EXPRESSION OF CYCLIN-B IN MEGAKARYOCYTES AND CELLS OF OTHER HEMATOPOIETIC LINEAGES

Megakaryocytes are normal bone marrow cells which have the unique abil ity to become polyploid. This phenomenon is armed endomitosis and its mechanism remains poorly understood at present. It is known that the c ell cycle of eukaryotes, particularly at G2/M transition, is regulated by a complex with histone HI kinase activity, the maturation- or M-ph ase promoting factor (MPF). We have therefore studied the expression o f subunits of MPF, the p34(cdc2) and cyclin B in normal bone marrow cu lture megakaryocytic cells blood leukocytes and platelets ac well as i n human megakaryoblastic cell lines Dami, Meg-01, HEL and the promyelo cytic cell line HL60. Using immunohistochemistry and electron microsco py we have observed that cyclin B was virtually undetectable in megaka ryocytes and platelets, but war abundant in granulocytes, monocytes/ma crophage and HL60. Studies by RT-PCR showed the presence in large quan tities of mRNA of cyclin B in all cell types studied, even in megakary ocytic-like cells. These observations suggest some important implicati ons in the understanding of the mechanisms of megakaryocyte polyploidi zation and related endomitosis.