Fig. 1. Weibel-Palade bodies forming in HUVECs show some connections to the Golgi and are often clathrin-coated. Cultures of HUVECs prepared for conventional transmission EM after chemical fixation give hints of the early biogenesis of WPBs. (A) Swollen Golgi cisternae (arrows) may be the earliest stage of WPB biogenesis, although no striations are apparent. Bar, 200 nm. (B) Weibel-Palade bodies juxta-Golgi have striations (arrows) and a clathrin coat (arrowheads) but the relationship to the Golgi is unclear. Bar, 200 nm. (C) Candidate WPBs (arrows) contain striations and have varying degrees of clathrin coat, but direct connections to the Golgi are unclear. Bar, 200 nm. (D) A single image showing the connections of a WPB that is connected (arrow) to the Golgi. Bar, 200 nm.