Fibroblast proliferation induced by blood cells.

Growth factors have been isolated from three cell types associated with the vascular system; the platelet, the monocyte/macrophage, and the endothelial cell. Each of these mitogens appears to have different characteristics. The platelet-derived growth factor is a cationic protein (pI 9.8) that is heat stable, of molecular weight approx. 30,000 and is located in the alpha granules of the platelet. The growth factors derived from the monocyte/macrophage and the endothelial cell each appear to be distinct and different proteins. Each of these factors was discovered as a result of the ability to induce quiescence in the G1/G0 state of cells in culture, that can be readily induced in the presence of cell free plasma-derived serum in the culture medium. Cells such as fibroblasts, smooth muscle, 3T3 cells and glial cells will remain quiescent in culture medium containing plasma-derived serum for periods up to 6 weeks. Stimulation by the platelet-derived growth factor or macrophage-derived growth factor, will commit these cells to enter S and begin cell cycle traverse. Inhibition of platelet function in vivo can be shown to inhibit intimal smooth muscle proliferation in response to endothelial injury. Thus, each of these factors may play important roles in the process of initiation of fibroplasia in wound repair, smooth muscle proliferation in atherogenesis and endothelial proliferation in response to injury.