Wound-healing is often divided into three or four phases, although the phases overlap considerably:
- Inflammation/debridement Phase: Injury to blood vessels in the skin triggers the haemostatic pathways that result in coagulation and clot formation. Other chemicals, such as cytokine and growth factors, are released and these draw inflammatory cells into the area. The blood clot helps to stabilize the wound edges and limits ingress of bacteria. As inflammation progresses, the inflammatory cells (macrophages and neutrophils) aid in debridement by phagocytosis of bacteria and debris, release of collagenases and formation of purulent exudates. This phase starts within minutes of injury and becomes maximal at 2-3 days after injury.
- Proliferation Phase: Growth factors and cytokines released by macrophages stimulate the migration and reproduction of fibroblasts. The fibroblasts produce collagen and other components of fibrous tissue. Angiogenesis is also stimulated by macrophages, directing growth of capillaries into the area. It is this mix of fibroblasts, collagen and new blood vessels that comprises granulation tissue, which usually becomes apparent 3-5 days after injury. Once granulation tissue has developed, epithelialization begins, although this process occurs much earlier (within 24-48 hours ) with sutured wounds. Epithelial cells migrate over the surface of the wound to cover it completely. Once this has happened, further cell division occurs and stratified epithelium begins to be produced. Specialized cells, called myofibroblasts, help wound contraction at this time, reducing the size of the wound.
- Maturation Phase: the number of collagen fibres deposited by the fibroblasts usually reaches its peak 2-3 weeks after injury. After this time, the apparently random deposition of collagen fibres reorganizes to increase wound strength. Fibroblasts and capillaries decrease in number, causing the scar to become paler and flatter. The process of maturation is ongoing, but scars rarely attain more than 80% of the skin's original strength.