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.
A Barr body (named after discoverer Murray Barr)[1] or X-chromatin is an inactive X chromosome. In species with XY sex-determination (including humans), females typically have two X chromosomes,[2] and one is rendered inactive in a process called lyonization. Errors in chromosome separation can also result in male and female individuals with extra X chromosomes. The Lyon hypothesis states that in cells with multiple X chromosomes, all but one are inactivated early in embryonic development in mammals.[3][4] The X chromosomes that become inactivated are chosen randomly, except in marsupials and in some extra-embryonic tissues of some placental mammals, in which the X chromosome from the sperm is always deactivated.[5]
In humans with euploidy, a genotypical female (46, XX karyotype) has one Barr body per somatic cell nucleus, while a genotypical male (46, XY) has none. The Barr body can be seen in the interphase nucleus as a darkly staining small mass in contact with the nucleus membrane. Barr bodies can be seen in neutrophils at the rim of the nucleus.
In humans with more than one X chromosome, the number of Barr bodies visible at interphase is always one fewer than the total number of X chromosomes. For example, people with Klinefelter syndrome (47, XXY) have a single Barr body, and people with a 47, XXX karyotype have two Barr bodies.
Mechanism
All individuals with two X chromosomes (such as the majority of human females) possesses only one Barr body per somatic cell, while all individuals with one X chromosome (such as most human males) has none.
The Barr body allows for equal expression of X chromosomes in the majority of human males and females.[6] If X inactivation did not occur, females (XX) would be expressing two X chromosomes, and males (XY) would only be expressing one. The disappearance of a Barr body in females (expressing both X chromosomes) can result in misregulation of heterochromatin. This misregulation leaves the potential of epigenetic instability and irregular gene expression.[7] Autosomal genes can be silenced when there is translocation of the X inactivation complex on the X chromosome to an autosome.[8]
Mammalian X-chromosome inactivation is initiated from the X inactivation centre or Xic, usually found near the centromere.[9] The centre contains twelve genes, seven of which code for proteins and five for untranslated RNAs. From the untranslated RNAs, only two are known to play an active role in the X inactivation process, Xist and Tsix.[9] The centre also appears to be important in chromosome counting: ensuring that random inactivation only takes place when two or more X-chromosomes are present. The provision of an extra artificial Xic in early embryogenesis can induce inactivation of the single X found in male cells.[9]
The roles of Xist and Tsix appear to be antagonistic. The loss of Tsix expression on the future inactive X chromosome results in an increase in levels of Xist around the Xic. Meanwhile, on the future active X Tsix levels are maintained; thus the levels of Xist remain low.[10] This shift allows Xist to begin coating the future inactive chromosome, spreading out from the Xic.[2] In non-random inactivation this choice appears to be fixed and current evidence suggests that the maternally inherited gene may be imprinted.[3] Variations in Xi frequency have been reported with age, pregnancy, the use of oral contraceptives, fluctuations in menstrual cycle and neoplasia.[11]
It is thought that this constitutes the mechanism of choice, and allows downstream processes to establish the compact state of the Barr body. These changes include histone modifications, such as histone H3 methylation (i.e. H3K27me3 by PRC2 which is recruited by Xist)[12] and histone H2A ubiquitination,[13] as well as direct modification of the DNA itself, via the methylation of CpG sites.[14] These changes help inactivate gene expression on the inactive X-chromosome and to bring about its compaction to form the Barr body.
Reactivation of a Barr body is possible, and has been seen in breast cancer patients.[15] One study showed that the frequency of Barr bodies in breast carcinoma was significantly lower than in healthy controls, indicating reactivation of previously inactivated X chromosomes.[15] In breast cancer cell lines, a loss of the repressive histone mark H3K27me3 was observed on the inactive X chromosome, disrupting its silenced state and leading to the expression of genes that are typically repressed.[16] This includes the bi-allelic expression of X-linked genes such as TBL1X and HDAC8. The abnormal expression of these genes may contribute to tumor progression by altering key pathways of transcriptional regulation.[16]
Uses
Barr Bodies in Ancient Samples: Observation and Relevance in Gender Identification of Extinct Species
Barr bodies are condensed, inactive X chromosomes found in the somatic cells of female mammals. Their detection in ancient samples provides a powerful tool for gender identification in extinct species, offering insights into population dynamics, biology, and evolution.
In forensic science, gender determination can be determined by analyzing dental pulp in Barr bodies.[17] Teeth are durable in the human body and is commonly used in forensics because of its characteristic of being less vulnerable to contamination by external DNA and its abundance in the body.[18] The presence of Barr bodies in dental pulp can be examined using histopathological and cytopathological techniques, where mean Barr body count is more in females than in male samples.[17] While the presence of Barr bodies is indicative of female sex, their absence is not sufficient to confirm male sex due to the possibility of chromosomal abnormalities or variations.[19]
Recent advancements in histological and genomic techniques have made it possible to observe Barr bodies in ancient remains, including fossilized bones and tissues:
Histological Staining: Techniques like hematoxylin-eosin staining can highlight chromatin structures, including Barr bodies, in well-preserved osteocytes embedded within bone matrix.[20]
Fluorescence Microscopy: Fluorescent dyes can differentiate X-chromosome condensation patterns, aiding in the visualization of Barr bodies.[20]
Integration with Genomic Tools: Techniques such as PaleoHi-C enable the spatial reconstruction of chromosomal interactions, confirming the presence of inactivated X chromosomes in ancient samples.
In a notable example, Barr bodies were detected in osteocytes from ancient mammalian remains, demonstrating the potential of this approach for studying gender in extinct populations.[20]
Relevance in Gender Identification
Population Studies:
Identifying sex ratios in extinct species sheds light on social structures, reproductive strategies, and extinction dynamics.
Reconstruction of Lifeways:
Understanding the distribution of genders within ancient populations allows bioarchaeologists to analyze sex-based differences in diet, health, and activity patterns.[21]
Preservation of Chromatin:
The discovery of intact Barr bodies in fossils underscores the potential for studying chromosomal and epigenetic features in ancient samples
Limitations and Challenges
Degradation of Samples: The fragmentation and chemical damage of ancient DNA and chromatin often hinder Barr body detection.
Sample Availability: Successful detection depends on the preservation of osteocytes or other cells within the sample matrix.
Replicability: Variability in preservation conditions can limit the reproducibility of results across samples.
Future Directions
Further research into the detection of Barr bodies may enhance our ability to:
Identify sex in a broader range of extinct species.
Study X-chromosome inactivation patterns across evolutionary timescales.
Integrate histological and genomic methods to reconstruct detailed population dynamics.
1 2 Brown, C.J., Robinson, W.P., (1997), XIST Expression and X-Chromosome Inactivation in Human Preimplantation Embryos Am. J. Hum. Genet. 61, 5–8 (Full Text PDF)
1 2 3 Rougeulle, C.; Avner, P. (2003). "Controlling X-inactivation in mammals: what does the centre hold?". Seminars in Cell & Developmental Biology. 14 (6): 331–340. doi:10.1016/j.semcdb.2003.09.014. PMID15015740.
↑ Lee, J. T.; Davidow, L. S.; Warshawsky, D. (1999). "Tisx, a gene antisense to Xist at the X-inactivation centre". Nat. Genet. 21 (4): 400–404. doi:10.1038/7734. PMID10192391. S2CID30636065.
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