Ultrastructural cytochemistry of leukaemic cells: characterization of the early small granules of monoblasts.

An ultrastructural study of blast cells showing either monocytic or granulocytic differentiation was carried out with the acid phosphatase (AP) and myeloperoxidase (MPO) reactions. Eight cases of acute myeloid leukaemia (AML) and three of chronic granulocytic leukaemia 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 six monoblastic leukaemias. These granules ranged from 0.05 to 0.2 micron 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; however, MPO positive granules were predominant in late promonocytes. Larger granules (0.2--0.6 micron) with MPO reactivity were characteristic of myeloblasts. In only two out of four cases did these granules show AP positivity, suggesting that, in contrast to monoblasts, AP activity is a late feature of myeloblastic differentiation. This study shows that ultrastructural cytochemistry may be helpful in the recognition and classification of acute leukaemias by demonstrating the early differentiation features of monocytic and granulocytic precursors.