Dr. Grumbach: Barr and associates have demonstrated that in the human the majority of somatic cells of females contain a conspicuous, heterochromatic mass of chromatin in the resting nuclei. Their discovery of a sex-difference in intermitotic nuclei of a number of vertebrate species, including man, has provided a relatively simple method for assessing the sex-chromosome constitution. This chromatin mass is about 1 micron in diameter and often plano-convex in configuration. It is usually located against the inner surface of the nuclear membrane and contains desoxyribonucleic acid. In males, a comparable chromatin mass is rarely found, never in more than a few per cent of the nuclei. There is good evidence that this so-called "sexchromatin" represents the fusion of heterochromatic portions of two X-chromosomes.
The sex chromatin can be conveniently determined by examination of specimens of skin obtained by biopsy (Fig. 1). Recently, more practical methods for determining cytologic sex have been described employing smears from readily available tissues, such as the oral and vaginal mucosa (Fig. 2) and the blood. Davidson and Smith have shown that there is a sex difference in the morphology of polymorphonuclear neutrophils.
Cytologic examination of chromosomal sex has provided an important tool for the investigation of anomalies of sex development. Apart from its ancillary role in diagnosis, cytologic examination of sex chromatin has made a significant contribution to our understanding of the disordered development in these afflictions. However, the results of this determination should not be regarded as an especial indication of the psychosexual orientation of patients with such abnormalities, nor, in the case of infants, of the sex to which they should be assigned.