The nucleolus is the site within the eukaryotic nucleus of transcription of rDNA, of processing of the rDNA transcripts, and of the formation of pre-ribosomal particles. We review current ideas for the molecular organization of these processes. The earliest transcriptional events take place near the junction of the fibrillar centers and the dense fibrillar component; nascent transcripts occupy the dense fibrillar component, and the later stages of pre-ribosome formation take place in the granular component. We review current knowledge of non-ribosomal nucleolar proteins. Nucleoli contain a group of proteins that bind RNA and are likely to act as chaperones to facilitate the molecular structural events in the folding and interaction of the many components of ribosomes. Some of these nucleolar proteins are also engaged in a shuttling cycle between the nucleus and the cytoplasm and may serve to transport other proteins.

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The nucleolus, the site of transcription and processing of the major ribosomal genes, generally reveals three distinct ultrastructural components in conventional thin‐section electron micrographs (fibrillar centres, dense fibrillar component and granular component). We show here that different parts of the transcription and transcript processing pathway can be mapped to the different nucleolar components in pea root cells. This study shows the full three‐dimensional arrangement of the different domains by in situ hybridization and confocal microscopy, and their correspondence with the major ultrastructural components of the nucleolus is revealed by parallel serial section electron microscopy. The active rDNA is widely dispersed in discrete foci, the larger of which, at least, correspond to well‐defined fibrillar centres. A probe to the external transcribed spacer (ETS) sequence of the pre‐rRNA transcripts labels clearly demarcated regions surrounding the foci of rDNA, and which we show correspond to the dense fibrillar component. Finally, a probe to the entire 45S transcript shows a higher concentration in regions corresponding to the granular component, surrounding the dense fibrillar component labelled by the ETS probe. The changes in structure that occur with heat shock show that nucleolar organization is dynamic and dependent upon transcriptional activity. These results show that the various RNA processing events are spatially highly organized and suggest a vectorial or radial model of transcription and transcript processing, where nascent and newly completed transcripts occupy zones surrounding the genes, which are in turn surrounded by regions containing the older more mature transcripts.

The luminal epithelium of adult ovariectomized mice responds to estradiol-17 beta with a synchronised wave of DNA synthesis and mitosis. Estriol, however, although producing a similar DNA-synthetic and mitotic response fails to cause an increase in cell number owing to a wave of cell death occurring at mitosis. In the present study it was shown that cells died by two different routes. The majority died by apoptosis but, unusually, a minority also died by necrosis. In the apoptotic cells the cytoplasm became dense, the endoplasmic reticulum and nuclear cisternae dilated; chromatin became marginated the nucleus shrank and became deeply infolded and contorted. Apoptosis, however, was uncharacteristic in that the nucleus failed to fragment, form caps or show disruption before the cells died by membrane rupture. Furthermore, the cells were frequently lost in sheets from the epithelium into the lumen. Part of the biochemical explanation for this onset of cell death comes from the accelerated loss from the tissue of estriol when compared to estradiol-17 beta. This resulted in a decline in protein and rRNA biosynthesis and a failure to complete ribosomal maturation. Evidence in favour of this explanation came from experiments that showed a return to the estradiol-17 beta level of response and an inhibition of cell death when the occupancy of the estriol receptor was maintained.