Barr body

Nucleus of a female amniotic fluid cell. Top: Both X-chromosome territories are detected by FISH. Shown is a single optical section made with a confocal microscope. Bottom: Same nucleus stained with DAPI and recorded with a CCD camera. The Barr body is indicated by the arrow, it identifies the inactive X (Xi).

Left: DAPI stained female human fibroblast with Barr body (arrow). Right: histone macroH2A1 staining. Arrow points to sex chromatin in DAPI-stained cell nucleus, and to the corresponding sex chromatin site in the histone macroH2A1-staining.

File:Barr body visualization.jpg

Visualization of a Barr body in a somatic cell nucleus.

Barr Body

The Barr body, named in honor of its discoverer, Murray Barr,[1] represents the inactivated X chromosome located within the nucleus of female somatic cells.[2] This phenomenon is a part of the lyonization process, which occurs in species where the Y or W chromosome determines sex instead of the diploidy of the X or Z chromosomes. The Lyon hypothesis proposes that in cells possessing multiple X chromosomes, all but one become inactivated during the embryogenesis of mammals.[3] This inactivation typically unfolds randomly in mammals early in their embryonic development.[4] However, there are exceptions like marsupials and certain placental mammals' extra-embryonic tissues, where the paternal X chromosome undergoes consistent deactivation.[5]

For humans possessing multiple X chromosomes, the count of Barr bodies discernible during interphase is always less by one than the total X chromosomes. As an illustration, men diagnosed with Klinefelter syndrome (having a 47,XXY karyotype) exhibit one Barr body, whereas women with a 47,XXX karyotype present with two Barr bodies. One can commonly observe Barr bodies on neutrophils' nuclei.

Mechanism

Standard human females showcase a single Barr body in every somatic cell, while their male counterparts typically exhibit none.

Initiation of mammalian X-chromosome inactivation originates from the X inactivation centre (or Xic), usually situated in proximity to the centromere.[6] This center hosts twelve genes; out of these, seven code proteins and five are designated for untranslated RNAs. Out of these, only Xist and Tsix have been identified as active participants in the X inactivation process.[6] Furthermore, the center seems pivotal for chromosome counting, ensuring the random inactivation process is exclusive to the presence of two or more X chromosomes.

Xist and Tsix functions appear to contrast. A decline in Tsix expression on the impending inactive X chromosome leads to enhanced Xist levels around the Xic. On the prospective active X, Tsix levels remain consistent, causing Xist levels to stay low.[7] This shift paves the way for Xist to commence covering the soon-to-be inactive chromosome, expanding from the Xic.[8] In non-random inactivation, this choice seems predetermined, suggesting potential imprinting of the maternally inherited gene.[4]

This mechanism is believed to drive the choice, initiating subsequent processes to establish the compact state of the Barr body. Accompanying changes involve histone modifications, such as histone H3 methylation[9] and histone H2A ubiquitination,[10] as well as direct DNA modifications through CpG site methylation.[11] These modifications contribute to gene expression inactivation on the inactive X-chromosome and promote its condensation into the Barr body.

See also

• X-inactivation
• Sex-determination system
• Demethylation
• Acetylation

Barr body Resources
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