Simian virus SV40 large T Antigen expression in the islets of Langerhans of transgenic mice results in beta-cell hyperproliferation, onset of new blood vessel formation and the development of highly vascularized solid tumors. Angiogenesis in the RIPTag mouse model, as well as in human cancer, is a hallmark of multistage tumorigenesis and precedes the development of solid tumors. In our study, intravital microscopy was used to monitor changes in the blood vessel phenotype, microcirculation and leukocyte adhesion during the progression from normal islets to angiogenic islets and solid tumors. In RIP I-Tag5 mice, an aberrant microangioarchitecture becomes apparent in early stages during spontaneous tumor development. Notably, the transition from normal to angiogenic islets is characterized by an increase in vessel diameter rather than vessel numbers. Thus, dilatation of existing vessels precedes vessel sprouting. Once initiated, neovascularization in angiogenic islets results in loss of vessel hierarchy and differentiation. Solid insulinomas display a higher vessel density and even more dramatic vessel heterogeneity as revealed by local "hot spots" of neovascularization and irregular vessel diameters. Strikingly, profound changes in the microangioarchitecture are already observed in early angiogenic islets suggesting that key features of the angiogenic vasculature are established prior to the expansion of tumor mass. Moreover, adhesion of leukocytes was found to be dramatically decreased in both angiogenic islets and solid tumors and correlates with morphological alterations of the vasculature. Thus, vessel transformation and reduced leukocyte-endothelium interactions are not exclusively features of solid tumors but represent early events during tumorigenesis. (C) 2002 Wiley-Liss, Inc.