Morphology of VWF strings. (A) Immunofluorescence image of a stimulated HUVEC labeled with an antibody against VWF showing numerous VWF strings branching at the surface of the cell. Scale bar represents 5 μm. (B) Scanning EM of VWF strings at the surface of stimulated HUVECs displaying numerous branching VWF strings. Note the presence of pore in the membrane (right top corner). Scale bar, 5 μm. (C) Scanning EM of a pore in the membrane representing the site of exocytosis of WPB. VWF strings are branching and bundling at the surface of the cell. Scale bar, 1 μm.