sebastien_vigneau 's Profile

X-inactivation patterns are known to be relatively consistent within the tissues of an individual mouse, but do vary slightly
(Nesbitt 1971; Johnston and Cattanach 1981; Krietsch et al. 1986; Plenge et al. 2000) because each tissue is derived from an independent sampling event in progenitor cells. We considered the possibility that
sampling whole-embryo RNA rather than adult tissue samples could reduce the variation in X-inactivation patterns observed
not directly attributable to the initial choice between chromosomes, making them a more suitable system for studies of X chromosome
choice. To test this hypothesis, we compared the X-inactivation patterns of F₁ adults to those of F₁ embryos. As shown in Fig. 4B, the distribution of X-inactivation patterns in F₁ whole embryos was significantly tighter than we observed in the F₁ adult ear biopsies; the mean did not change significantly, but the variance was significantly reduced in the whole embryos
(F-test p = 0.007). Although this finding may not be surprising given that the whole embryo is a more representative sample than a
specific tissue with respect to X inactivation, it does suggest that embryos may be an ideal system for future studies of
X chromosome choice.

The variance observed in X-inactivation patterns can be used to estimate the approximate number of cells present at the time
of X chromosome choice, as in previous studies (McMahon et al. 1983; Baader et al. 1996). The whole-embryo data are a useful sample with which to readdress this question because the variance in patterns is attributed
to just one sampling event (choosing between the two chromosomes), rather than two (choosing a subset of X-inactivated cells
that will then give rise to the tissue in question). Using an approach outlined previously (McMahon et al. 1983; see Materials and methods), we used the F₁ whole-embryo data to determine the number of precursor cells present. Our data indicate that approximately 70 cells were
present at the initial choice (95% CI, 48–96 cells), suggesting that X chromosome choice in the cells of the embryo proper
occurs between 4.5 and 5.5 days post coitum, in agreement with previous estimates (Monk and Harper 1979; Rastan et al. 1980; McMahon et al. 1983; McMahon and Monk 1983; Baader et al. 1996)