Barr's corpuscle

The Barr corpuscles or bodies, also called sex chromatin X, are a heterochromatic, flat and convex mass, measuring 0.7x1.2 microns, found in the nucleus of the somatic cells of the females of some animals. They are formed by the condensation of the sexual chromatin of one of the X chromosomes, which is inactivated due to the process called lionization in animals where sex is determined with the presence or absence of the Y chromosome.

They were discovered by the Canadian Murray Barr and Ewart George Bertram in 1949, who demonstrated that it is possible to genetically determine the sex of an individual depending on whether or not this mass of chromatin exists on the inner surface of the nuclear membrane (sex chromatin).).

The study of sex chromatin, especially of cells of the oral mucosa, allows us to identify the presence of the X chromosome in newborns with undefined external genitalia to obtain the diagnosis of sex in an individual with Disorders of sexual development.

Formation mechanism

According to the Lyon hypothesis (proposed by Mary Lyon in 1966), one of the two X chromosomes in each female somatic cell is genetically inactive. Barr's body represents the inactive X chromosome. She determined 4 principles for sex chromatin:

1. sexual chromatin is genetically inactive,
2. Inactivation occurs at random,
3. Inactivation can be in the parent chromosome.
4. Inactivation occurs on the 16th day of the embryonic period.

The number of Barr masses is determined by the formula B = X – (P/2), where P symbolizes the ploidy of the cell. Thus, the number of Barr bodies observed in a normal human female somatic cell will be B = X – (1). A normal female human cell has a single Barr Body per cell.

Inactivation of the X chromosome in mammals starts from a zone near the centromere. This zone contains twelve genes, of which seven code for proteins, five for RNAs that will not be translated (only two are known to play a role). of them in chromosome inactivation). This area also seems to be essential in decision making, that is, it ensures that chromosome inactivation only occurs when there are two or more X chromosomes in the cell. It is thought that the mechanism of choice is based on the antagonistic action of the two RNAs that will not be translated that are encoded in the region near the centromere.

Achieving chromosome condensation requires histone modifications (H3 methylations), histone H2A ubiquitinations, as well as direct DNA modifications mediated by CpG site methylations. All of these modifications aid in the inactivation of the expression of the X chromosome genes, so that their condensation and compaction is achieved until the Barr Body is achieved.

Structure of X chromosomes in chromatin

Dip-C studies, based on chromosome conformation studies, affirm that the inactive X chromosome tends to exhibit a compact conformation, while the active X chromosome usually presents in an expanded form.

Regarding the compartmentalization of chromosomes in the chromatin structure, the active X chromosome occurs in both euchromatin and heterochromatin regions (as occurs naturally on the male X chromosome) whereas the compartmentalization of the Inactive X chromosome is more homogeneous. Furthermore, these studies show the formation of multiple superloops in the inactive X chromosome.