Expression of interphasic nucleolar organizer regions in normal, dysplastic and neoplastic colorectal mucosa.

Silver-binding nucleolar organizer region (Ag-NOR) expression in interphasic nuclei was studied in normal, dysplastic and neoplastic colorectal mucosa at the light microscope level and by means of an image analyser (IBAS II). Both methods showed a progressive increase in the mean number of Ag-NOR sites per nucleus from mild dysplasia to invasive carcinoma. Ag-NOR counts differed significantly in the various classes of lesions (P less than 0.001), except between moderate and severe dysplasia (P greater than 0.05). Severe dysplasia showed a mean number of NORs lower than that for invasive carcinoma though an overlap in the respective frequency distributions was observed. The mean of the variances of the mean dot areas per cell nucleus (pooled variance) also showed a step-wise increase from normal to neoplastic lesions, indicating a greater variability in NOR size as a characteristic of malignant cells. A similar increase was observed in the percentage of nuclear area occupied by Ag-NORs. The mean area per silver-stained dot was also measured in the different classes of lesions by IBAS II. Data obtained showed no significant differences among the values. In conclusion, the wide overlap between the frequency distributions does not allow consideration of the Ag-NOR count alone to be a reliable marker of malignant transformation in a single cell. It appears that the study of Ag-NOR number needs to be evaluated together with dot anisometry in order to be a useful criterion in distinguishing the biological behaviour of neoplastic lesions in colorectal mucosa.