Barr body
Barr body is the small darkly stained structure, in the nucleus. In the majority of cases, it is found in the periphery of the nucleus, in minority of cases, it is seen in other parts , sometimes it is observed in proximity to nucleolus.
It is also called as X /Sex Chromatin. Basically this is deactivated or inactive X chromosome
.
This darkly stained structure is named after Dr Murray Barr who was a canadian physician who along with his student Dr Bertram way back in 1948 identified this structure.

Now we need to understand as to why there is inactivation of the x chromosome. To understand the this first , we should understand Lyon hypothesis, proposed by Dr Lyon who was a british geneticist.

It states

In female mammals, one of the two X-chromosomes in the somatic cell is inactive.
Inactivation of X-chromosome is random.
Inactivation occurs during development.
Inactive X chromosome remains inactivated in all the generations of the cell.

So, the inactivation of X chromosome is to
Ensure females, like males, have one functional copy of the X chromosome in each body cell.
Equalize dosage of gene products from the X chromosome between XX females and XY males

so, basically one X-chromosome is disabled to avoid unwanted information from being passed down to future generations.

Mechanism of X chromosome inactivation:
The X chromosome inactivated this is by a process called transcriptional silencing. Each X chromosome has X inactivation center in which there are genes. One set of genes X-inactive specific transcript (Xist), is responsible for inactivation of X-chromosome. This process is called LYONIZATION. The other set of gene Tsix (Xist reversed) prevents inactivation!

This inactive X chromosome is silenced by being packaged into a transcriptionally inactive structure called HETEROCHROMATIN. This is a tightly packed chromatin with no or little transcriptional activity and this is seen morphologically as Barr Body. Note that normally, active chromosome is enclosed within euchromatin which is a loosely packed chromatin which participates in the transcriptional activity.

Total number of Barr bodies: The no. of Barr bodies at interphase is nX-1
The no. of Barr bodies is one less than the total no. of X chromosomes as illustrated in the table below

Demonstration of Barr body
It is best demonstrated by any nuclear dyes like Pap, Cresyl violet, thionin, orcein, Feulgen, carbol fuschin and flouroscent staining. It can also be demonstrated in unstained and in living cells by Phase contrast microscopy.

Why Barr Body testing is done ? INDICATIONS

1.Method to identify person’s gender and to establish sexual identity in newborns and adults
2.Was earlier used in competitive sports to identify males masquerading as females (to gain competitive advantage!)
3. As a screening test for ambiguous genitalia (sexual characters are not clear )
4. Delayed onset of puberty
5. Crime Scene! (Buccal epithelial cells in saliva traces )

Procedure:
After thoroughly rinsing the mouth with water, the buccal smears are taken with the help of spatula of cytobrush whiich is them smeared on the glass slide. The smears are fixed, stained and then observed under microscope.

Normally, Barr body is seen in 30% to 60% of the female somatic cells

Other tissue from which Barr bodies can be studied include- Corneal epithelium, vaginal mucosa, fibroblasts etc.

Also seen in polymorphonuclear leukocytes in peripheral blood smears- appears as small deeply stained nuclear mass of about 1.5 μ in diameter, attached to the body of the nucleus by means of a thin stalk. This is the classical DRUMSTICK appearance, which is the most common morphological form. Seen in 1% to 17% of neutrophils with an average of 2.9%. This is also referred to as DAVIDSON BODY, named after Prof William Mackay Davidson (1909-1991) a Scottish pathologist, hematologist who first doscoverd it. The other morphological forms are Racquet forms, sessile nodules, small clubs and minor lobes.

The difference between Barr body and Davidson body is summarised below.

To watch the video tutorial on Barr body click on the link below