HGF and neoplasm: Mechanisms driving acquired resistance include the recruitment of perivascular cells, such as pericytes, which protect immature tumor blood vessels from destruction by anti-angiogenic agents191; the up-regulation of compensatory pro-angiogenic signaling pathways, such as hepatocyte growth factor (HGF), bFGF, PlGF, and Dll4192, 193, 194; and the mobilization of bone marrow-derived cells, including EPCs,195 pericyte progenitor cells,196 and tumor-associated macrophages.197