Figure 1.

Schematic representation of system of PDEs given in (1), showing the interactions between growth factors, receptors, ligand-receptor complexes, and cell types. The ligand VEGF is denoted by V, Ang-1 by A1, and Ang-2 by A2. Curved arrows indicate the cell type that produced the referenced protein (e.g., hypoxic cells produce VEGF and Ang-2, whereas ECs produce Ang-1 and Ang-2), and straight arrows indicate the physiological response to ligand-receptor binding (e.g., VEGF binding to VEGFR-2 induces angiogenesis). Notice how VEGF and Ang-2 diffuse in the extracellular space, whereas Ang-1 only acts locally.