Platelet-derived growth factor (PDGF) is one among numerous growth factors that regulate cell growth and division. In particular, PDGF plays a significant role in blood vessel formation, the growth of blood vessels from already-existing blood vessel tissue, mitogenesis, i.e. proliferation, of mesenchymal cells such as fibroblasts, osteoblasts, tenocytes, vascular smooth muscle cells and mesenchymal stem cells as well as chemotaxis, the directed migration, of mesenchymal cells. Platelet-derived growth factor is a dimeric glycoprotein that can be composed of two A subunits (PDGF-AA), two B subunits (PDGF-BB), or one of each (PDGF-AB). PDGF is a potent mitogen for cells of mesenchymal origin, including fibroblasts, smooth muscle cells and glial cells. In both mouse and human, the PDGF signalling network consists of five ligands, PDGF-AA through -DD (including -AB), and two receptors, PDGFRalpha and PDGFRbeta. All PDGFs function as secreted, disulphide-linked homodimers, but only PDGFA and B can form functional heterodimers. Though PDGF is synthesized, stored (in the alpha granules of platelets), and released by platelets upon activation, it is also produced by other cells including smooth muscle cells, activated macrophages, and endothelial cells Recombinant PDGF is used in medicine to help heal chronic ulcers and in orthopedic surgery and periodontics as an alternative to bone autograft to stimulate bone regeneration and repair. Types and classification There are five different isoforms of PDGF that activate cellular response through two different receptors. Known ligands include: PDGF-AA (PDGFA), -BB (PDGFB), -CC (PDGFC), and -DD (PDGFD), and -AB (a PDGFA and PDGFB heterodimer). The ligands interact with the two tyrosine kinase receptor monomers, PDGFRα (PDGFRA) and -Rβ (PDGFRB). The PDGF family also includes a few other members of the family, including the VEGF sub-family. Mechanisms The receptor for PDGF, PDGFR is classified as a receptor tyrosine kinase (RTK), a type of cell surface receptor. Two types of PDGFRs have been identified: alpha-type and beta-type PDGFRs. The alpha type binds to PDGF-AA, PDGF-BB and PDGF-AB, whereas the beta type PDGFR binds with high affinity to PDGF-BB and PDGF-AB. PDGF binds to the PDGFR ligand binding pocket located within the second and third immunoglobulin domains. Upon activation by PDGF, these receptors dimerise, and are "switched on" by auto-phosphorylation of several sites on their cytosolic domains, which serve to mediate binding of cofactors and subsequently activate signal transduction, for example, through the PI3K pathway or through reactive oxygen species (ROS)-mediated activation of the STAT3 pathway. Downstream effects of