Proposed model of platelet exocytosis. Platelets release the contents of their secretory granules through two modes of exocytosis, single or compound, depending on the strength of the stimulation. The initial fusion routes occur through the OCS and thin narrow channels (blue lines) and form small pores at the platelet surface, whereas later fusion pathways involve direct interaction with the cell surface and result in the formation of large pores. In case of weak stimulation, fusion of individual α and δ granules with the plasma membrane is observed (ie, single exocytosis). After strong stimulation, VAMP8-mediated homotypic fusion of α granules (ie, compound exocytosis) takes place. Two types of compound exocytosis may occur: (1) α granules fuse with each other and then fuse with the plasma membrane or (2) a single α granule fuses first with the plasma membrane and then other granules subsequently fuse with this granule. Highly activated platelets are completely degranulated, retaining only mitochondria and the dense tubular system in their cytoplasm. DTS, the dense tubular system.