title: ELCS in ice: cryo-electron microscopy of nuclear envelope-limited chromatin sheets.
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.
authors.affiliation: Cell Biology and Biophysics Unit, European Molecular Biology Laboratory, Meyerhofstr. 1, 69117, Heidelberg, Germany, eltsov@embl.de.
authors.forename: Mikhail
authors.initials: M
authors.lastname: Eltsov
authors.forename: Sergey
authors.initials: S
authors.lastname: Sosnovski
authors.forename: Ada L
authors.initials: AL
authors.lastname: Olins
authors.forename: Donald E
authors.initials: DE
authors.lastname: Olins
mesh_terms: D002843:Chromatin / Q000378:metabolism / Q000648:ultrastructure*; D020285:Cryoelectron Microscopy; D018922:HL-60 Cells; D006801:Humans; D007399:Interphase; D009685:Nuclear Envelope / Q000378:metabolism / Q000648:ultrastructure*
chemical_list: D002843:Chromatin