WPB growth is achieved by adding nontubulated VWF to the vesicle lumen. WPB elongation and condensation occurs by clathrin-mediated membrane remodeling. Electron tomograms of WPBs localized by CLEM revealed insights in mechanisms that seem to contribute to WPB growth, elongation, and membrane remodeling. (A-Ai) Tomogram sections of a WPB formed 4 hours after transfection (hpt). In the transition from Golgi to the forming WPB, a dense cluster of nontubulated VWF is observed, whereas VWF in the WPB lumen is tubulated (t). Also note the clathrin coat on the membrane of the WPB (arrowheads). (B-Bii) Tomogram sections of a WPB 8 hpt. In the Golgi, a dense cluster of nontubulated VWF is visible, which seems to be delivered to the forming WPB via the connection visible between the Golgi and WPB membrane. In the WPB tubules of VWF (t) are observed. The arrowheads indicate the extensive clathrin coat observed on the WPB membrane. This WPB is also presented in Figiugre 4E-Ei. (C) Tomogram section of a WPB 8 hpt. Close to the WPB, a small vesicle is observed that contains one tubule (arrow). On the WPB membrane, an extensive clathrin coat is observed (arrowheads). (D) Tomogram section of a WPB 8 hpt. The irregular shaped membrane (arrow) suggests fusion between the WPB and vesicles as shown in C. Also, an abundant clathrin coating (arrowheads) was observed on the WPB membrane. (E) Tomogram section of a WPB 6 hpt. In the WPB membrane, clathrin-coated pits are observed (arrowheads), which suggest clathrin-mediated compensatory membrane retrieval. (F) Tomogram section of a WPB 8 hpt close to the Golgi apparatus. On the WPB membrane, more pronounced clathrin-coated pits are observed as indicated by the arrowheads. Scale bars: 250 nm. Tilt series for tomography were acquired using an FEI Tecnai 12 at 120 kV and using an FEI Eagle 4k × 4k CCD camera. Tilt series for tomography were collected with SerialEM. Tomogram reconstructions were performed in IMOD. Go, Golgi; nt, nontabulated.