Ultrastructure and cytochemistry of the tumorous blood cells in the mutant Lethal(3)malignant blood neoplasm of Drosophila melanogaster

In the recessive mutant lethal(3)malignant blood neoplasm, (l(3)mbn), the precursors of the plasmatocytes, the proplasmatocytes, are malignantly transformed. The hypertrophied hematopoietic organs produce excessive amounts of plasmatocytes which are released into the hemolymph throughout the third larval instar. As a result, their number in the hemolymph increases up to 15-fold compared with that in the wild-type hemolymph. Most free blood cells in the mutant hemolymph are plasmatocytes. Podocytes and lamellocytes of normal size are rare. Approximately 10–12% of the blood cells are giant plasmato-, podo-, and lamellocytes which show numerous primary and secondary lysosomes in their cytoplasm. Furthermore, the malignant plasmatocytes invade all larval tissues which become histolyzed by a yet unknown process. The high level of polyphenol oxidase expression, coupled with the inability of the cells to distinguish between their own and foreign tissue, leads to the formation of melanotic masses and eventually to the melanization of the entire larva.