Platelet-derived growth factors (PDGF) play a major role in pericyte recruitment in tumor capillaries. Pericytes are required for proper vessel development, and contribute to tumor angiogenesis by promoting stabilization and maturation of newly formed vessels. To investigate the effects of pericyte coverage on tumor vessel morphology and function in vivo, tumors derived from B16 melanoma cells transfected with either control plasmid (B16/ctr) or plasmid encoding full-length PDGF-BB (B16/PDGF), the latter previously shown to have enhanced blood vessel pericyte coverage and an increased tumor growth rate, were assessed using histopathological methods, Hoechst 33342-based perfusion analyses, and two noninvasive susceptibility magnetic resonance imaging (MRI) methods. Susceptibility-contrast MRI, incorporating the use of ultrasmall superparamagnetic iron oxide particles, revealed a significant (p <0.05) reduction in vessel size index (R-nu) of B16/PDGF tumors, and which was validated histologically by the presence of significantly smaller (p <0.001), more punctate blood vessels identified by fluorescence microscopy of the perfusion marker Hoechst 33342. Intrinsic-susceptibility MRI was used to measure the transverse MRI relaxation rate R-2*, sensitive to changes in endogenous paramagnetic [deoxyhaemoglobin], and used to probe for vascular maturation and function. Hypercapnia (5% CO2/95% air) induced a negligible Delta R-2* response in the B16/ctr and B16/PDGF tumors. In contrast, hyperoxia (5% CO2/95% O-2) induced a significantly greater R2* reduction in the B16/PDGF tumors (p <0.02). Together the susceptibility MRI-derived biomarkers reveal novel pericyte-dependent changes in the morphology and function of the perfused tumor vasculature in vivo. (c) 2007 Wiley-Liss, Inc.