Different dimeric forms of platelet-derived growth factor (PDGF) are key regulators of connective tissue cells in embryogenesis and in the pathogenesis of a number of diseases. During development. PDGF-B/ PDGF β receptor signaling controls mesangial cell development in kidney glomeruli (Leveen et aL, 1994: Lindahl et aL, 1998) and the recruitment of vascular smooth muscle cells (SMCs) and pericytes in developing blood vessels (Lindahl et aL, 1997a; Crosby et al., 1998; Hellstrom et aL, 1999), while PDGF-A/ PDGF a receptor signaling is critical for the recruitment of alveolar SMCs and alveogenesis (Bostrom et dl., 1996; Lindahl et aL, 1997b) and for 0-2A progenitor cell expansion in developing rat optic nerve, which gives rise to oligodendrocytes and type 2 astrocytes (Noble et aL, 1988; Raffe/ al., 1988). The newly defined PDGF-C also targets mesenchymal cells, and may be particularly important in the development of the kidney mesenchyme and physiological and pathophysiological remodeling of the cardiac interstitium (Li et aL, 2000). The potent effects of PDGF on mesenchymal cells in development, particularly SMCs, is recapitulated in wound healing and in multiple inflammatory diseases, where PDGF appears to be important for the recruitment of connective tissue cells and matrix deposition in the fibrotic response.