Nature 288, 172 - 173 (13 November 1980); doi:10.1038/288172a0

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

S. Rastan^*, M. H. Kaufman^†, A. H. Handyside^† & M. F. Lyon^*

^*MRG Radiobiology Unit, Harwell, Didcot, Oxon OX11 0RD, UK
^†Department of Anatomy, University of Cambridge, Cambridge CB2 3DY, UK

In somatic cells of female mammals one of the two X chromosomes is genetically inactive and heterochromatic, resulting in dosage compensation for X-linked genes¹⁻³. 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.

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