Timing of X-chromosome inactivation in postimplantation mouse embryos.

The onset of X-chromosome inactivation was investigated cytologically in postimplantation female mouse embryos of age 5£, 6\ and 1\ days post-coitum (d.p.c.) and in the isolated epiblasts of 6 d.p.c. embryos before primitive streak formation using a heat/hypotonic technique to reveal the inactive X chromosome by differentially dark staining with Giemsa. The results indicate that X inactivation has taken place in all the cells of the so-called ' undifferentiated' epiblast by 6 d.p.c. before primitive streak formation. Further evidence is presented to suggest that X inactivation is complete in all cells of the mouse embryo by 5i d.p.c.