Clear cell sarcoma of the kidney with emphasis on ultrastructural studies.

Clear cell sarcoma of the kidney (CCSK) is a malignant childhood tumor distinguished from Wilms' tumor by its gross, microscopic, and ultrastructural morphologic features and its propensity to metastasize to bone. Some clinical features of 75 cases of CCSK registered in the National Wilms' Tumor Studies I-III are presented along with the results of electron microscopic study of 12 tumors. Ultrastructural features of the tumor are compared to those of tumors of pericytes, Schwann's cells, renomedullary interstitial cells, fibroblasts, other renal tumors, and other clear cell tumors. Fine structural properties of primitive cells with variable numbers of organelles do not strongly suggest a histogenetic origin. The effect of formalin fixation on perinuclear cytoplasmic filaments and the tendency of tumor cell extensions to enfold the pale extracellular matrix explain the vacuolated appearance of some tumor cells.