Acute nonlymphocytic leukemia showing abnormal nuclear lobulation.

An ultrastructural study of four cases of acute nonlymphocytic leukemia with abnormal nuclear lobulation was done. These cases were classified as M3 (Case 1), M3 variant (Cases 2 and 3), and M5, differentiated type (Case 4), on the basis of light microscopy according to the FAB classification. Ultrastructural observation of the leukemic cells of Cases 2 and 3 showed large bundles of fibrils in addition to abnormalities in the endoplasmic reticulum and granules, as had been reported in cases of acute promyelocytic leukemia. Large bundles of fibrils and abnormal nuclear lobulation also seemed to be characteristic of the M3 variant. However, the relationship between these structures was not identified. The exact nature of nuclear lobulation is not known, but seems to be related to some kind of intrinsic force and may be a presentation of the premature occurrence of lobulation intrinsic to the granulocyte or monocyte nucleus.