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 ␣-granules revealed large heterogeneity in their membrane organization. On the basis of their divergent morphology, we categorized ␣-granules into the following subtypes: spherical granules with electron-dense and electron-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 ␣-granule subtypes. Tubular ␣-granules represent approximately 16% of the total ␣-granule population and are detected in approximately half of the platelet population. They express membrane-bound proteins GLUT3 and ␣IIb-3 integrin and contain abundant fibrinogen and albumin but low levels of -thromboglobulin and no von Willebrand factor. Our 3D study demonstrates that, besides the existence of morphologically different ␣-granule subtypes, high spatial segregation of cargo exists within individual ␣-granules. (Blood. 2010;116(7):1147-1156)
IntroductionBlood platelets are the smallest cells in our circulation. They play a central role in the arrest of bleeding after damage of a blood vessel and are crucial elements in the development of thrombosis. 1,2 On injury, platelets rapidly adhere to components of the subendothelium, followed by shape change and subsequent granule secretion. 3 These rapid membrane dynamics are crucial for the progression of platelet-substrate interaction (spreading) and subsequent plateletplatelet interaction (aggregation), ultimately leading to the formation of a platelet plug and the arrest of bleeding. 4 Platelets contain several distinct membrane systems: (1) the open canalicular system (OCS), which is continuous with the cell surface and serves as a membrane reservoir during shape change and spreading 5,6 ; (2) the dense tubular system (DTS), representing the platelet smooth endoplasmic reticulum 7 ; and (3) secretory organelles. Four types of secretory organelles have been identified in platelets, based on their ultrastructure and selective protein composition: ␣-granules, dense granules, multivesicular bodies, and lysosomes. 8-10 ␣-Granules are the major secretory organelles and appear in electron microscopy cross sections as 200-to 500-nm spherical organelles. Platelet ␣-granules and dense granules are differentially released and play crucial roles in the secondary platelet response. 11,12 Recent studies have suggested the existence of ␣-granule subclasses with dif...