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CD41+/CD45+ Cells Without Acetylcholinesterase Activity Are Immature and a Major Megakaryocytic Population in Murine Bone Marrow

Murine megakaryocytes (MKs) are defined by CD41/CD61 expression and acetylcholinesterase (AChE) activity; however, their stages of differentiation in bone marrow (BM) have not been fully elucidated. In murine lineage‐negative (Lin−)/CD45+ BM cells, we found CD41+ MKs without AChE activity (AChE−) except for CD41++ MKs with AChE activity (AChE+), in which CD61 expression was similar to their CD41 level. Lin−/CD41+/CD45+/AChE− MKs could differentiate into AChE+, with an accompanying increase in CD41/CD61 during in vitro culture. Both proplatelet formation (PPF) and platelet (PLT) production for Lin−/CD41+/CD45+/AChE− MKs were observed later than for Lin−/CD41++/CD45+/AChE+ MKs, whereas MK progenitors were scarcely detected in both subpopulations. GeneChip and semiquantitative polymerase chain reaction analyses revealed that the Lin−/CD41+/CD45+/AChE− MKs are assigned at the stage between the progenitor and PPF preparation phases in respect to the many MK/PLT‐specific gene expressions, including β1‐tubulin. In normal mice, the number of Lin−/CD41+/CD45+/AChE− MKs was 100 times higher than that of AChE+ MKs in BM. When MK destruction and consequent thrombocytopenia were caused by an antitumor agent, mitomycin‐C, Lin−/CD41+/CD45+/AChE− MKs led to an increase in AChE+ MKs and subsequent PLT recovery with interleukin‐11 administration. It was concluded that MKs in murine BM at least in part consist of immature Lin−/CD41+/CD45+/AChE− MKs and more differentiated Lin−/CD41++/CD45+/AChE+ MKs. Immature Lin−/CD41+/CD45+/AChE− MKs are a major MK population compared with AChE+ MKs in BM and play an important role in rapid PLT recovery in vivo.