ReviewThe perichromatin region: A functional compartment in the nucleus that determines large-scale chromatin folding
Section snippets
Nuclear compartmentalization
The nucleus is a cellular compartment that contains various structurally and functionally different subcompartments. In contrast to the cytoplasm, compartmentalization does not depend on diffusion barriers, such as membranes. Rather, it seems at least in part related to self-organizing properties of the chromatin fibre and adhering proteins and RNA molecules. Various nuclear structures have been rather well characterized, especially thanks to methods of ultrastructural cytochemistry, starting
In situ chromatin structure based on electron microscopy
Transmission electron microscopic studies of ultrathin sections of intact cells reveal two morphological forms of chromatin. One is condensed or dense chromatin, which represents most of the chromatin in the nucleus of most cells. This, among others contains classic heterochromatin, such as that of the Barr body of the inactive X chromosome and pericentromeric heterochromatin [15]. The dispersed or diffuse chromatin that is localized in the periphery of condensed chromatin domains corresponds
Nuclear localization of the sites of RNA synthesis
Studies aiming at visualizing the sites of synthesis of nucleoplasmic RNA by electron microscopic methods in different cell types, using short pulses of either radioactive [24], [25], [26] or halogenated [27], [28] precursors identified the border region of condensed chromatin areas, called the perichromatin region, as the major site of newly synthesized RNA. Nascent RNA has been predominantly associated with RNP fibrils originally described by Monneron and Bernhard [6] and visualized by a
Chromosome territories and the perichromatin region
It has been established, during the last two decades, that chromatin originating from different chromosomes does not mix considerably. Chromatin domains belonging to the same chromosome give rise to chromosome territories [37]. The visualization of individual chromosome territories has mainly been based on the application of FISH methods, using chromosome-specific DNA probes.
As to the possibility of revealing, at an ultrastructural level, individual chromosome territories in the interphase
How is the perichromatin region organized?
From the different data mentioned above it appears that a number of essential nuclear functions are exerted in the perichromatin domain. This consists of dispersed chromatin, the degree of dispersion probably depending on local gene activity. In this context it is interesting to mention the work of Derenzini et al. [43] on regenerating rat liver, suggesting that the chromatin ultrastructural pattern may be a consequence and not a cause of gene transcription (see also [44]).
The association of
The perichromatin domain and the folding of the chromatin fibre
How can we envisage that the chromatin fibre is folded inside the nucleus? Whatever the condensation state is of the fibre (30 nm fibre or higher), the electron microscopy studies summarized here show two important aspects of it. First, electron microscopy indicates that an interphase chromosome is made up of a number of irregularly shaped condensed chromatin domains with dimensions in the range of 100–800 nm or sometimes even more. We can make an estimate of how much chromatin on average is
Acknowledgements
We thank Dr. Cédric Bouchet-Marquis for kindly providing his micrograph. We are indebted to Mrs. Liliane Hautle for her help with the preparation of the manuscript, to Mr. Willy Blanchard for photographic assistance, and to Dr. Jacques Rouquette for critical reading of the manuscript. The work has been supported by the Swiss National Science Foundation and by the Dutch National Research Council NWO.
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