Fig. 2. Biogenesis of WPBs in cultured human endothelial cells seen by HPF/FS. (A) Tubule formation in the TGN and the cooperation of AP1 and clathrin in initial formation. (B) Immature WPB with electron-lucent interior, but with a membranous stalk still attaching it to the TGN. (C) Immature WPB with clathrin-coated bud, which is presumably involved in retrieval of material not required in the mature WPBs. (D) Mature WPB showing the remarkable increase in electron density that occurs during maturation. Bars, 200 nm.