Proposed model for macrophage-mediated angiogenesis. M1 macrophages promote sprouting of blood vessels via secretion of VEGF, bFGF, IL8, RANTES, and TNFa. M2a macrophages promote fusion of blood vessels through as-yet unidentified secreted factors. M2a macrophages may also regulate the actions of M1 macrophages via production of TIMP3, and may recruit pericytes via secretion of PDGF-BB, although this was not directly assessed in this study. M2c macrophages may function in vascular remodeling, given their high levels of production of MMP9. Reproduced with permission from .