Proplatelet fission model of platelet release. Model of platelet production suggested by these experiments and previous studies (see Results). (A) Released proplatelets undergo successive rounds of fission along their midbody and at their ends. This process is mediated by the formation of a cleavage furrow at the point of division and results in platelet release from proplatelet ends at an increasing rate in time as more ends become available after each fission event. Shear promotes proplatelet fission and drives platelet release. (B) Barbell proplatelets of ∼30–50-µm perimeter reversibly convert into preplatelets. This process is driven by twisting microtubule-based forces and may represent a novel mechanism of microtubule reorganization and granule redistribution after each fission event. (C) During barbell proplatelet formation, dynamic and bidirectional assembly and reorganization of microtubule coils mediate platelet cytoskeleton arrangement as α- and dense granules track to distal proplatelet tips. Platelets release from proplatelet ends after the final fission event.