Journal
JOURNAL OF CELL SCIENCE
Volume 121, Issue 7, Pages 1119-1127Publisher
COMPANY BIOLOGISTS LTD
DOI: 10.1242/jcs.026104
Keywords
facultative heterochromatin; large-scale chromatin; electron microscopy; chromocenters; the X chromosome
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Funding
- NIGMS NIH HHS [R01 GM42516, R01 GM042516] Funding Source: Medline
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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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