ULTRASTRUCTURE OF CELLULAR CONGENITAL MESOBLASTIC NEPHROMA

A detailed ultrastructural description of the cellular variant of cong enital mesoblastic nephroma (CMN) is presented and compared to the cla ssical form. Studied were 9 cases of the cellular variant, 6 mixed (ce llular/classical) tumors, and 1 classical CMN. The occurrence of a bro ad selection of ultrastructural features was assessed using a semiquan titative scoring system. The results indicate that cellular CMNs are c omposed mainly of primitive mesenchymal cells, but also usually contai n varying numbers of differentiating fibroblasts and myofibroblasts. T his entity thus bears a closer resemblance at the ultrastructural leve l of organization to infantile fibrosarcoma than to conventional fibro sarcoma. Electron microscopy can be useful in distinguishing this rela tively benign entity from the several malignancies with which it is so metimes confused.