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.

The nucleolus.

Nucleoli are the sites of biosynthesis of the ribosomal precursors. They contain may copies of the genes for the main rRNAs (18S- and 28 S-rRNA) in the form of tandemly arranged repeats at the chromosomal nucleolar organizer regions (NORs). They also contain the small rRNA (5S-rRNA) that is synthesized outside the nucleolus, specific nucleolar proteins, among them the factors and enzymes necessary for transcription and transcript processing, and the precursor units of the ribosomes.

Nucleolar morphology and rDNA in situ hybridisation in monocytes.

The aim of this study was to correlate morphological changes of nucleoli of non-proliferating monocytes to their functional activity, since nucleolar morphology is currently considered as a diagnostic marker for cell proliferation. Monocytes from healthy donors were fractionated by current counterflow centrifugation and kept in culture for 6 days. Cells were stimulated by the addition of 200 units/ml interferon gamma (IFN gamma).

Human ribosomal RNA gene repeats are localized in the dense fibrillar component of nucleoli: light and electron microscopic in situ hybridization in human Sertoli cells.

The distribution of the human ribosomal gene repeat within human Sertoli cell nucleoli was investigated with the help of DNA-DNA in situ hybridization at the light and electron microscopic level. Probes from both the transcribed part of the gene repeat and the "non-transcribed" spacer were found to hybridize predominantly to the dense fibrillar component of nucleoli. It therefore can be concluded that the dense fibrillar component of nucleoli is the major site of the intranucleolar location of the ribosomal DNA.

Nucleoli, nucleolar chromosomes and ribosomal genes in the human spermatocyte.

The formation and development of nucleoli and their connections with the nucleolar chromosomes were studied in human spermatocytes using electron microscopy, silver staining of nucleolus organizer regions (NORs), high resolution autoradiography and in situ hybridization in order to localize rRNA genes and their transcription in the different stages of meiotic prophase I. At leptotene, new nucleoli were formed, consisting of a fibrillar centre surrounded by a cap of dense fibrillar component. Following [3H]uridine uptake, label was found only over the dense fibrillar component.