Actin filaments are associated with one end of an individual WPB. (A) Untransfected HUVECs were stimulated for 5 min with 100 ng/ml PMA and then fixed in formaldehyde with a procedure optimal for the actin cytoskeleton (see Materials and methods), costained for VWF (red) and phalloidin (green), and imaged on a confocal microscope. Images shown are maximum intensity projections. (B) Box region in A is shown at a higher magnification and illustrates WPB fusion events associated with actin filament rings. Dotted lines represent regions in which xzy sections were taken. (C and D) Xzy sections of the zoomed in region showing position of actin filaments relative to VWF. Bars: (A) 10 µm; (C and D) 2 µm.