Schematic diagram of platelet. The platelet is a 2–3 μm discoid cell that contains α-granules, dense granules, and lysosomes. Platelets also contain mitochondria. Tunnel invaginations of the plasma membrane forms a complex membrane network, termed the open canalicular system, that courses throughout the platelet interior. Platelet granule secretion is thought to occur through fusion of granules with either the plasma membrane or the open canalicular system.