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Megakaryocyte maturation involves DNA replication without cytokinesis, resulting in large bone marrow cells (w65mm) of high ploidy (8N, 16N, 32N and 64N). It has long been appreciated that as megakaryocytes mature and increase in ploidy, they increase in size and as a consequence, also increase cell surface antigen presentation. We now report that megakaryocytes of individual ploidies exist as two distinct CD41/CD61 populations; bright and dim. While these populations are similar in size (forward scatter), CD41/CD61bright megakaryocytes have a 3.6 fold increase in sidescatter, which is indicative of increased cellular complexity and an increase in granularity that is associated with a megakaryocyte primed for platelet release. We have demonstrated high ploidy CD41/CD61bright megakaryocytes also have well developed demarcation membrane system, increased a-granules and when transplanted into …