Fusion of WPBs with secretory pods revealed by electron tomography. (A) Digital slice through an electron tomogram of a WPB (asterisk) fusing with an SP. Note the presence of VWF tubules (short arrow), one of which appears to unfurl (long arrow). (B) Three-dimensional model of the tomogram shown in panel A. (C-D) Example of an SP making contact with a WPB via nanovesicles. (C,C′) Two digital slices through a tomogram showing the nanovesicles (arrowhead) intercalated between the WPB (asterisk) and SP. (D) Three-dimensional model of the tomogram shown in panels C and C′. (E-F) Nanovesicles fusing with a WPB and SP. (E-E″) Series of digital slices through a tomogram of a WPB (asterisk) in the process of fusing with an SP. Open arrowheads indicate nanovesicles whereas the black arrowhead points toward an electron lucent area interposed between the WPB delimiting membrane and the tubular matrix. (F) Three-dimensional model of the tomogram shown in panels E to E″. Scale bars represent 100 nm. Tilt series for tomographic reconstruction were acquired with an FEI Tecnai 12 TEM at 120 kV and using an FEI Eagle 4kx4k CCD camera.