Morphologic and flow cytometric analysis of circulating megakaryoblasts in chronic myeloid leukaemia.

The immunophenotype (a), ultrastructural features (b) and cell kinetics (c) of circulating megakaryoblasts have been studied in two cases of pure megakaryoblastic and one case of mixed (myeloblastic, megakaryoblastic) cell proliferation in chronic myeloid leukaemia (CML). (a) The blast cells showed early megakaryocyte differentiation antigen (HLA-DR), platelet specific GpIIIa (CD61) and GpIIb-IIIa (CD41) antigens in different percentages. (b) The megakaryoblasts were recognized by the presence of platelet GpIIIa (CD61) demonstrated by an immunoelectron microscopic method. The labelled cells were "lymphocyte-like" megakaryoblasts and cells with features of cytoplasmic maturation (demarcation membranes, alpha granules and vacuoles). (c) Cellular DNA content of the megakaryoblasts was measured by propidium iodide (PI) staining of cells expressing platelet GpIIIa (CD61). Flow cytometric (FC) DNA analysis revealed no aneuploidy and high ploidy (greater than 4N) cell population. In the two cases of pure megakaryoblastic proliferation a high percentage of the megakaryoblasts were in the S-phase, while the non-megakaryoblastic cell fraction showed no elevated S-phase compartment. It is concluded that in CML the circulating megakaryoblasts (1) have a nuclear maturation arrest and accumulation at the level of tetraploid DNA content, (2) surface antigen expression and cytoplasmic organelles show a tendency to mature and (3) in pure megakaryoblastic proliferation the myeloid cells are not in the cell compartment showing high proliferation.