Wound Dressing

Figure:Hand abrasion, 30 minutes after injury.
Any injury results in disruption of blood vessels leading to haemorrhage which is controlled by clot formation which contains fibrin mesh with aggregated platelets embedded in it. Fibrin is the end product of coagulation pathway and besides causing heamostasis it is also the primary component of the provisional tissue matrix seen in the early phases of wound healing. It provides a scaffold for the migration of inflammatory and mesenchymal cells. Platelet aggregation is a vicious cycle and leads to release of cytokines, which includes PDGF, TGF-a, FGFb, PDEGF. These cytokines influence wound healing directly or indirectly. The processes of clot formation and platelet aggregation terminate when stimuli for clot formation dissipate. Lysis of clot starts along with clot formation and is mediated by plasminogen activator, which converts plasminogen to plasmin.

Figure 2 : 2 days after injury.
Tissue trauma stimulates the inflammatory response. Immediately after injury intense local vasoconstriction occurs, mediated by circulating catecholeamines and prostaglandins released by injured cells. This is followed by vasodilatation and increased capillary permeability resulting in local edema. This is mediated by histamine, kinins, prostaglandins, leukotrienes, and endothelial cell products. Neutrophils are the first leukocytes to be found in wounded tissues. They phagocytose damaged tissue or bacteria. Neutrophils themselves are phagocytosed by macrophages. Pain in the area of injury is due to changes in pH due to break down of tissues and bacteria along with swelling and decreased tissue oxygenation due to disruption of blood vessels. Neutrophil count of the wound increases for 24-48 hrs and then declines unless wound contamination has occurred.
Monocytes transform into macrophages as they migrate from capillaries into extra vascular space. Macrocytes phagocytose bacteria and tissue debris and secrete enzymes (collagenase and elastase) responsible for breaking down damaged matrix. They also cytokines, P.G.s, oxygren free radicals and other regulators of wound healing.
Lymphocytes produce various factors like HB-EGF(Heparin binding epidermal growth factor), basic fibroblast growth factor and they are also involved in cellular and humoral immunity. Initially for 24-48 hrs neutrophils dominate but 48-72hrs later they are outnumbered by macrophages which persist for few days. After 5-7 days fibroblasts are the predominant cell type.