Wound Healing Phases and Factors

Wound healing is an overlapping cascade of four phases: hemostasis, inflammation, proliferation and remodelling. The phases are not separated by hard dates. Each one starts before the previous one ends, so the nurse reads the wound bed and the patient rather than counting days. Hemostasis starts immediately. Damaged vessels contract and a clot of fibrin forms to plug the wound and stop bleeding. The clot also shields the wound against bacteria, gives incoming immune cells a scaffold and stores growth factors. Platelets release platelet-derived growth factor (PDGF) and other signals that draw immune cells and skin cells to the injury. Inflammation starts early and overlaps with proliferation. Neutrophils arrive early, remove necrotic tissue and bacteria, and in the absence of infection decline within a few days. Monocytes enter the wound and become macrophages, which in humans peak about 7 days after injury. Macrophages are master effector cells in repair: they clear debris, release growth factors and shift from a pro-inflammatory to a reparative role that lets the wound move into proliferation. The expected local signs of this phase are redness,...