Rounded, electron-dense structures are positive for both FVIII and VWF. (A-C) TEM views of immunogold-labeled WPBs from FVIII-GFP–transduced BOECs, double-labeled with anti–VWF (10 nm gold), and anti-GFP (15 nm gold) to detect FVIII-GFP. Scale bars represent 200 nm. (A) WPBs positive for VWF and FVIII mostly had a rounded morphology. (B) Intermediate, “pear”-shaped WPBs containing both FVIII and VWF. (C) Elongated WPBs are only positive for VWF. (D) Size distribution of WPBs found in nontransduced and FVIII-transduced BOECs. FVIII-negative WPBs (●) had an average length and width of 701 ± 29 nm and 186 ± 58 nm (mean ± SD), respectively. On the other hand, FVIII-positive WPBs (○) were 360 ± 111 nm by 269 ± 81 nm (mean ± SD). Data includes both round and intermediate shaped FVIII-positive WPBs. (Inset) Diameter range of 88 selected FVIII-containing WPBs from panel D that were truly round.