The facultative heterochromatin of the inactive X chromosome has a distinctive condensed ultrastructure.
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.
Rego
Alena
A
Department of Cell and Developmental Biology, University of Illinois, 601 South Goodwin Avenue, Urbana, IL 61801, USA.
Sinclair
Paul B
PB
Tao
Wei
W
Kireev
Igor
I
Belmont
Andrew S
AS
D002478:Cells, Cultured; D041321:Chromosomes, Human, X / Q000235:genetics / Q000648:ultrastructure*; D005260:Female; D005455:Fluorescent Antibody Technique; D006570:Heterochromatin / Q000235:genetics / Q000648:ultrastructure*; D006801:Humans; D016253:Microscopy, Immunoelectron; D049951:X Chromosome Inactivation
D006570:Heterochromatin