Cytological and histochemical studies on the mechanism of the selective silver staining of nucleolus organizer regions (NORs).

A new silver staining method is presented (Ag-II staining) providing a rapid and reproducible way to selective silver staining of nucleolus organizer regions (NORs). In comparison with other techniques, such as the Ag-AS method and the Ag-I method, factors influencing silver stainability are discussed. Histochemical studies on the nature of the NOR-specific silver precipitate were performed either by employing various pretreatments or by inhibiting the participation ("blocking") of the various proteins or protein compounds in the staining reaction. The results would seem to indicate that the interactions of silver-ions with the carboxyl groups of acidic proteins which are involved in the rRNA-transcription process are mainly responsible for the selective silver staining of NORs.