Figure 1.

A: A PDGF receptor-bearing cell that does not produce PDGF itself responds to PDGF via activation of receptors at the cell surface. This initiates activation of intracellular signaling pathways leading to cell growth, proliferation, and survival. B: In a PDGF receptor-bearing cell that produces PDGF-A or -B, the ligands will meet and activate the receptors already in the ER, Golgi, and secretary vesicles. Whereas some intracellular signaling pathways are activated intracellularly, other pathways important for mitogenesis are not activated until the ligand–receptor complex reaches the cell membrane.