Ribosome biogenesis in man: Current views on nucleolar structures and function

Nucleoli develop when preribosomes are synthesized at the chromosomal nucleolar organizer regions. Typically they consist of at least three nucleolar subcompartments, the fibrillar center (FC), the dense fibrillar component (DF), and the granular component (GC). The understanding of the functional arrangements of these subcompartments relates to aspects in cell biology, pathology, and virus research. In the present review morphological studies are discussed in the light of molecular findings. The available data confirm the hypothesis that rDNA transcription is connected with the DF but not necessarily with the presence of an FC. Within the DF, rDNA transcription is restricted to foci, possibly representing single transcribing genes. FCs may serve to store inactive transcription factors, to initiate rDNA transcription, and may provide structural support for transcription. The GC can be interpreted as a collection of maturing preribosomes. More recently the nucleolar subcompartments were focused on in the context of virus research and tumor biology.