Megakaryoblastic micromegakaryocytic crisis in chronic myeloid leukemia.

Atypical megakaryoblasts (MKB) or megakaryocytes (MK) are occasionally present in the peripheral blood during the terminal development of chronic myeloid leukemia (CML). We report on a 49-year-old female suffering from Ph1 chromosome-positive CML with typical megakaryoblastic transformation in the peripheral blood and in the bone marrow. The small "blasts" were at the most only slightly larger and were occasionally even smaller than lymphocytes but showed megakaryoblastic or atypical megakaryocytic differentiation. The cytoplasmic cytochemical pattern of the atypical megakaryocytic cells was identical to that of large atypical thrombocytes. Platelet peroxidase was detected upon electron-microscopic (EM) examination. Immunologic characterization disclosed the presence of MK-specific antigens. When cultured in vitro on agar, the blasts transformed spontaneously into large mature MK, exhibiting characteristic cytochemical and immunological patterns. Cytogenetic examination of peripheral blood showed severe abnormalities. The patient did not respond to therapy and died 3 months after manifestation of the blast crisis.