II. Pituitary Chromophobe Adenoma
III. Large Granular Lymphocyte Leukemia or Fischer rat leukemia
This is the most common tumor of rats. Because of the extensive
mammary tissue present in rats, tumors may be found practically anywhere--behind
the shoulder blades, at the base of the tail, etc. Tumors may occur
in males as well as in females. The tumors, which appear as subcutaneous
lumps, may be quite extensive (photo on left). Ulceration and subsequent
dermatitis occur frequently. Mammary tumors are usually encapsulated
and benign. Occasionally mammary adenocarcinomas are diagnosed from excisional
biopsies. With surgical removal, the prognosis is good, although recurrence
in another location is common. Histologically, both fibrous an dglandular
components are hyperplastic (photo on right). The progression of
either the glandular or fibrous component to malignancy is rare.
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II. Pituitary Chromophobe Adenoma
This is an extremely common tumor of rats, although incidence
varies with strain. Grossly, tumors are soft with an irregular surface
and may have prominent hemorrhagic areas. They are generally well
circumscribed and compress adjacent brain tissue, occasionally causing
hydrocephalus resulting in depression or head tilt. The tumors commonly
secrete prolactin.
III. Large Granular Lymphocyte Leukemia or Fischer Rat Leukemia
A spontaneous leukemia with circulating atypical mononuclear cells
containing azurophilic cytoplasmic granules has been reported in aged Wistar
Furth (WF) and Fischer 344 (F344) rat strains with an incidence of 17%
and 25%, respectively, in aged rats. The mononuclear cell leukemia is one
of the most common neoplasms in aged F344 rats and is frequently referred
to as Fischer Rat Leukemia. Clinically, leukocytosis with up to 90%
atypical mononuclear cells and anemia are observed. Splenomegaly,
hepatomegaly and lymphadenopathy are seen at necropsy. Tissue enlargement
is due to massive infiltration of the sinusoids with large, undifferentiated
lymphocytes with oval nuclei. The kidneys are often grey due to the
deposition of brown, iron positive granules in the proximal tubules.
Leukemic cells appear to be of natural killer (NK) cell origin.
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