A protruding mass was identified in the papilla of the right kidney of a 10-week-old male Sprague-Dawley rat. Microscopically, the neoplastic tissues were consisted of epithelial elements, where basophilic neoplastic cells displayed a high nucleus-to-cytoplasm ratio and formed tubular growth patterns characterized by small, elongated, or convoluted tubules. Blastemal elements were often arranged in aggregates or nests, composed of tightly packed basophilic polygonal to spindloid primitive cells. The surrounding interstitial tissue appeared loose and myxomatous. Based on these histological features, the diagnosis was nephroblastoma. Nephroblastoma is considered as an embryonic tumor originated from metanephric blastemal elements in the renal cortex and typically displays characteristic triphasic patterns. Also, this tumor seldom arises from or remains localized to the renal pelvis. To our literature review, this is the first nephroblastoma occurred at renal papilla in a rat.

Abstract
INTRODUCTION
CASE REPORT
DISCUSSION
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• Tae-Woo Kim(Laboratory of Veterinary Pathology, College of Veterinary Medicine, Chungbuk National University, Cheongju 28644, Korea, ABSolution Co., Ltd., Hwaseong 18514, Korea)
• Tae-Kyung Kim(Laboratory of Veterinary Pathology, College of Veterinary Medicine, Chungbuk National University, Cheongju 28644, Korea, ABSolution Co., Ltd., Hwaseong 18514, Korea)
• Yong-Hoon Lee(ABSolution Co., Ltd., Hwaseong 18514, Korea)
• Hye-Joon Park(ABSolution Co., Ltd., Hwaseong 18514, Korea)
• Mu-Jin Lee(ABSolution Co., Ltd., Hwaseong 18514, Korea)
• Sung-Jin Park(ABSolution Co., Ltd., Hwaseong 18514, Korea)
• Laehong Jo(ABSolution Co., Ltd., Hwaseong 18514, Korea)
• Yong-Seok Kim(ABSolution Co., Ltd., Hwaseong 18514, Korea)
• Byeongwoo Ahn(Laboratory of Veterinary Pathology, College of Veterinary Medicine, Chungbuk National University, Cheongju 28644, Korea) Corresponding author