Hypoxia in tumors increases HIF1, which, partly by inducing VEGF and SDF1α in tumor cells, recruits BMDC including EPC, PPC, and CD45+ monocytic vascular modulatory cells to endorse vascular remodeling in glioblastomas. SDF1α serves as a retention factor of CXCR4+ vascular progenitor and monocytic BMDC in GBM. HIF1 not only induces VEGF transcription in GBM, but also increases VEGF activity by recruiting CD45+ BMDC that carry and secrete MMP-9 to the tumor site, which in turn makes sequestered VEGF bioavailable for its receptor VEGFR2.