title: Regional and temporal changes in the pattern of X-chromosome replication during the early post-implantation development of the female mouse.
abstract: We have made a detailed study of the X-chromosome replication pattern during the period when X-inactivation is occurring in the mouse embryo. Our observations show unequivocal regionalization of the embryo with respect to the temporal X-chromosome. The switch from isocyclic to allocyclic replication occurs in the embryonic ectoderm at approximately 6 days of development and is random with respect to parental origin of the X-chromosome. In the extra-embryonic tissues, however, the switch to allocyclic replication has apparently occurred prior to 5.3 days of development and almost exclusively involves the paternally-derived X-chromosome. Since these findings are consistent with results obtained in biochemical studies of X-chromosome activity in female embryos, we conclude that there is a close temporal relationship between the cytogenetic and biochemical manifestations of the X-inactivation process. In addition, we have observed a pattern of early paternal X-chromosome replication, transitory in some cases, that is unique to extra-embryonic tissues. These results suggest that there may be some differences in the mechanism by which X-inactivation occurs in the extra-embryonic tissues as compared with the embryonic ectoderm.
mesh_terms: D000818:Animals; D001973:Bromodeoxyuridine / Q000494:pharmacology; D002453:Cell Cycle; D004261:DNA Replication / Q000187:drug effects; D004622:Embryo, Mammalian / Q000502:physiology*; D005260:Female; D007621:Karyotyping; D051379:Mice; D011247:Pregnancy; D012730:Sex Chromosomes / Q000502:physiology*; D013194:Staining and Labeling; D013997:Time Factors; D014960:X Chromosome / Q000502:physiology*
chemical_list: D001973:Bromodeoxyuridine