Nephroma, Mesoblastic: 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.Wilms Tumor: A malignant kidney tumor, caused by the uncontrolled multiplication of renal stem (blastemal), stromal (STROMAL CELLS), and epithelial (EPITHELIAL CELLS) elements. However, not all three are present in every case. Several genes or chromosomal areas have been associated with Wilms tumor which is usually found in childhood as a firm lump in a child's side or ABDOMEN.Kidney Diseases, Cystic: A heterogeneous group of hereditary and acquired disorders in which the KIDNEY contains one or more CYSTS unilaterally or bilaterally (KIDNEY, CYSTIC).Kidney Neoplasms: Tumors or cancers of the KIDNEY.Nephrectomy: Excision of kidney.