Studies on the activity of the X chromosomes in female teratocarcinoma cells in culture.

Embryonal carcinoma cells derived from murine teratocarcinomas are able to differentiate into the same variety of tissue types as early embryonic cells. Because embryonal carcinoma cells resemble those of the embryo at a stage before X chromosome inactivation has occurred in females embryonal carcinoma cells containing two X chromosomes were examined to determine whether both are genetically active. The specific activities of X-linked enzymes were measured in embryonal carcinoma cells containing either one or two X chromosomes. The activities in both cell types were similar, suggesting that only one X chromosome was active in the female cells. Further support for this conclusion came from experiments in which azaguanine-resistant mutants were recovered with similar frequencies from embryonal carcinoma cell lines containing one and two X chromosomes. Late replication of an X chromosome DNA was detected in one embryonal carcinoma cell line with two X chromosomes but not in another. This suggests that cells of these two lines were arrested at different developmental stages, and that late DNA replication may not be a necessary adjunct of X inactivation. Evidence is presented which suggests that X chromosome reactivation does not occur during differentiation of the cells in vitro.