Simultaneous ultrastructural localization of Ag-NOR (nucleolar organizer region) proteins and ribonucleoproteins during mitosis, in human breast cancerous tissues.

The simultaneous ultrastructural localization of the Ag-NOR proteins and ribonucleoproteins (RNP) was carried out during mitosis in human breast cancerous tissues using a one-step silver staining method followed by acetylation. Owing to the good preservation of the cells and to the high precision of the Ag staining we were able to follow the behaviour of the argyrophilic nucleolar components from prophase to late telophase. During all the phases of mitosis the Ag-NOR proteins were associated with fibrillar RNP component (without visible fibrillar centre), which originated, during prophase, from the dense fibrillar component of the nucleolus and then integrated within the periphery of some metaphasic chromosomes as irregular structures. The Ag staining of these structures was homogeneous but lower compared to those of nucleoli during prophase. The RNP sheath that covered the chromosomes was never silver stained. From anaphase to early telophase the argyrophilic RNP structures disposed within the periphery of the chromatin mass formed roundish and then spherical structures in which the Ag-NOR proteins were clustered as centrally localized spheres. These findings showed the usefulness of our method for the precise study of the spatial relationship between the Ag-NOR proteins and the chromosomes. Thus we were able to show striking segregation of both the argyrophilic structures at the periphery of the chromatin mass and of the Ag-NOR proteins within these structures. This suggests that a high degree of the structural organization in the future nucleus is already present during anaphase. This could be related to the rapid resumption of ribosomal RNA synthesis.