Fine Structure and Peroxidase Activity of Circulating Micromegakaryoblasts and Platelets in a Case of Acute Myelofibrosis

S ummary . In a case of acute myelofibrosis with thrombocytosis, electron microscopical examination of the buffy coat revealed the existence of numerous mononuclear cells with a blastic appearance. They were considered blasts because nucleoli were prominent, and ribosomes, free or linked to short saccules of rough endoplasmic reticulum (RER), were numerous. The presence of peroxidase activity in the perinuclear space and RER of these cells was identical with the occurrence and location of this enzyme reaction product in normal bone marrow megakaryocytes. The results indicated that the atypical cells were circulating micromegakaryoblasts. Highly abnormal platelets were also present in the patient's peripheral blood. The peroxidase test provides a useful adjunct for distinguishing cells of the megakaryocyte line which in the past may have been mistaken for atypical lymphocytes or myeloblasts by light or electron microscopy.