Ultrastructural Observations on a Variant of Acute Promyelocytic Leukemia

A patient with acute leukemia is presented in whom the leukemic cells, as seen by light microscopy were typical promyelocytes. The cells had normal or slightly invaginated nuclei with typical cytoplasmic granules and the diagnosis was confirmed by cytochemistry. The clinical course was rapid and the patient died of disseminated intravascular coagulation and urosepsis within a few days of diagnosis. However, electron microscopic examination showed cells with extremely convoluted and lobulated nuclei with nuclear pockets and cytoplasmic bridges as well as the complete absence of cytoplasmic granules in the majority of the cells. Furthermore, the urine lysozyme (muramidase) was elevated. These findings suggest that the leukemia in this patient may be classified as a hypogranular variant of acute promyelocytic leukemia (APL), with monocytoid ultrastructural appearances.