Ultrastructural localization of peroxidases in 'undifferentiated' blasts during the blast crisis of chronic granulocytic leukaemia.

Twelve cases of Philadelphia chromosome positive chronic granulocytic leukaemia (CGL) in blast transformation have been investigated using ultrastructural peroxidase detection. In all cases, the leukaemic blasts were negative for myeloperoxidase on the basis of standard cytochemistry. In nine cases a variable proportion of blasts contained peroxidase activity detectable only by electron microscopy, permitting definition of their myeloid nature. By their distinct characteristics and localization, different peroxidase activities were recognized. Thus, several types of blasts were identified: megakaryoblasts (MKB), basophil promyelocytes (BPM), myeloid blasts with small granules containing peroxidase (MyB), and proerythroblasts (ProE). MKB were predominant in two cases and present in four cases, mixed with other myeloid blasts. BPM were abundant in one case and present in seven cases. MyB were identified as a majority in four cases. Three cases remained without any peroxidase. It is concluded that ultrastructural detection of peroxidases is of value for the identification of early myeloid blasts. Their high incidence and the simultaneous presence of several myeloid precursors suggest that during the blast crisis the target cell is frequently a pluripotent myeloid stem cell.