Factors influencing mammalian X chromosome condensation and sex chromatin formation

Abstract

The aim of this study is to determine why, in contrast to expectations based on the Lyon hypothesis, a variable number of nuclei of cells from mammalian females are sex chromatin negative. The frequency of sex chromatin positive nuclei was determined in cell cultures of varying cell densities. The cells were derived from seven chromosomally normal human female embryos, one newborn female with an extra E group chromosome and two normal male embryos. In all cultures of females the frequency of sex chromatin positive nuclei increased linearly from about 35% to 60% at cell densities of less than one cell per 0.01 mm² of culture surface to 90% to 100% at densities of 20 to 125 cells per 0.01 mm². This frequency-to-density relationship was independent of the mitotic rate and the rate at which cell density increased. When large variations in cell density were produced intentionally on the same glass coverslip, sex chromatin frequency was related to the density of cells in any one area of a coverslip and seemed to be largely independent of the cell density in other parts of the coverslip. The frequency of sex-chromatin-like bodies of male cultures remained very low at all cell densities. These and other preliminary observations described suggest that, in the nucleus of the female, sex chromatin formation resulting from the condensation of an X chromosome at interphase is not directly related to the mitotic cycle but may be related to the metabolic state of the cell.

X‐chromosome Inactivation and Developmental Patterns in Mammals

“…Similarly, Therkelsen & Petersen (1967), again working with cultured fibroblasts, plotted curves of the change with time of both G-6-PD activity and sex chromatin count, and found no evidence that cells without sex chromatin had more than one X active. Klinger, Davis, Goldhuber & Ditta (1968) found one cause of the variation in sex chromatin count among cultured cells to be the number of cells per unit volume (cell density), the count increasing with density. After a change in density the sex-chromatin count altered so rapidly that it seemed most unlikely that any change in chromosome activity was involved.…”

Sex Chromatin Formation in the Blastocyst of the Roe Deer (Capreolus Capreolus) During Delayed Implantation

“…An analysis of the factors controlling the onset of sex chromatin formation in the rabbit (Klinger, Kosseff & Plotnick, 1971) suggested that absolute time from fertilization was the most important single factor, although a lowering of cell metabolism may also be involved (Klinger, Davis, Goldhuber & Ditta, 1968;Issa et al, 1969). It was, therefore, of interest to study the formation of sex chromatin during the development of a species such as the roe deer in which the blastocyst is maintained unattached and metabolically inactive in the uterine lumen for about 5 months (Short & Hay, 1966;Aitken, Burton, Hawkins, Kerr-Wilson, Short & Steven, 1973 In the second group of embryos, the presumptive males, large condensations of chromatin resembling sex chromatin were only observed in 7-6 + 4-5% (mean + S.D.)…”

5-azacytidine-induced decrease in the frequency of Barr body in human fibroblasts

“…Control cultures were treated identically except for omission of 5-azaC. Since it has been shown that the frequency of Barr body" 1 " nuclei in cultured human fibroblasts is related to cell density (Klinger et al 1968;Mukherjee & San Sebastian, 1978), the average cell densities for a particular set of treated and control cultures were kept more or less constant by seeding the same number of cells in each culture tube. The cell viability and growth were not adversely affected in the experimental cultures as determined by trypan blue test and by determination of cell numbers at various time intervals.…”