Correlation between X-chromosome inactivation and cell differentiation in female preimplantation mouse embryos

By means of a cytological method involving BrdU incorporation and acridine orange fluorescence staining in combination with embryo manipulation, we studied X-chromosome activity in female preimplantation mouse embryos with special reference to the correlation between X-chromosome inactivation and cell differentiation. There was no sign of asynchronous replication between the two X chromosomes from the one-cell to intermediate blastocyst stage. The allocyclic X chromosome, first detected in late blastocysts, was paternal in origin, mostly replicating early in the S phase and limited to the trophectoderm. Subsequent X-chromosome inactivation occurring in the primary endoderm was also characterized by the involvement of the paternal X and early replication. Both X chromosomes continued to replicate synchronously in the embryonic ectoderm or epiblast at this stage. It was evident that overt cell differentiation preceded the appearance of the asynchronously replicating X chromosome in the trophectoderm and primary endoderm. This finding seems to support the view that cell differentiation is an important correlate of X-chromosome inactivation.