**6. Platelet growth factors**

A number of growth factors are located in platelet alpha-granules. In order to be released into the injured tissue monocytes, neutrophils, fibroblasts, mesenchymal stem cells and osteoblasts need the chemotactic effect of an available growth factor. Platelet-derived growth factor (PDGF) produces such effect. This growth factor influences mitogenesis of fibroblasts and smooth muscle cells. PDGF improves the formation of fibrous tissue, assists in the three phases of the healing cascade, and also significantly affects angiogenesis and re-epithelialization. Another important growth factor is transforming growth factor beta (TGF-b). It impacts the connection between fibronectins. It influences cell migration, proliferation and replication, and is present during inflammation processes [41].

Chronic wounds' and endochondral ossification's healing processes are highly influenced by vascular endothelial growth factor (VEGF). It is active in angiogenesis, too [13,42]. The next factor taking part in the recovery of chronic wounds is epidermal growth factor (EGF). It also influences the mitogenesis of endothelial cells, fibroblasts, and keratinocytes [42]. Hepatocite growth factor (HGF) is located in various tissues (several types of epithelium, liver, lung, kidney and tumor). It affects tissue regeneration and possesses morphogenic, mitogenic, antiapoptotic, and neurotrophic qualities [38,43-46].
