Myeloid precursor cells adhere to angiogenic endothelium via activated α4β1 or β2 integrins. Stromal derived growth factor 1 (SDF-1), colony stimulating factors (GM-CSF and others) or β-defensin mobilize specific subsets of myeloid precursor cells. In the presence of specific cytokines and growth factors, these cells differentiate either into endothelial-like cells, which are directly incorporated into new blood vessels (green arrow), or into monocytes / macrophages, which support tumor growth in a indirect manner (blue arrow). While interleukin-10 promotes differentiation of monocytes into M2 macrophages, VEGF and other factors promote endothelial cell differentiation from progenitor cells.