X-chromosome inactivation in extra-embryonic membranes of diploid parthenogenetic mouse embryos demonstrated by differential staining

Abstract

In somatic cells of female mammals one of the two X chromosomes is genetically inactive and heterochromatic, resulting in dosage compensation for X-linked genes^1–3. In marsupials the paternally derived X chromosome is preferentially inactivated. In eutherian mammals, although either X chromosome can be inactivated at random in somatic cells, preferential inactivation of the paternally derived X chromosome has been demonstrated cytologically in mouse and rat yolk sac^5,6 and mouse chorion⁵ and biochemically in mouse yolk sac⁷, chorionic ectoderm⁸ and trophoblast. In mouse yolk sac the non-random element has been shown both biochemically⁷ and cytologically⁹ to be confined to the endoderm layer in which there is almost total paternal X-chromosome inactivation. We have therefore looked at X-chromosome activity in the separated yolk sac layers of diploid parthenogenetic mouse embryos in which both X chromosomes are maternally derived. Kaufman et al.¹⁰ have demonstrated X inactivation in somatic cells of diploid parthenogenetic embryos, and we have used a modification of Kanda's method¹¹, which renders the presumptive inactive X dark staining, to reveal an inactive X chromosome in both endoderm and mesoderm layers of separated yolk sacs from parthenogenones. Thus even in tissues in which there is normally total non-random paternal X inactivation, in the absence of a paternally derived X chromosome a maternally derived X can be inactivated.