Megakaryocytes (MKs) are differentiated through linage commitment from hematopoietic stem cells and terminally differentiated MKs release platelets. The processes of megakaryopoiesis and subsequent thrombopoiesis are unique and complex. The underlying mechanisms of MK development and platelet production, beginning with stem cells, are not fully understood because of the difficulty in obtaining suf ficient numbers of MKs for analysis. However, recent advances in cell biology have furthered

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... tand ing of megakaryopoiesis and thrombopoiesis and allowed for great progress toward elucidating the mech anisms for MK differentiation and subsequent platelet production. In addition, a recently developed in vi tro culture system for MKs and the technology for ex vivo production of platelets have resulted in a bet ter understanding of MK and platelet biology and have highlighted their possible clinical applications. Platelet transfusions are widely used for patients with severe thrombocytopenia, and the supply of platelets is solely dependent on volunteer donors. Thus, the development of an efficient and stable sys tem of ex vivo platelet generation from donorindependent sources is needed.