Nucleolar organizer regions in congenital mesoblastic nephroma.

A review of 78 renal tumors in patients under 6 months of age revealed 35 congenital mesoblastic nephromas (CMNs). Based on cellular criteria, 14 were classified as classical, 4 as partly cellular, and 17 as cellular CMN. The mean ages were 24, 11, and 70 days, respectively. There were 13 intrarenal tumors (stage I) but 9 classical, 3 partly cellular, and 5 cellular CMNs extended to the perirenal fat (stage II) and 5 cellular tumors ruptured (stage III). In order to assess cellular proliferative activity, silver staining of nucleolar organizer region (Ag-NOR) proteins was performed on 19 CMNs. The number of Ag-NOR dots per cell was significantly lower in classical and partly cellular CMN than in cellular CMN, whatever the stage (P < .01). Within the cellular CMNs, the mean number of Ag-NOR dots was statistically higher in the single case that recurred with fatal outcome (P < .02). Counting of Ag-NOR dots appears to be a reproducible method for evaluating the biologic potential of CMNs. The number of Ag-NOR dots, DNA content measurements, the histological subclassification, and the presence or absence of tumor at the surgical margins may be useful features in selecting those patients who will benefit from further treatment after nephrectomy.