Wound healing is the process in which the skin repairs itself after injury. Wound healing is divided into four sequential and overlapping states :
The skin consists of the dermis and epidermis that exist in equilibrium. Once the skin is broken the process of wound healing begins by the aggregation of platelets to the injury site to form a fibrin clot. The purpose of the clot is to control bleeding. This process is called homeostasis. It involves three concepts: vasoconstriction, platelet activation, and coagulation. In the inflammatory phase, phagocytes remove debris and bacteria. Platelets become activated by various mediators released during injury.
As the inflammatory phase subsides, the proliferative phase takes over. After homeostasis and inflammation have stabilized the injury, the repair process begins. This can be divided into four phases: revascularization/angiogensis, granulation, epithelialization and contraction. In angiogenesis, vascular endothelial cells form new blood vessels. This is necessary in order to bring oxygen and nutrients to cells involved in the reconstruction process. In granulation tissue formation fibroblasts, attracted by mediators migrate to the affected area becoming the predominant cells. Fibroblasts are necessary to form the new extracellular matrix by supplying collagen and fibronectin. At the same time the epidermis is reepithelialized. In contraction, the final stage, the open wound is closed. This is caused by fibroblast expressing protein that causes the sides of the wound to close. Even after the wound is closed remodeling continues over the next few months.
In the maturation and remodeling phase, collagen is remodeled and realigned along tension lines and cells that are no longer needed are removed by apoptosis. This process may be interrupted leading to the formation of non-healing chronic wounds. This may be seen in diabetes, venous of arterial disease, old age, and infection.