Immunofluorescence analysis of tumor microvessels compared with healthy control skin in ret transgenic mice. Cryosections were stained for VWF and CD31 (A,C). An anti-GPIb antibody was used to identify platelets (B,D). Nuclei were stained with DAPI. Representative images of control skin show VWF localized within the vessel wall, lacking ULVWF fibers within the lumen (A-B, arrowheads) and only few platelets are visible (B, red). By contrast, in tumor microvessels, ULVWF fibers are detectable within the vessel lumen indicating EC activation (C, arrows). These ULVWF fibers bind platelets as shown in the same vessel (D, red, arrows). Insets, A higher magnification of the presented images (n = 4 to 10 animals; scale bars = 20 µm). See also supplemental Figures 1-2.