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  • Title: The facultative heterochromatin of the inactive X chromosome has a distinctive condensed ultrastructure.
    Author: Rego A, Sinclair PB, Tao W, Kireev I, Belmont AS.
    Journal: J Cell Sci; 2008 Apr 01; 121(Pt 7):1119-27. PubMed ID: 18334550.
    Abstract:
    The mammalian inactive X chromosome (Xi) is a model for facultative heterochromatin. Increased DNA compaction for the Xi, and for facultative heterochromatin in general, has long been assumed based on recognition of a distinct Barr body using nucleic-acid staining. This conclusion has been challenged by a report revealing equal volumes occupied by the inactive and active X chromosomes. Here, we use light and electron microscopy to demonstrate in mouse and human fibroblasts a unique Xi ultrastructure, distinct from euchromatin and constitutive heterochromatin, containing tightly packed, heterochromatic fibers/domains with diameters in some cases approaching that of prophase chromatids. Significant space between these packed structures is observed even within condensed regions of the Xi. Serial-section analysis also reveals extensive contacts of the Xi with the nuclear envelope and/or nucleolus, with nuclear envelope association being observed in all cells. Implications of our results for models of Xi gene silencing and chromosome territory organization are discussed.
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