Electron microscopic localization of silver staining NOR-proteins in rat liver nucleoli upon D-galactosamine block of transcription.

The method for electron microscopy of the Ag-staining of NOR-specific proteins was adapted to tissue sections from solid organs. The distribution of the Ag-staining proteins in rat liver nucleoli after complete block of transcription caused by D-galactosamine was investigated. In control animals, the Ag-staining proteins are associated with the fibrillar components of nucleoli. After block of transcription, positive Ag-staining is observed in the condensed fibrillar components of segregated nucleoli and later in the derived dense nucleolar fibrillar remnants. The granular components and the spherical bodies in segregated nucleoli are negative. It is concluded that in interphase nucleoli the Ag-staining NOR proteins are associated with the fibrillar components and with the derived nucleolar fibrillar remnants. The positive Ag-staining does not reflect the actual transcription of rRNA genes since it is present in both transcribed and non-transcribed r-chromatin.