Sex chromatin bodies

Comings, D. E. 1967. The duration of replication of the inactive X chromosome in humans based on the persistance of the heterochromatic sex chromatin body during DNA synthesis. Cytogenetics (Basel), 6 20-37. 

XXX Type of Trisomy. Patients have been observed with duplicate sex chromatin bodies in the cells of the buccal mucosa, with resistant amenorrhea, infantile external genitalia, underdeveloped breasts, and high levels of urinary gonadotropins. Chromosome studies on cultured bone marrow cells demonstrated the presence of 47 chromosomes with the XXX type of trisomy. 

One index of inactivation is the formation of sex chromatin bodies. In man and macaque, these bodies were identified in trophoblast cells, in 10-12-day-old blastocysts and in 16-19-day-old embryos (Glenister, 1956 Park, 1957 Liberman, 1966 Zacharov, 1968) - i.e., at the 2600-5000 cell stage (Hamerton, 1964). At the same time, sex chromatin was observed in 14% of the trophoblast cells on the 13th day of embryonic development, but is absent in the mesoderm of the chorion. 

Sex chromatin formation does not occur in all cells of the embryo. In 3-months-old human embryos, the sex chromatin body is present in the nuclei of somatic cells, while it is absent in oogonia and oocyte nuclei (Ohno et al. 1961). 

Uncultured cells have also been used to a limited extent for fetal sex determination. This Is possible because the cells can be appropriately stained and examined for the presence of sex chromatin, characteristic of female cells, and for the fluorescent Y-body, characteristic of male cells (33,34). However, these cytologic techniques are not completely reliable, and It Is still desirable to determine fetal sex by direct chromosome analysis (35). 

When a sample of suspect blood arrives at the laboratory, it may be useful to determine the sex of the donor (for example, in the case of criminal abortion). Low-performance older techniques (search as for the sex chromatin, Barr s body, Y-chromosome fluorescence, etc) and other methods such as the determination of sex hormone levels (progesterone/ testosterone) have been replaced by procedures for detecting sites on the X and Y chromosomes using PCR. At present, the sex determination is carried out at the same time that the genetic profile is determined when using multiplex STR kits. 

In support of Mary Lyon s hypothesis is the fact that the number of chromatin bodies found in individuals is equal to the number of X chromosomes in the karyotype minus one. However, the second X chromosome cannot be considered to be totally unnecessary. If it were, the XO karyotype would not lead to the clinical symptomatology found in Turner s syndrome, nor would an XXY karyotype result in a phenotype different from XY. It has been proposed that the second X chromosome might affect the expression of the genotype in the phenotype simply by its presence. The number of ridges in the fingerprint decreases as the number of genetically inactive X chromosome increases. The Y chromosome unquestionably determines the male sex characteristics, and the male sex characteristics develop in the phenotype even if the karyotypes contain as many as three X chromosomes. Whether the Y chromosome determines the appearance of the phenotype because it stores specific genes or simply because of its presence in the karyotype has not been established. 

Belgian cytologist Edouard van Beneden (1846-1910) discovers that the number of chromatin-containing threadlike bodies (subsequently named chromosomes) in the cells of a given species is always the same and that the sex cells contain half this number. 

---