January 2018
U-N06 (NP)

SIGNALMENT:  9-month-old female Fisher 344 rat.

HISTORY:  This rat received a single injection of dimethylnitrosamine at one day of age.

HISTOPATHOLOGIC DESCRIPTION:  Kidney:  Effacing and replacing approximately 70% of the cortex and medulla is an unencapsulated, infiltrative, poorly differentiated neoplasm composed of spindle to stellate cells.  Spindles cells are arranged in streams and tightly packed whorls that surround and entrap tubules, glomeruli and cystic spaces, and infiltrate adjacent normal cortex on a fine collagenous to myxomatous matrix. Stellate cells are arranged in loose streams widely separate and replace preexisting tubules, glomeruli and cystic spaces, and extend to the renal pelvis.  Neoplastic cells have indistinct cell borders, scant to moderate eosinophilic fibrillar, occasionally vacuolated cytoplasm, and oval to elongate nucleus that is vesiculate, and has 1-2 variably distinct nucleoli.  Mitoses within the stellate cells average 1 per 10 HPF. Mitoses within the spindle cells average 1 per HPF and there is occasional single cell necrosis characterized by shrunken cells with hypereosinophilic cytoplasm and pyknosis.  Entrapped glomeruli are atrophied and surrounded by a dilated Bowman’s space. Entrapped tubules are scattered throughout the neoplasm are variably dilated and cystic, lined by epithelium that is either attenuated or hyperplastic, and multifocally contain homogenous eosinophilic material (proteinaceous fluid) or basophilic granular material (mineral) and occasionally neutrophils and cellular debris (cellular casts). Within the adjacent renal cortex, tubules contain one or more of the following changes: degeneration, characterized by swollen vacuolated cytoplasm and faded nuclei; or regeneration, characterized by basophilic nuclei and rare mitotic figures.  Multifocally, there are moderate numbers of mildly dilated tubules, and the interstitium is mildly expanded by small numbers of lymphocytes and plasma cells (interstitial nephritis).

MORPHOLOGIC DIAGNOSIS:  Kidney:  Renal mesenchymal tumor (RMT), Fisher 344 rat, rodent.

SYNONYMS:  Renomedullary interstitial cell tumor, malignant mesenchymal tumor








This tumor is found only in laboratory rodents and humans


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