As we all know, the four phases of wound healing include the hemostatic phase, the inflammatory phase, the proliferative phase and tissue remodeling.
The hemostatic phase begins immediately following tissue injury. Platelets migrate and bind to the wound site, reducing or stopping blood flow. Fibrinogen is converted to fibrin by thrombin and a complex platelet/fibrin structure is formed. Growth factors released from platelets trapped in this structure initiate the wound healing process.
The inflammatory phase, which can last up to seven days, involves the infiltration of white cells into the wound site. This phase facilitates the removal of foreign material and the release of additional growth factors. Subsequently, the proliferative phase, which is also known as the fibroblastic phase, involves tissue regeneration, angiogenesis, matrix formation and epithelialization.5 The final phase of wound healing, remodeling, can last up to 24 months after the proliferative phase. In this phase of wound healing, enzyme complexes proteolytically remove excess tissue matrix while there is a continual process of collagen synthesis and breakdown.6 It is important to note these phases of wound healing overlap each other and are not separately identifiable chronological events.
Human platelets play an important role in controlling bleeding. They interact with the fibrin network and create a platelet/fibrin “plug” that achieves hemostasis. Perhaps more importantly, alpha granules on the surface of human platelets store most of the growth factors needed to start the healing process. These growth factors are immediately released from platelets activated by the clotting process occurring in the wound. These growth factors are involved in every phase of wound healing and are critical for any wound to heal.5
Researchers have identified a number of growth factors, including: PDGF (platelet-derived growth factor); EGF (epidermal growth factor); TGF-Beta (transforming growth factor-beta); VEGF (vascular endothelial growth factor); and IGF-I (insulin growth factor-I). Platelet-derived growth factors affect chemotaxis and migration (recruiting stem cells to the wound site). They also attach to cell receptors and control the genetic expression of stem cells via modulation of signal transduction pathways of secondary proteins, resulting in cellular division and differentiation. They promote angiogenesis and start the tissue regeneration and remodeling process.5