Congenital Mesoblastic Nephroma in a Dog: A Benign Variant of Nephroblastoma

A nephroblastoma consisting predominantly of mature mesenchymal components developed in a 7-month-old male beagle dog. The dog was necropsied after experimental use. There were no abnormal signs or laboratory tests prior to necropsy. The left kidney was slightly enlarged, and the tumorous area of the kidney was gray-white. The cut surface of the tumor, occupying the upper one-third of the kidney, revealed a solid, rubbery, yellow-white mass. The tumor was about 3.0 cm in its greatest diameter, showed replacement growth pattern, and lacked a capsule (Fig. 1). Histologically, the tumor was mainly composed of spindle cells arranged in interlacing bundles that were intermingled with the normal renal tissue; however, there was no sharp demarcation between bundles and the surrounding renal parenchyma. The greater part of the tumor had a loose myxomatous pattern with an alcian blue-positive matrix, but dense, cellular, fibromatous areas with transition to the myxomatous areas were also noted (Fig. 2). Spindle cells (fibroblasts) had scanty cytoplasm with cigar-shaped nuclei and were negative for phosphotungstic acid-hematoxylin staining. Neither nuclear atypia nor mitotic figures were detected. Within the myxomatous area, cysts of various sizes were recognized in addition to entrapped nephrons. The small cysts were dilated