Preferential paternal X inactivation in extraembryonic tissues of early mouse embryos

The preferential expression of the maternal X chromosome seen in certain extraembryonic membranes of the mouse was studied by investigating the tissues from which these membranes are derived during early development. The electrophoretic variant of the X-coded enzyme PGK-1 (phosphoglycerate kinase) was used to distinguish the expression of the maternal from the paternal X chromosome in heterozygous females. Both the extraembryonic ectoderm and primary endoderm of 6 1/2-day female egg cylinders gave almost exclusive expression of the maternal form of the enzyme whereas the epiblast gave near equal expression of the two parental alleles. No paternal PGK-1 band could be detected in samples of pooled 3 1/2-day blastocysts, but after 3 or 4 days of culture in vitro a faint paternal band was seen in the resultant outgrowths. The activity of the maternal band in these latter samples had increased greatly from that of the blastocysts, consistent with preferential expression of the maternal Pgk-1 allele in the trophoblastic cells of the outgrowths, while both alleles are expressed in inner-cell-mass cells. The results strongly support the idea that non-random X-chromosome expression is due to preferential paternal X inactivation in trophectoderm (from which extraembryonic ectoderm is derived) and in primary endoderm, and not to cell selection.