Fibroblasts migrate to the wound, and there will be repeated response to mediators released during the inflammation phase, such as C5a, fibronectin, PDGF, FGF and TGF.
There have been a lot of studies focused on extracellular matrix, which plays a very important role in the process of wound healing. There is accumulating evidence that cells themselves are having various sensitive responses by means of receptors in the surrounding environment and that cytoskeleton, which is essential for cellular migration, undergoes a certain change. Hyauronate and fibrolast, which are one of extracellular matrix, helps cellular migration of chemotactic factors, produced during the fibroplasia. The former helps the formation of chemical gradients that have affinity to fibroblast. The latter can combine with proteins and fibroblast inside the matrix, thus preparing for the route along which fibroblast can move. Fibroblast combines with arginine-glycine-aspartic acid sequence on fibronectin, while fibroblast fibronectin receptor changes through cell membrane and combines with actin filament. In this manner, cells can move along the fibronectin strand according to chemotactic gradients. Fibroblast also produces other matrix proteins, such as proteoglycans or structural protein; proteoglycan is a protein where polysaccaride can adhere to.
PDGF is capable of stimulating fibroblast replication and chemotaxis. TGF-β induces fibroblast to synthesize fibronectin and collagen, and EGF can also stimulate the production of collagen.
Collagen is a type of protein most commonly present in mammals and is rich in glycin and proline, released by fibroblast. There are at least about 12 types of collagens discovered so far. Collagen is made of solid α-helical structure, and the collagen strands are cross-linked to each other so well that they hardly breakdown. Collagen is therefore a very important element for wound healing in mammal tissue structure.
Macrophage controls the release of collagen from fibroblast with the use of growth factors, such as PDGF, EGF, FGF and TGF-β. Collagen remodeling, by degradation and synthesis, continues for almost about 2-3 years. Among other structural proteins, elastin is made up of random coil, which is capable of contracting to some degree, unlike collagen made up of regular strands rich in proline and lysine. Elastins are found in less quantity than collagens in wounds.
Angiogenesis refers to the migration and proliferation of endothelial cells. Angiogenesis factors, such as FGF and TGF-α, and endothelial cell growth stimulates the budding of capillaries from nearby intact existing capillary network.
Endothelial cell connects between capillaries and makes capillary network for wound healing. Angiogenesis is a very important step for wound healing as a mechanism for transferring new healing factors to the wound site.
When the wound has adequate blood supply, the process of angiogenesis stops, possibly mediated by oxygen tension. This can be explained by the stimulation of angiogenesis when the wound is in hypoxia and cessation of angiogenesis in normal oxygen tension.
Epithelializaton occurs when epithelial cells proliferate on the edge of wound. These epithelial cells use contractile protein to migrate upon the collagen and fibronectin on the wound surface, the process of which continues until the wound is covered by epithelial cells.
Scar contracture occurs when the wound starts to mature, in which case myofibroblast should be considered. These cell are detectable in the process of contraction, but not after the contraction is complete.
In the maturation phase of wound healing, wound repair is followed by wound remodeling. With reduced vascularity, the scar becomes less hyperemic, and the tissue organization and maturation continues for about 2 years, increasing the wound strength. Despite increased wound strength, collagen is not increased in the wound.
Hyaluronidase, plasminogen activators, collagenase and elastase are involved in wound remodeling. Hyaluronate is replaced by dermatan sulfate and chondroitin sulfate in the matrix, and cell migration and differentiation is reduced in the process.
Plasminogen becomes plasmin, which degrades fibrin, and urokinase, released by fibroblast, endothelial cell, keratinocytes and leukocyte, activates collagenase and elastase. Collagenase is released by macrophage, fibroblast, epithelial cell and leukocyte and can breakdown the collagen triple helix. Collagenase activity lasts for several months after tissue injury.
- To be continued -
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