The platelet interior revisited: Electron tomography reveals tubular α-granule subtypes

We have used (cryo) electron tomography to provide a 3-dimensional (3D) map of the intracellular membrane organization of human platelets at high spatial resolution. Our study shows that the open canalicular system and dense tubular system are highly intertwined and form close associations in specialized membrane regions. 3D reconstructions of individual α-granules revealed large heterogeneity in their membrane organization. On the basis of their divergent morphology, we categorized α-granules into the following subtypes: spherical granules with electron-denseandelectron-lucent zone containing 12-nm von Willebrand factor tubules, subtypes containing a multitude of luminal vesicles, 50-nm-wide tubular organelles, and a population with 18.4-nm crystalline cross-striations. Low-dose (cryo) electron tomography and 3D reconstruction of whole vitrified platelets confirmed the existence of long tubular granules with a remarkably curved architecture. Immunoelectron microscopy confirmed that these extended structures represent α-granule subtypes. Tubular α-granules represent approximately 16% of the total α-granule population and are detected in approximately half of the platelet population. They express membrane-bound proteins GLUT3 and αIIb-β3 integrin and contain abundant fibrinogen and albumin but low levels of β-thromboglobulin and no von Willebrand factor. Our 3D study demonstrates that, besides the existence of morphologically different α-granule subtypes, high spatial segregation of cargo exists within individual α-granules.