ULVWF strings colocalized with P-selectin on endothelial cells. HUVECs were stimulated with histamine and perfused with PBS buffer containing 1% paraformaldehyde at a flow rate of 0.1 mL/min for 20 minutes. After perfusion, cells were stained with FITC-conjugated monoclonal anti-VWF and Texas Red–conjugated polyclonal anti–P-selectin antibodies. ULVWF formed stringlike structures in the absence of platelets (A). P-selectin expression was in clusters (B), and most of these P-selectin clusters were located along the ULVWF strings (C). The figure represents 8 independent experiments (bar = 100 μm).