Multigranular compartments result from the homotypic fusion of α granules. (A-B) Ultrastructural TEM analysis of multigranular compartments in platelets stimulated with 1 U/mL thrombin for 10 seconds. Two representative TEM images illustrate the presence of nucleoids (arrows) in the aggregates. (C-D) IEM characterization of the multigranular compartments. Electron micrographs of ultrathin cryosections of platelets stimulated with 0.1 U/mL thrombin. Antibodies were detected with protein A coupled to 10-nm gold particles. (C) Multigranular compartments (*) were identified as fused α granules by immunolabeling with an antifibrinogen antibody. Labeling with antiserotonin was absent from these structures (*), whereas δ granules (δ) were positive. (E-F) Representative TEM images showing that the limiting membrane of the multigranular compartments (*) is not electron-dense (white arrowheads) whereas the plasma membrane is positive for tannic acid (black arrowheads). Note the presence of a δ granule close to the multigranular compartment in (E) without any fusing profile. (G-H) FIB-SEM analysis of the unstained multigranular compartments in platelets stimulated with 1 U/mL thrombin for 10 seconds. An example of the FIB-SEM raw observations and the corresponding 3D reconstruction of stained granules (in red) and an unstained multigranular compartment (in yellow) are shown. Scale bars, 500 nm.