Wound Healing

What is wound healing?

Wound healing and wound repair, is an intricate process in which the skin (or some other organ) repairs itself after injury.

In normal skin, the epidermis (outermost layer) and dermis (inner or deeper layer) exists in a steady-stated equilibrium, forming a protective barrier against the external environment.

Once the protective barrier is broken, the normal (physiologic) process of wound healing is immediately set in motion.

The classic model of wound healing is divided into three or four sequential, yet overlapping, phases:
Healing Phase 1 - Haemostasis
Upon injury to the skin, a set of complex biochemical events takes place in a closely orchestrated cascade to repair the damage.
Within minutes post-injury, platelets (thrombocytes) aggregate at the injury site to form a fibrin clot. This clot acts to control active bleeding (haemostasis).

Healing Phase 2 - Inflammatory
In the inflammatory phase, bacteria and debris are phagocytized and removed, and factors are released that cause the migration and division of cells involved in the proliferative phase.

Healing Phase 3 - Proliferative
The proliferative phase is characterized by angiogenesis, collagen deposition, granulation tissue formation, epithelialization, and wound contraction. In angiogenesis, new blood vessels are formed by vascular endothelial cells.
In fibroplasia and granulation tissue formation, fibroblasts grow and form a new, provisional extracellular matrix (ECM) by excreting collagen and fibronectin. Concurrently, re-epithelialization of the epidermis occurs, in which epithelial cells proliferate and 'crawl' atop the wound bed, providing cover for the new tissue.
In contraction, the wound is made smaller by the action of myofibroblasts, which establish a grip on the wound edges and contract themselves using a mechanism similar to that in smooth muscle cells. When the cells' roles are close to complete, unneeded cells undergo apoptosis.

Healing Phase 4 - Remodelling
In the maturation and remodelling phase, collagen is remodelled and realigned along tension lines and cells that are no longer needed are removed by apoptosis.
However, this process is not only complex but fragile, and susceptible to interruption or failure leading to the formation of chronic non-healing wounds. Factors which may contribute to this include diabetes, venous or arterial disease, old age, and infection