Immunoelectron microscopy of VWF strings. HUVECs were incubated with medium containing 100 μM histamine for 10 minutes with gentle rocking, washed with DPBS, and fixed with 3% paraformaldehyde. Cell-surface VWF multimers were labeled with 12-nm gold-conjugated antibody and visualized by quick-freeze deep-etch electron microscopy. Multiple VWF strands (A) form twisted bundles that sometimes bifurcate (B) and connect with one another to form networks (C). Arrows indicate branching points. Bars represent 500 nm.