Analysis of organelle content supports a membrane budding model of platelet biogenesis

Understanding how in vivo platelet biogenesis is undertaken is critical to making on-demand platelet production for clinical use feasible. We recently described the discovery of plasma membrane budding as a major in vivo platelet-producing pathway. In vitro recapitulation of this finding could pave the way towards efficient laboratory-based platelet production. The plausibility of the plasma membrane budding model has been called into question. The foundation of this is the contention that the size and payload composition of plasma membrane buds are not consistent with bona fide platelets. Thus, membrane buds likely represent stages in megakaryocyte-derived microparticle formation. Using 3D super-resolution imaging, we have performed a quantitative comparison of size and organelle content of plasma membrane buds, platelets, and microparticles in the adult mouse bone marrow. We unequivocally demonstrate that the structures we previously described as membrane buds exhibit the same size range as free platelets, that all buds contain organelles, and that membrane buds and free platelets contained an equivalent number of organelles. Crucially, membrane buds and microparticles are completely distinct from each other. To prevent future confusion between the processes of microparticle formation and platelet biogenesis, we propose using the more specific term "pre-platelet membrane buds".