The expression of genes is regulated at many levels. Perhaps the area in which least is known is how nuclear organization influences gene expression. Studies of higher-order chromatin arrangements and their dynamic interactions with other nuclear components have been boosted by recent technical advances. The emerging view is that chromosomes are compartmentalized into discrete territories. The location of a gene within a chromosome territory seems to influence its access to the machinery responsible for specific nuclear functions, such as transcription and splicing. This view is consistent with a topological model for gene regulation.

Studies of higher-order chromatin arrangements are an essential part of ongoing attempts to explore changes in epigenome structure and their functional implications during development and cell differentiation. However, the extent and cell-type-specificity of three-dimensional (3D) chromosome arrangements has remained controversial. In order to overcome technical limitations of previous studies, we have developed tools that allow the quantitative 3D positional mapping of all chromosomes simultaneously. We present unequivocal evidence for a probabilistic 3D order of prometaphase chromosomes, as well as of chromosome territories (CTs) in nuclei of quiescent (G0) and cycling (early S-phase) human diploid fibroblasts (46, XY). Radial distance measurements showed a probabilistic, highly nonrandom correlation with chromosome size: small chromosomes—independently of their gene density—were distributed significantly closer to the center of the nucleus or prometaphase rosette, while large chromosomes were located closer to the nuclear or rosette rim. This arrangement was independently confirmed in both human fibroblast and amniotic fluid cell nuclei. Notably, these cell types exhibit flat-ellipsoidal cell nuclei, in contrast to the spherical nuclei of lymphocytes and several other human cell types, for which we and others previously demonstrated gene-density-correlated radial 3D CT arrangements. Modeling of 3D CT arrangements suggests that cell-type-specific differences in radial CT arrangements are not solely due to geometrical constraints that result from nuclear shape differences. We also found gene-density-correlated arrangements of higher-order chromatin shared by all human cell types studied so far. Chromatin domains, which are gene-poor, form a layer beneath the nuclear envelope, while gene-dense chromatin is enriched in the nuclear interior. We discuss the possible functional implications of this finding.

SUMMARY The effect of refractive‐index mismatch, as encountered in the observation of biological specimens, on the image acquisition process in confocal fluorescence microscopy is investigated theoretically. The analysis takes the vectorial properties of light into account and is valid for high numerical apertures. Quantitative predictions on the decrease of resolution, intensity drop and shift of focus are given for practical situations. When observing with a numerical aperture of 1·3 (oil immersion) and an excitation wavelength of 514 nm the centre of the focus shifts 1·7 μm per 10 μm of axial displacement in an aqueous medium, thus yielding an image that is scaled by a factor of 1·2 in the axial direction. Furthermore, it can be expected that for a fluorescent plane 20 μm deep inside an aqueous medium the peak intensity is 40% less than for a plane which is 10 μm deep. In addition, the axial resolution is decreased by a factor of 1·4. The theory was experimentally verified for test samples with different refractive indices.

No abstract

The resolution of optical microscopy is limited by the numerical aperture and the wavelength of light. Many strategies for improving resolution such as 4Pi and I5M have focused on an increase of the numerical aperture. Other approaches have based resolution improvement in fluorescence microscopy on the establishment of a nonlinear relationship between local excitation light intensity in the sample and in the emitted light. However, despite their innovative character, current techniques such as stimulated emission depletion (STED) and ground-state depletion (GSD) microscopy require complex optical configurations and instrumentation to narrow the point-spread function. We develop the theory of nonlinear patterned excitation microscopy for achieving a substantial improvement in resolution by deliberate saturation of the fluorophore excited state. The postacquisition manipulation of the acquired data is computationally more complex than in STED or GSD, but the experimental requirements are simple. Simulations comparing saturated patterned excitation microscopy with linear patterned excitation microscopy (also referred to in the literature as structured illumination or harmonic excitation light microscopy) and ordinary widefield microscopy are presented and discussed. The effects of photon noise are included in the simulations.

No abstract

We demonstrate that the nuclear topological arrangement of chromosome territories (CTs) has been conserved during primate evolution over a period of about 30 million years. Recent evidence shows that the positioning of chromatin in human lymphocyte nuclei is correlated with gene density. For example, human chromosome 19 territories, which contain mainly gene-dense and early replicating chromatin, are located toward the nuclear center, whereas chromosome 18 territories, which consist mainly of genepoor and later replicating chromatin, is located close to the nuclear border. In this study, we subjected seven different primate species to comparative analysis of the radial distribution pattern of human chromosome 18-and 19-homologous chromatin by three-dimensional fluorescence in situ hybridization. Our data demonstrate that gene-density-correlated radial chromatin arrangements were conserved during higher-primate genome evolution, irrespective of the major karyotypic rearrangements that occurred in different phylogenetic lineages. The evolutionarily conserved positioning of homologous chromosomes or chromosome segments in related species supports evidence for a functionally relevant higher-order chromatin arrangement that is correlated with gene-density.

a b s t r a c tRecent methodological advancements in microscopy and DNA sequencing-based methods provide unprecedented new insights into the spatio-temporal relationships between chromatin and nuclear machineries. We discuss a model of the underlying functional nuclear organization derived mostly from electron and super-resolved fluorescence microscopy studies. It is based on two spatially co-aligned, active and inactive nuclear compartments (ANC and INC). The INC comprises the compact, transcriptionally inactive core of chromatin domain clusters (CDCs). The ANC is formed by the transcriptionally active periphery of CDCs, called the perichromatin region (PR), and the interchromatin compartment (IC). The IC is connected to nuclear pores and serves nuclear import and export functions. The ANC is the major site of RNA synthesis. It is highly enriched in epigenetic marks for transcriptionally competent chromatin and RNA Polymerase II. Marks for silent chromatin are enriched in the INC. Multi-scale cross-correlation spectroscopy suggests that nuclear architecture resembles a random obstacle network for diffusing proteins. An increased dwell time of proteins and protein complexes within the ANC may help to limit genome scanning by factors or factor complexes to DNA exposed within the ANC.