Cytochemical demonstration of 'platelet' peroxidase at the light microscope level.

A reliable and reproducible modified method was developed for the cytochemical demonstration of the so-called 'platelet' peroxidase at the light microscopy (LM) level. Interestingly, not only normal platelets and megakaryocytes showed the peroxidase activity, but also normal lymphocytes. As with the ultrastructural platelet peroxidase (PPO) method, myeloperoxidase (MPO) was also demonstrated in granulocytes. Peroxidase activity was exhibited by blast cells of acute myeloid leukaemia, including megakaryoblasts of megakaryoblastic leukaemia, by hairy cells of hairy cell leukaemia, by centroblasts of centroblastic lymphoma and by plasma cells of plasma cell leukaemia. The enzyme activity was also demonstrated in the mast cells of systemic mastocytosis. It seems that different forms of peroxidase are present in most haemic cells. Fixation and conditions of incubation are probably the determining factors for their demonstration. The method may allow further cytochemical discrimination at the LM level between blast cell populations which are negative for MPO by standard methods.