Megakaryoblastic acute leukemia: identification by the ultrastructural demonstration of platelet peroxidase

Abstract

In some acute leukemics, blast cells may lack morphologic and cytochemical characteristics indicating their original cell line. Whether these cells are in fact undifferentiated or derived from early precursors of lymphocytes or of other cell lines remains open to question. Leukemias with megakaryoblast (MKB) predominance have been considered rare. In these cases, the recognition of the MKB has been based on the large size of the cells and on their morphologic characteristics. Until now however, the identification of early small MKBs has been uncertain when conventional staining and ultrastructural methods have been employed. It has previously been shown that platelet peroxidase (PPO), which is distinct from granulocytic peroxidases, can be employed as a marker of normal small MKB. A new, sensitive cytochemical method for the demonstration of PPO has been applied to the study of a case of acute leukemia with thrombopenia. The majority of marrow and circulating small blasts that exhibit an undifferentiated or lymphoid appearance upon light and electron microscopic examination have been shown to possess PPO. The morphology of PPO-positive blasts is quite variable. They show no granule or demarcation membrane production usually associated with the beginning of normal megakaryocytic maturation. The presence of PPO alone has permitted us to classify this case as a pure MKB leukemia.