X inactivation-specific transcript expression in mouse oocytes and zygotes

Expression of the X inactivation-specific transcript (Xist) gene has previously been shown by reverse transcription-polymerase chain reaction (RT- PCR) to be present at the 4-cell stage of female mouse embryos. This early expression, which is followed by X inactivation in the extra-embryonic tissues, is maternally imprinted. By the blastocyst stage, as the embryonic lineages begin to form, the imprint is lost and expression becomes random. By applying in-situ RT-PCR, we showed that Xist is expressed even earlier in development, in unfertilized mouse oocytes as well as in pronuclei stage zygotes. Our data demonstrate Xist expression in oocytes and suggest that Xist transcripts may occur in both XX and XY zygotes. A difference in the pattern of expression (rod-like or rounded punctate signal) is found among pronucleai-stage embryos. Early expression is in agreement with findings reported in human embryos.