Three-dimensional co-localization of nucleolar argyrophilic components and DNA in cell nuclei by confocal microscopy

Silver dots deposited specifically on proteins of the nucleolar organizer regions (Ag-NOR proteins) after a one-step silver staining technique were visualized in cells in culture, in cells in smears, and in tissue sections, with a scanning laser confocal microscope working in the reflectance mode. After specific labeling of DNA with the fluorescent dye chromomycin A3, DNA and silver dots could be observed either individually or simultaneously. Therefore, it was possible to study the three-dimensional organization of nucleolar silver-stained structures relative to DNA with a high X, Y, and Z resolution. Our results showed that the argyrophilic components are organized as a twisted necklace structure within interphase nucleoli of cells in culture. We also demonstrated a striking three-dimensional symmetric disposition of NORs within the two sets of chromosomes in telophase cells. Similar results were obtained for cells in smears, although their three-dimensional organization was somewhat disturbed due to air-drying. We also demonstrated that silver dots cannot be visualized in the reflectance mode within sections of paraffin-embedded tissues. However, their simultaneous demonstration in non-confocal transmitted light, together with that of DNA in confocal mode, appeared very useful to study their localization within nuclei and mitotic chromosomes.