AgNOR (nucleolar organizer regions) staining in malignant melanoma

Nucleolar organizer regions (NORs) are loops of ribosomal DNA seen in nuclei, which are demonstrable as black dots (AgNOR) in tissue sections by silver (Ag) colloid staining. The number of such AgNORs is correlated with cellular activity and is an indicator of the degree of malignancy. In this study, 76 melanocytic lesions were analyzed by AgNOR staining, and the clinical and histopathological characteristics of malignant melanoma and melanocytic nevi were considered. Although the AgNOR counts for melanocytic nevi were significantly different from those in malignant melanoma, an obvious overlap between them was detected. The number of AgNORs in melanocytic nevi per cell was usually 1 or 2. On the other hand, the number of AgNORs per malignant melanoma cell was variable. Morphologically, malignant melanoma cells often showed dispersal of AgNORs throughout the nucleus as well as multiple nucleoli containing clustered AgNORs, whereas melanocytic nevus cells tended to have a regular nucleolus with tightly clustered AgNORs. The correlation between AgNOR count and pathological staging was uncertain, but a slight correlation between AgNOR count and thickness of the primary lesion was obtained. However, the AgNOR count in malignant melanoma was not a prognostic factor for the disease. Therefore, the AgNOR method is difficult to use for differential diagnosis between benign pigmented lesions and malignant melanoma. Nonetheless, an AgNOR count of more than two per cell favors a diagnosis of malignant melanoma.