Ultrastructural Cytochemistry of Leukaemic Cells: Characterization of the Early Small Granules of Monoblasts

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Abstract
An ultrastructural study of [human] blast cells showing monocytic or granulocytic differentiation was performed with the acid phosphatase (AP) and myeloperoxidase (MPO) reactions. Eight cases of acute myeloid leukemia (AML) and 3 of chronic granulocytic leukemia in blast crisis were studied. A hitherto unrecognized small lysosomal granule characterized by AP activity and lack of MPO was present in the majority of cells of all 6 monoblastic leukemias. These granules were 0.05-0.2 .mu.m in size and were distributed throughout the cytoplasm, frequently at the periphery of the cells. A small proportion of monoblasts showed AP reactivity in the Golgi cisternae. Both AP and MPO were positive in the granules of promonocytes; MPO-positive granules were predominant in late promonocytes. Larger granules (0.2-0.6 .mu.m) with MPO reactivity were characteristic of myeloblasts. In 2 of 4 cases these granules showed AP positivity, suggesting that, in contrast to monoblasts, AP activity is a late feature of myeloblastic differentiation. Ultrastructural cytochemistry may be helpful in the recognition and classification of acute leukemias by demonstrating the early differentiation features of monocytic and granulocytic precursors.