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First published online 11 March 2008
doi: 10.1242/jcs.026104

Journal of Cell Science 121, 1119-1127 (2008)
Published by The Company of Biologists 2008
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Research Article

The facultative heterochromatin of the inactive X chromosome has a distinctive condensed ultrastructure

Alena Rego, Paul B. Sinclair, Wei Tao*, Igor Kireev and Andrew S. Belmont{ddagger}

Department of Cell and Developmental Biology, University of Illinois, 601 South Goodwin Avenue, Urbana, IL 61801, USA

{ddagger} Author for correspondence (e-mail: asbel{at}uiuc.edu)

Accepted 8 January 2008

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.

Key words: Facultative heterochromatin, Large-scale chromatin, Electron microscopy, Chromocenters, The X chromosome


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