Peculiar cytoplasmic inclusions in acute lymphoblastic leukemia: cytochemical and electron microscopic observations

In acute lymphoblastic leukemia (ALL) of a 19-year-old woman, blast cells in the blood and bone marrow contained peculiar large cytoplasmic inclusions. The inclusions, measuring 0.5-3.5 micron in diameter, were stained with methylene blue, toluidin blue, pyronin, and periodic acid-Schiff. Activities for acid phosphatase and alpha naphthyl butyrate esterase were not detected in the inclusions. The cytochemical findings indicated that the inclusions contained basophilic material and polysaccharides and that they were not lysosomal in nature. In electron microscopy, the inclusions were membrane bound and contained densely packed microvesicles, measuring 40-70 nm in diameter; amorphous electron-dense material; and whorled membranous figures. The microvesicles often had a nucleoid-like core and resembled viruses in size and structure.