Periendothelial location of bone marrow-derived cells in angiogenic vessels. Angiogenesis was induced in the ears of mice that underwent BM transplantation by repeated subcutaneous VEGF protein injections (A) or by subcutaneous implantation of B16 melanoma (B-D). The tissues were studied by immunofluorescence microscopy (C) or by laser scanning confocal microscopy (A-B, D). (A-B) Vascular endothelial cells stained for von Willebrand factor (VWF, A) or CD105 (B) demonstrate the periendothelial location of the GFP⁺ BM-derived cells. Note that no GFP⁺ endothelial cells can be detected. (C) The abundant number and close association of BM-derived cells to endothelial cells (CD31 staining, red) can also be seen in the peritumoral area where larger arterioles with no apparent angiogenic activity are seen. (D) A 3-dimensional projection digitally reconstituted from stacks of confocal optical slices demonstrates the periendothelial location of the BM-derived GFP⁺ cells bordering the vascular endothelial cells expressing VWF (red). Bars represent 40 μm.