The possibility that growth factors may represent important constituents in normal growth and development received its impetus with the discovery of epidermal growth factor (EGF) and the demonstration that EGF was responsible for premature eyelid opening and tooth eruption.'Subsequent to the discovery, purification, and characterization of EGF, a number of other growth factors have been discovered, many of which are in the process of purification and characterization. Two of these, platelet-derived growth factor (PDGF) and macrophage-derived growth factor (MDGF), were postulated to exist, based upon the knowledge that cells in culture required whole blood serum for continuing proliferation'and upon the observation that when healing wounds were deprived of activated macrophages, the process of wound fibroplasia was markedly de~ reased.~ Several attempts were made to purify the growth factors present in whole blood serum by chromatographic fractionation, which yielded different molecular weight constitutents that appeared to have mitogenic activity. 2 The discovery that one of the principal growth factors present in whole blood serum was derived from the platelet, was based upon the observation that serum prepared from cell-free plasma lacked any mitogenic activity. 495 This led to a series of experiments demonstrating that platelets were the only blood cells that contained significant mitogenic activity, which was released during the process of blood coagulation. 5* 6 This mitogen, PDGF, has been highly purified:-" and has received recent widespread attention based on its extensive homology with the product of an oncogene.'

PDGF is a highly cationic (pl-c 10) 30,000 molecular weight glycoprotein that is composed of two subunits of approximately 17,000 and 14,000 molecular weight each, which are disulfide bonded. It is released from platelets at sites where platelet adherence or aggregation will lead to platelet shape change and to secretion of the contents of the platelet's alpha-granules.