X-chromosome inactivation in the early mouse embryo Inner cell mass Extraembryonic tissue of placenta Embryonic cells Extraembryonic The lacz reporter gene has been fused to 1 X-chromosome( paternal )in this precursors female XX mouse embryo(C, D) The paternally derived X chromosome con- tained a lacz transgene that is active in the early embryo. Those cells in which the chromosome is active make B-galactosidease and can be stained blue. The other cells are counterstained and appear pink. C) In the early blastocyst stage at day 4, both X chromo- random inactivation of one of the chromosomes occurs. Thuys gi somes are active in all cells; thus all cells appear blue. (D)At da those where the paternal X is active stain blue. In the mouse(but not the human) trophoblast, the paternally derived X chromosome in preferentially inactivated Thus, the trophoblast cells are uni- lly pink. (A, B, photographs courtesy of M. L. Barr; C, D after Sugimoto et al. 2000, photographs courtesy of N. TakagiX-chromosome inactivation in the early mouse embryo The lacZ reporter gene has been fused to 1 X-chromosome ( paternal ) in this female XX mouse embryo ( C, D)