VWF and neoplasm: Furthermore, immunofluorescence staining of von Willebrand factor (vWF), a microvascular endothelial marker, also revealed reduced average blood vessel density in tumours obtained from the metformin-treated animals (69 ± 29% over control group, P < 0.02; Figures 3(b) and 3(d)), implicating an antiangiogenic impact of metformin.