[Silver staining of nucleolus organizer regions (NORs). Application to the study of nucleolar structure and value in pathology].

Nucleolus is the morphologic expression of synthesis and maturation of ribosomal RNA (rRNA) from amplified ribosomal DNA (rDNA). Nucleolar Organizer Regions (NORs) are functional subunits of the nucleolus in which actively transcribed rDNA is surrounded with numerous regulatory proteins. Some of them are argyrophilic non-histone proteins (Ag-NOR proteins). By using a cytochemical reaction based on this argyrophilia, active NORs may be stained by the precipitation, at their level, of metallic silver granules whose quantity is directly related to the nucleolar activity. In the present paper, we described various applications of a silver-staining method we developed in our laboratory. The Ag-NOR proteins were ultrastructurally localized within precise nucleolar components. Moreover by viewing thick-sections of silver-stained cells with high-voltage microscopes we were able to describe the three-dimensional structure of nucleoli. Finally, this silver-staining method may be applied at the optical level, to sections of routinely fixed and paraffin-embedded human tissues. With this simple staining method, it is now possible to study the relationships of the number of nucleolar argyrophilic structures with the diagnostic of neoplasms.