A 5-year-old boy with Down's syndrome presented with a brief history of cough, fever and lethargy. The white cell count was elevated at 133 × 109/l, haemoglobin concentration was 11·9 g/dl and the platelet count was 89 × 109/l. A Romanovsky stained blood film (top) showed 95% blast cells, which appeared lymphoid except for the presence of azurophilic intracytoplasmic inclusions that resembled the granules of myeloblasts. The bone marrow aspirate showed 96% blast cells. Again these appeared lymphoid but for the presence of the cytoplasmic inclusions. Flow cytometry showed the blast cells to be precursor B cells (CD10, 19, 34, 79a, terminal deoxynucleotidyl transferase positive). There was no expression of myeloid-associated antigens (CD13, 33, 117, myeloperoxidase). A diagnosis of precursor B-cell acute lymphoblastic leukaemia (ALL) was made. Electron microscopy (bottom) showed abundant mitochondria but no other cytoplasmic organelles or granules.
Cytoplasmic inclusions and Auer rod-like structures have occasionally been observed in ALL with an estimated frequency of around 2% of cases. Different authors have drawn different conclusions as to their origins; viral inclusion particles, lysosomal-derived structures and dysplastic mitochondria have been proposed. Here, electron microscopy clearly demonstrated that the inclusions in this case were mitochondria.