Figure 5.

A−F: Blood vessel lining cells (arrows) that protrude into the lumen of vessels in MCa-IV tumors. Vasculature emptied of blood by perfusion of fixative. A: Protruding cells in a large, thin-walled vessel in a toluidine blue-stained 0.5-μm epoxy section photographed at low magnification. B and C: Protruding cells (arrow) in a tumor vessel (black lumen) labeled with FITC-lectin (green) and rhodamine-cationic liposomes (red). D: Scanning EM view of a vessel with lining cells that protrude into the lumen as in A−C. Extensive cellular overlap, numerous cytoplasmic projections (arrows), and openings between cells (arrowheads) are present. White box marks region shown at higher magnification in Figure 7, B and C ▶ . E: Lining cell with a thread-like process that spans the vessel lumen. F: Overlapping lining cells in tumor vessel. One region (*) of the cell marked 1 is located above cell 2, which in turn is superficial to cells 3 and 4, but another region (**) of cell 1 is beneath cells 2 and 3. Projections from cell 3 appear to penetrate cells 1 and 4 (arrows). Inset shows outline of endothelial cell borders. G: Lining cells with numerous transluminal cytoplasmic processes (arrows) in a blood vessel of a Shionogi male mammary carcinoma. H: Openings (arrowheads) and other defects in the vessel lining of a LS174T human colon carcinoma grown in a SCID mouse. I: Openings (arrowheads) and other defects in the vessel lining of an islet cell tumor in a transgenic RIP-Tag2 mouse. Scale bar in I applies to all figures; bar length represents 25 μm in A and G−I; 50 μm in B and C; 20 μm in D; 10 μm in E; 5 μm in F.