Cytoplasmic dynein drives microtubule sliding and proplatelet elongation. Dynein-mediated sliding of antiparallel microtubule doublets within proplatelet shafts drives elongation under static and physiological shear force in vitro. Presumably, the same occurs in vivo, where shear force applied to megakaryocyte fragments extruded into sinusoidal blood vessels triggers the highly dynamic process of proplatelet formation and platelet biogenesis.