[Expression of cyclin B in megakaryocytes and cells of other hematopoietic lines]

Megakaryocytes are normal bone marrow cells which have the unique ability to become polyploid. This phenomenon is termed endomitosis and its mechanism remains poorly understood at present. It is known that the cell cycle of eukaryotes, particularly at G2/M transition, is regulated by a complex with histone H1 kinase activity, the maturation- or M-phase promoting factor (MPF). We have therefore studied the expression of subunits of MPF, the p34cdc2 and cyclin B in normal bone marrow culture megakaryocytic cells, blood leukocytes and platelets as well as in human megakaryoblastic cell lines Dami, Meg-01, HEL and the promyelocytic cell line HL60. Using immunohistochemistry and electron microscopy we have observed that cyclin B was virtually undetectable in megakaryocytes and platelets, but was abundant in granulocytes, monocytes/macrophages and HL60. Studies by RT-PCR showed the presence in large quantities of mRNA of cyclin B in all cell types studied, even in megakaryocytic-like cells. These observations suggest some important implications in the understanding of the mechanisms of megakaryocyte polyploidization and related endomitosis.