Influence of nuclear selection criteria on sex chromatin frequency in oral mucosa cells of newborn females

The frequency of sex chromatin positive nuclei was determined in oral mucosa smears of 47 newborn females during their first three days of life. In contrast to some previous reports, the lowest sex chromatin frequency found was 21 %, and the means for days 1, 2 and 3 were 33.5 %, 34.1 %and 37.0% respectively. The mean value for day 3 was nearly identical with that obtained for six normal adult women. The slight increase over the three days was statistically significant, although the magnitude of this increase was much less than that reported by other workers. By determining the ratio of nuclei with morphologic characteristics suitable for sex chromatin determinations to those which do not meet minimal morphologic criteria, it was possible to show that there are more unsuitable, probably degenerating nuclei present shortly after birth than later on. This factor may explain the low initial frequencies other workers have reported. It may be difficult to differentiate some degenerated nuclei from viable nuclei. The former may be ‘false negatives’, and their inadvertent inclusion in a count would decrease the frequency of sex chromatin positive nuclei. This argument is supported by our findings on sex chromatin counts performed in the absence of selection criteria. Smears with many unsuitable nuclei had a much lower frequency than those with few unsuitable nuclei. This bias can be reduced, if not eliminated, when stringent selection criteria are applied. With standard criteria, reproducible and reliable sex chromatin frequency determinations can be made on oral smears of newborn infants.

DNA content and size of sex chromatin positive female nuclei during the cell cycle

Relative nuclear DNA content and nuclear area were determined in fast growing, karyotypically normal, fibroblastic cell cultures derived from five human females. In two cultures H³ thymidine autoradiographic and photometric determinations were combined on the same nuclei. This allowed the separation of nuclei into three groups: those with the lowest mean DNA values which did not take up H³ thymidine are in the G₁ period (2d group); those with intermediate DNA values which did take up thymidine are presumably synthesizing DNA and are in the S period; those which have double the mean 2d group DNA value and did not take up thymidine are a mixture of nuclei in the G₂ period with some true tetraploids 4d group). The overall frequency of sex chromatin positive nuclei in the cultures studied ranges from 64 to 78%. Sex chromatin positive nuclei occur with about the same frequency in 2d and S nuclei but are somewhat more frequent in 4d nuclei. In three cultures the relative DNA content of sex chromatin was determined. Sex chromatin of 4d nuclei contain about twice as much DNA as those of 2d nuclei; the DNA content of sex chromatin of S nuclei is between that of 2d and 4a nuclei but closer to the mean 2d value. Sex chromatin can be identified in labelled S nuclei. These observations clearly indicate that the condensed, sex chromatin forming X chromosome does not despiralize completely during DNA replication as some authors have suggested. The DNA content of sex chromatin positive and negative nuclei is identical within each nuclear group. The areas of sex chromatin positive 2d group nuclei are larger than for negative nuclei, but this difference is only significant in one of the cultures where a large number of nuclei were measured. It is concluded that failure of the X chromosome to condense in some nuclei of females is not related to DNA