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

Alena Rego, Paul B. Sinclair, Wei Tao, Igor Kireev, Andrew S. Belmont

Research output: Contribution to journalArticlepeer-review


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.

Original languageEnglish (US)
Pages (from-to)1119-1127
Number of pages9
JournalJournal of cell science
Issue number7
StatePublished - Apr 1 2008


  • Chromocenters
  • Electron microscopy
  • Facultative heterochromatin
  • Large-scale chromatin
  • The X chromosome

ASJC Scopus subject areas

  • Cell Biology


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