A solid, unencapsulated tumor of the kidney composed of spindle mesenchymal cells that resemble fibroblasts or muscle cells. The homogeneous mass typically extends into the renal parenchyma and replaces most of the kidney. In most cases, mesoblastic nephroma is benign and occurs in the fetus or newborn, and rarely in the older child or the adult. The disease is MONDO_0003130 (mesoblastic nephroma). Also known as: mesoblastic nephroma (morphologic abnormality).