Platelets attach to ULVWF strings secreted from HUVECs or GMVECs stimulated by histamine in millimolar or Stxs in nanomolar concentrations. Washed normal human platelets in Ca²⁺/Mg²⁺-free Tyrode buffer were perfused over HUVECs (A-B) or GMVECs (C-D) and attached to the ULVWF strings formed as a result of cell stimulation by 20 mM histamine (A), or 10 nM of either Stx-1 (B-C) or Stx-2 (D). White arrows point to ULVWF strings with adherent platelets. Photographs of ULVWF-platelet strings were taken under flowing conditions 2 minutes after HUVEC stimulation. Each of these photos is representative of 3 to 20 experiments.