Nucleolus: the ribosome factory.

The nucleolus is a nuclear compartment and represents the most obvious and clearly differentiated nuclear structure seen in the microscope. Within nucleoli most events of ribosome biogenesis, such as ribosomal RNA synthesis, processing, and ribosome subunit assembly, take place. Several lines of evidence now show that the nucleolus has also numerous non-ribosomal functions. This review is focused on the recent progress in our knowledge of how to correlate the known biochemical processes taking place in the nucleolus with nucleolar structures observed in the microscope. We still lack detailed enough information to understand fully the organization and regulation of the processes taking place in the nucleolar sub-structures. However, the present power of microscopy techniques should allow for an in situ description of the organization of nucleolar processes at the molecular level in the years to come.