The Healing Cascade of Platelets. Dynamic Regenerative Medicine
The healing of soft or hard tissues involves a sequence of events that begin at the moment of injury and continue for several subsequent months and can be divided into three stages:
1) inflammation, 2) proliferation and 3) remodeling
The first, inflammation stage, involves activation of platelets. During this process of the platelets begin the coagulation cascade and release cytokines (Inflammatory mediators), which begin the healing process. The these inflammatory mediators/cells attract white blood cells (WBC), which begin to migrate to the injured area. The first WBC to be recruited are responsible for the initial local cleansing by removing bacteria and cellular debris.
Over the next few days, a proliferative phase occurs in which further inflammatory cells migrate to the injured area, attracted by chemical signals from the growth factors. These circulating cells are differentiated into further inflammatory cells known as macrophages and begin to perform the signaling and modulation function that to this point had been per- formed by the platelets, which begin to vacate the area. The macrophages debride the area and secrete growth factors responsible for initiating new healing events such as the formation of new tissue. The development depends on the recruitment of vascular cells.
The arrival of stem cells and their differentiation into specific tissues such as bone, cartilaginous and vascular tissue begins in this phase and is dictated by chemical signaling.
During the final, or remodeling, phase, the collagen contracts and the edges of the injury are drawn together. Cell density and vascularisation decrease, excess repair matrix is removed and the collagen fibers are aligned along the stress lines, which increases the strength of the newly formed tissue. Granulation tissue accumulates and either slowly remodels the scar tissue or is transformed into specific tissues such as skin and bone.