Spontaneous nephroblastoma with oncocytic differentiation in a Japanese White rabbit.

Spontaneous nephroblastoma is an uncommon tumor in laboratory rabbits. We recently encountered this tumor, and we describe its histological characteristics in this report. A male 3-year-old Japanese White rabbit (JW/kbs), maintained as a stock animal, suddenly showed poor condition and was found dead a few days later. At necropsy, a large mass was found that extended from one side of the renal pelvis. The cut surface of the mass was dark red in color and velvety to the touch. The kidney on the contralateral side was normal. Microscopically, the tumor mass consisted of biphasic components, which consisted of epithelial (tubular and glomerular) and blastemal (nodular) elements. No sarcomatous proliferation was observed. In addition, some of the tubules were lined by cells with a large amount of eosinophilic cytoplasm. The cells were confirmed as oncocytes by immunohistochemical and electron microscopic examinations. The present case was therefore diagnosed as a nephroblastoma with oncocytic differentiation.