Mammalian megakaryocyte development is characterized by a progressive accumulation of cells exhibiting a polylobated nucleus with a polyploid DNA content. In this study human megakaryocytes were obtained from CD34+ haemopoietic progenitors by in vitro liquid culture in the presence of 100 ng/ml of recombinant thrombopoietin (TPO). Ultrastructural examination of polyploid megakaryocytes showed the presence of a large number of centrioles, the breakdown of the nuclear envelope, and the progressive chromatin condensation, all aspects characteristic of mitosis. At both indirect immunofluorescence and Western blot analyses, cyclin B and its related cyclin-dependent kinase (CDK)1, which forms the mitosis promoting factor (MPF), showed an increased expression in maturating megakaryoblasts and megakaryocytes (day 8 of culture) with respect to freshly isolated CD34+ progenitors. This expression tended to decline in fully developed megakaryocytes (day 15 of culture). The amount of cyclin D and of the related CDK4, governing the G1 phase of the cell cycle, increased during megakaryocyte development, maintaining high levels of expression also in mature megakaryocytes. These results indicate that megakaryocyte polyploidization depends on a true, although incomplete, mitotic process, and that cyclin D/CDK4 probably plays a crucial role throughout megakaryocytopoiesis
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