1. Coagulation phase: 0-2hrs. At a site of injury, exposed basal lamina collagen (type IV) activates plasma thrombin to cleave fibrinogen and form a fibrin mesh. The fibrin mesh traps platelets, which aggregate, modify and degranulate, releasing pro-inflammatory agents that include platelet-derived growth factor (PDGF). PDGF is a chemoattractant for neutrophils, macrophages and fibroblasts.
2. Inflammatory phase: 0-3 days. Neutrophils, the first leukocytes to arrive in the wound space, remove necrotic debris and kill bacteria. In a respiratory burst, neutrophils consume oxygen and secrete peroxidases for microbial killing. After 24 hours, macrophages/monocytes predominate to regulate inflammation and tissue repair. They further remove bacteria and debris. Activated macrophages release growth factors that promote angiogenesis, granulation tissue formation and cross communication with B-cell and T-cell mediated immune responses. Studies show the need for neutrophil “priming” to facilitate proper macrophage growth factor effect.
3. Proliferation phase: 3-21 days. Macrophages secrete transforming growth factor beta (TGF-ß), which enhances fibroblast differentiation and proliferation. Fibroblasts, the predominate cell type found in granulation tissue, produce extracellular matrix and collagen scaffolding, a strut for granulation (type I in most tissues). Extracellular matrix (ECM), composed of hyaluronic acid, fibronectin and glycoproteins, regulates granulation tissue fluid composition and is fundamentally important in orchestrating cell-cell interaction, proliferation, migration, differentiation and adhesion.
As granulation tissue develops, blood vessels invade the collagen scaffolding. This process of angiogenesis is directed through macrophage-released vascular endothelial growth factor (VEGF). Macrophages also secrete matrix-modifying enzymes, matrix metalloproteases (MMP), which degrade local cellular attachments to collagen/gel matrix. Fibroblast and keratinocytes are freed to migrate. Epithelialization proceeds as keratinocytes divide and migrate, covering the wound bed.
4. Maturation phase: two weeks-years. The invasion of tissue-specific cells, wound contraction and collagen deposition typifies maturation. Maturing adult wounds heal by scarring with significant collagen deposition and a 20 percent reduction in strength. Conversely, neonatal wound maturation produces supple, stronger tissue with less collagen deposition. TGF-ß appears to regulate the amount of hyaluronic acid, collagen deposition and scarring. TGF-ß1 is associated with neonatal tissue regeneration while TGF- ß3 is more commonly associated with adult tissue repair.