Crystalline inclusions in the cytoplasm and nuclei of cells of acute myeloid leukaemia

In a survey by electron microscopy of peripheral blood and/or bone marrow from 230 adult patients with acute myeloid leukaemia, five were observed to contain crystalline inclusions in the cytoplasm of the leukaemic cells and a sixth contained crystals in the nuclei. In four cases, two of FAB type M2 and two of M4, the cytoplasmic crystals were hexagonal in section and 1-2 micron long. Two examples showed internal periodicities in the range 3.3-4.0 nm when the electronmicrographs were analysed by optical diffractometry. A single case of M1 contained smaller trapezoidal crystals with a 4.9nm periodicity. The sixth patient, with unusual cytological abnormalities and a rare t(3; 6) chromosomal translocation, contained six-sided crystals in the nuclei of some relatively undifferentiated cells. To the best of our knowledge such intranuclear crystals have not previously been reported in leukaemia. The relevance of the crystals to the leukaemic process is discussed.