Stereoscopic back-scattered electron imaging of silver-stained proteins in nucleoli

By using simultaneously the AgNOR silver staining method, back-scattered electron imaging mode and stereo-tilt in scanning electron microscopy (SEM), it is possible to observe the nucleus through the cell surface, the nucleolus, and the tri-dimensional distribution of the AgNOR-associated acidic proteins. In C3H10T1:2 cells and their 7-12-dimethylbenz--anthracene-treated transformants, the staining demonstrates several intranucleolar silver-staining granules (SSG), surrounded by a weakly staining region. The SSG may represent the fibrillar center (FC) and the weakly staining region, the fibrillar dense component (FD). This component can link several SSG together to form a rope-like structure. In cells with no visible nucleolus and inactive nucleolar organizer regions (NORs) the silver-staining granules are less numerous, close together and the presumed fibrillar dense components are not visible. The SSG are located more peripheraly, and the weakly staining region and the rope-like structure are less prominent in control cell nucleoli than in transformed cells with a comparatively high rate of RNA synthesis.