Pathology of the mononuclear cell leukemia of Fischer rats. I. Morphologic studies.

Pathological evaluations were done in 205 rats with mononuclear cell leukemia. Leukemia was diagnosed in 22.2% of males and 20.4% of females with significant risk beginning at 20 months of age. Mononuclear cell leukemia was responsible for 50% of early deaths in two-year studies. Clinically, rats became depressed, pale, icteric and had palpably enlarged spleens. Gross lesions included splenomegaly, enlarged mesenteric lymph nodes, and mottled livers. Hemorrhages occurred in the lungs, brain, and lymph nodes. Histological examination demonstrated that spleen and liver were most consistently and seriously involved, although numerous other organs contained leukemic infiltrates of variable severity. Spleens exhibited diffuse leukemic infiltration of the red pulp, follicular lymphoid depletion, and decrease in both extramedullary hematopoiesis and hemosiderin. Liver lesions consisted of diffuse centrilobular degeneration and necrosis. Erythrophagocytosis by tumor cells was common in the spleen and observed in liver, lymph nodes, and adrenals. The disease appeared to originate in the spleen. Bone marrow infiltration occurred late relative to spleen involvement and was present in less than half of the rats.