Sex of nuclei in ocular tissues.

BARR AND BERnTAM (1949), reporting their observations on small chromatin bodies lying adjacent to the large nucleoli in nerve cells of the cat, pointed out for the first time that these peculiar structures, known then as " nucleolar satellites", were related to the sex of the animal, being found frequently in the female and seldom in the male. It has since been shown that this chromatin mass, now renamed " sex chromatin", may readily be demonstrated in the nuclei of most tissues of the female, both human and animal (Barr and others, 1950; Graham and Barr, 1952; Prince, 1952; Moore and Barr, 1953, 1954; Emery and McMillan, 1954). In nuclei of somatic cells other than those of neurones, the sex chromatin appears as a small hemispherical mass with its flat surface lying against the nuclear membrane. It stains well with basic dyes, but is most clearly demonstrated in Feulgen-stained preparations of female skin, where it may be observed in up to 70 per cent. of nuclei (Fig. 1). In the male similar intranuclear bodies may occasionally be seen, but rarely in more than 10 per cent. of nuclei, and these are probably unrelated to the sex chromatin. Similar sex differences exist in the nuclei of neutrophil polymorphonuclears, as was first shown by Davidson and Smith (1954), who noted small drumstick projections arising from the nuclei of occasional polymorphonuclears in women. This appears to be a further variant of sex chromatin and they have found that sex determination from an ordinary blood-film examination is completely reliable. The exact nature of the special chromatin mass in the female cell is not yet finally determined, but the simplest explanation is that it consists of the XX pair of chromosomes, which in combination are sufficiently large and morphologically typical to be distinct from particulate chromatin, whereas the Y chromosome is too small to render visible the XY pair ofchromosomes in the male nucleus. The apparent existence of sex chromatin in a small proportion of male nuclei may possibly be accounted for by artefacts caused by coarse particles of chromatin, by indentations of the nuclear-membrane, and by overlapping of nuclei. In any event, whatever the nature of the chromatin mass, it is now generally accepted that sections of unknown origin, as far as sex is concerned, may be accurately classified as female or male;