Correlation of FVIII-positive WPBs with rounded, electron-dense structures containing short VWF tubules. (A) Fluorescence image of BOECs expressing FVIII-GFP showing FVIII fluorescence in green and 4,6-diamidino-2-phenylindole staining in blue. Scale bar represents 10 μm. (B) Overlay of transmission electron micrograph and the fluorescence image of the cells shown in panel A. The black line crossing the image represents a fold in the section. Scale bar represents 10 μm. (C) Zoom of panel B showing a FVIII negative and positive BOECs. (C′) Higher magnification TEM view of the boxed area in panel C showing an elongated WPB that does not contain FVIII. Scale bar represents 250 nm. (D) FVIII-positive structures correlating with FVIII-GFP fluorescence. Scale bar represents 4 μm. (D′-D″) Two consecutive sections of the structures boxed in overlay D. Scale bar represents 250 nm. (E) Digital slice through an electron tomogram of the electron-dense structures shown in D-D″ showing FVIII-positive, electron-dense structures with disorganized, short VWF tubules. Arrows indicate longitudinal sections of VWF tubules. Scale bar represents 250 nm. (F) Three-dimensional model of tomogram shown in panel E (see also supplemental Video 1, available on the Blood Web site; see the Supplemental Materials link at the top of the online article). Yellow represents VWF tubules; and green, vesicle membranes.