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Originally published as JHC exPRESS on October 3, 2005.
doi:10.1369/jhc.5R6780.2005
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Journal of Histochemistry and Cytochemistry
Volume 54 (2): 131-145, 2006
Copyright ©The Histochemical Society, Inc.

REVIEW

Structural and Functional Organization of Ribosomal Genes within the Mammalian Cell Nucleolus

Massimo Derenzini, Gianandrea Pasquinelli, Marie-Françoise O'Donohue, Dominique Ploton and Marc Thiry

Sezione di Patologia Clinica, Dipartimento di Patologia Sperimentale, Università di Bologna, Italy (MD,GP); Unité MéDIAN, CNRS UMR 6142, IFR 53, Reims, France (M-F O'D,DP); and Laboratory of Cell and Tissue Biology, Department of Life Sciences, Faculty of Sciences, University of Liège, Liège, Belgium (MT)

Correspondence to: Massimo Derenzini, Dipartimento di Patologia Sperimentale, Via S. Giacomo 14, 40126 Bologna, Italy. E-mail: massimo.derenzini{at}unibo.it

Data on the in situ structural–functional organization of ribosomal genes in the mammalian cell nucleolus are reviewed here. Major findings on chromatin structure in situ come from investigations carried out using the Feulgen-like osmium ammine reaction as a highly specific electron-opaque DNA tracer. Intranucleolar chromatin shows three different levels of organization: compact clumps, fibers ranging from 11 to 30 nm, and loose agglomerates of extended DNA filaments. Both clumps and fibers of chromatin exhibit a nucleosomal organization that is lacking in the loose agglomerates of extended DNA filaments. In fact, these filaments constantly show a thickness of 2–3 nm, the same as a DNA double-helix molecule. The loose agglomerates of DNA filaments are located in the fibrillar centers, the interphase counterpart of metaphase NORs, therefore being constituted by ribosomal DNA. The extended, non-nucleosomal configuration of this rDNA has been shown to be independent of transcriptional activity and characterizes ribosome genes that are either transcribed or transcriptionally silent. Data reviewed are consistent with a model of control for ribosome gene activity that is not mediated by changes in chromatin structure. The presence of rDNA in mammalian cells always structurally ready for transcription might facilitate a more rapid adjustment of the ribosome production in response to the metabolic needs of the cell. (J Histochem Cytochem 54:131–145, 2006)

Key Words: ribosomal genes • nucleolus • chromatin structure in situ • ribosomal transcription • electron microscopy • osmium ammine • mammalian cells


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