ELCS in ice: cryo-electron microscopy of nuclear envelope-limited chromatin sheets |
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Authors: | Mikhail Eltsov Sergey Sosnovski Ada L. Olins Donald E. Olins |
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Affiliation: | 1. Cell Biology and Biophysics Unit, European Molecular Biology Laboratory, Meyerhofstr. 1, 69117, Heidelberg, Germany 2. Neurophysiology & New Microscopies Laboratory, INSERM U603 - CNRS UMR 8154, 45 rue des Saints Pères, 75006, Paris, France 3. Department of Pharmaceutical Sciences, College of Pharmacy, University of New England, 716 Stevens Avenue, Portland, ME, 04103, USA
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Abstract: | Nuclear envelope-limited chromatin sheets (ELCS) form during excessive interphase nuclear envelope growth in a variety of cells. ELCS appear as extended sheets within the cytoplasm connecting distant nuclear lobes. Cross-section stained images of ELCS, viewed by transmission electron microscopy, resemble a sandwich of apposed nuclear envelopes separated by ~30 nm, containing a layer of parallel chromatin fibers. In this study, the ultrastructure of ELCS was compared by three different methods: (1) aldehyde fixation/dehydration/plastic embedding/sectioning and staining, (2) high-pressure freezing/freeze substitution into plastic/sectioning and staining, and (3) high-pressure freezing/cryo-sectioning/cryo-electron microscopy. ELCS could be clearly visualized by all three methods and, consequently, must exist in vivo and are not fixation artifacts. The ~30-nm chromatin fibers could only be observed following aldehyde fixation; none were seen in cryo-sections. Electron microscopic tomography tangential views of aldehyde-fixed ELCS suggested an ordering of the separate chromatin fibers adjacent to the nuclear envelope. Possible mechanisms of this chromatin ordering are discussed. |
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