Cellular events that follow tissue damage are controlled among e-book others by platelets and the released growth factors. Platelets release a large variety of growth factors and cytokines after they adhere, aggregate, and form a fibrin mesh [13]. Furthermore, artificial recombinant growth factors often require further synthetic or animal proteins as carriers. PRP in contrast could serve as a natural carrier itself [14].3. Mechanisms of PRP on Repair of Bone DefectsBone has a substantial capacity for repair and regeneration in response to injury occurs by surgery, various diseases, or trauma. Both processes involve a complex integration of cells, growth factors, and the extracellular matrix [15, 16]. PRP can potentially enhance healing by the delivery of various growth factors and cytokines from the ��-granules contained in platelets [17].

The basic cytokines, which identified platelets, play important roles in cell proliferation, chemotaxis, cell differentiation, and angiogenesis. Bioactive factors are also contained in the dense granules in platelets. The dense granules contain serotonin, histamine, dopamine, calcium, and adenosine [18]. These nongrowth factors have fundamental effects on the biologic aspects of wound healing. At present, the molecular mechanisms of bone defect repair studies have focused on three aspects of the inflammatory cytokines, growth factors, and angiogenic factors. The role of PRP works through these three aspects of bone repair [19]. Growth factors and cytokines in PRP associated with diferent mechanisms are showed in Table 1.

Table 1Growth factors and cytokines in PRP in different mechanisms.3.1. PRP in the Role of Inflammatory Cytokines Promotes Bone RepairThere is increasing evidence that inflammation plays a vital role in early fracture repair [20]. Consequently, platelets are stimulated to aggregate and secrete growth factors, cytokines, and hemostatic factors critical in the early stages of the intrinsic and extrinsic pathways of the clotting cascade. Inflammatory reactions involve a number of biochemical and cellular alterations, the extent of which correlates with the extent of the initial trauma [21, 22]. Histamine and serotonin are released by platelets and both function to increase capillary permeability, which allows inflammatory cells greater access to the wound site and activates macrophages [23, 24].

Adenosine receptor activation modulates inflammation during wound healing [25]. The major proinflammatory Batimastat cytokines that are responsible for early responses are IL1, IL6, and TNF-alpha [26, 27]. The expression of TNF-�� and IL-1 in fractures follows a biphasic pattern, with a peak during the initiation of fracture repair, followed by a second peak at the transition from chondrogenesis to osteogenesis during endochondral maturation [28, 29].