Mesenchymal Tumor, Kidney, Rat

Abstract

Small tumors are visible at one pole or at the curvature of the intact kidney as white discoloration on the surface. Such lesions are usually confined to the outer zones of the organ as an ill-defined infiltrative growth following the sagittal plane of the cortex and without the rounded form which typifies epithelial tumors of the kidney (Fig. 31). A poorly delineated tumor outline is characteristic and particularly evident when the kidney is prepared as a histologic section. The texture of the small to intermediate-sized tumors is usually fibrous and sometimes they are obviously cystic (Fig. 31). In some cases at autopsy the affected kidney appears uniformly enlarged, and when sectioned, the tumor tissue is found to be distributed throughout much of the organ. In larger tumors, proliferation occurs beyond the kidney outline to form bulging nodular masses (Fig.32) which may reach an immense size and fill the abdominal cavity. Very large tumors are usually multilobular, with prominent areas of hemorrhage, necrosis, and gelatinous tissue. Despite the destruction of kidney substance in these cases, invariably a small rim of normal renal parenchyma remains intact at some point on the surface of the tumor.