Atypical glycogen deposits in a plasmacytoma: an ultrastructural study

A solitary, benign plasmacytoma, arising from the gum of the lower jaw of a dog, was studied by light and electron microscopy. Some of the tumour cell nuclei showed the characteristic clockface or cartwheel pattern of heterochromatin distribution similar to that found in normal plasma cells. Like normal plasma cells the tumour cells were endowed with rough endoplasmic reticulum but this was invariably dilated or visiculated. Glycogen deposits were found within mitochondria and within the dilated and vesiculated rough endoplasmic reticulum. The idea that these deposits were derived secondarily from glycogen deposits in the cytoplasm seems untenable because no glycogen particles were found in the cytoplasmic matrix. It would therefore appear that most, perhaps all, stages of glycogen synthesis occurred within the mitochondria and rough endoplasmic reticulum of some of these tumour cells.