Electron-microscopic characterization of mixed granulated (hybridoid) leucocytes of chronic myeloid leukaemia

In chronic granulocytic leukaemia, hybridoid leucocytes can regularly be found. Light microscopically they contain a mixture of eosinophilic, basophilic and naphthol AS-D chloroacetate esterase-positive granules. The present study was done to clarify the ultrastructural composition of these cells. It could be clearly shown that in some leukaemic granulocytes primary and secondary eosinophilic as well as basophilic granules occur side by side. There were also basophils with additional tissue mast cell granules. Since normal mast cell granules as well as granules of normal eosinophilic promyelocytes are naphthol AS-D chloroacetate esterase-positive, it would appear possible that mastocytoid as well as primary eosinophilic granules within the leukaemic basophils are responsible for the atypical, granular naphthol AS-D chloroacetate esterase-positivity of these cells. The existence in chronic myeloid leukaemia both of mixed basophilic and eosinophilic granulated leucocytes and of mixed basophilic and mastocytoid granulated leucocytes may suggest a common myeloid precursor of eosinophils, basophils and tissue mast cells. In addition, the hybridoid granulocytes may be considered an expression of a neoplasia-related lineage infidelity.