A study on nucleolar silver staining in plant cells. The role of argyrophilic proteins in nucleolar physiology

Using a high resolution nucleolar silver staining technique, we have studied the distribution of silver grains at interphase and mitosis. The nucleolar fibrillar centers and the fibrillar component, as well as the mitotic nucleolar organizing region (NOR), are argyrophilic. Of the NOR structures, the fibrillar component is transcriptionally active, but the fibrillar centers and the mitotic NORs are not. Treatment with cycloheximide (CHM), which indirectly impairs rRNA synthesis, does not affect the pattern of silver staining. The currently assumed correlation between activity and Ag-NOR staining appears questionable. On the other hand, all Ag-NOR-stained structures contain decondensed chromatin, whereas condensed portions of this chromatin are not stained by silver. In CHM-treated cells there is apparently no chromatin condensation accompanying inhibition of rRNA synthesis.