3D ultrastructure of unstimulated platelets using FIB-SEM. (A-C) 3D representations of a human platelet showing (A) the openings of the OCS at the platelet surface (gray), (B) the spatial arrangement of the OCS (blue) and of the closed canalicular system (arrow), and (C) the distribution of α granules (yellow), δ granules (black), and mitochondria (dark blue) in the whole platelet (see supplemental Movie 1 for an animation). (D-G) Images from TEM thin sections and (E-G) 3D FIB-SEM reconstructions of δ granules. These granules are mostly spherical, contain 1 or 2 characteristic electron-dense substructures, and may display a tail extension. (H-L) Images from TEM thin sections and 3D FIB-SEM reconstructions of α granules. (I) Oval α granules contain a nucleoid (*) shown in green and VWF (arrowheads) shown in red. Also shown are (J) a haltere and (L) tubular α granules that has developed a long extension. Scale bars, 200 nm.