During mouse preimplantation development, the exclusive expression of the Xist gene from the paternally inherited allele is thought to play a role in the inactivation of the paternally inherited X chromosome in the extra-embryonic cell lineages of the developing female embryo. Recently, inactivation of the paternally inherited X chromosome has also been shown to occur in the extraembryonic cell lineages of the human female conceptus. In this paper, we determine whether the pattern of XIST expression in human preimplantation embryos is similarly correlated with paternal X inactivation. We developed procedures sensitive to the single cell, for the simultaneous analysis of XIST and HPRT expression and of sexing, initially using human fibroblast cells. Application of these procedures to human cleavage-stage embryos derived by in vitro fertilization revealed a pattern of XIST expression different from that in the mouse. Transcripts of the XIST gene were detected as early as the 1-cell zygote and, with increasing efficiency, through to the 8-cell stage of preimplantation development. In addition, transcripts of XIST were detected in both male (hence from the maternally inherited allele) and female preimplantation embryos. This pattern of expression is not consistent with a role for the early expression of the XIST gene in the choice of paternal X inactivation in the extraembryonic cell lineages of the developing human embryo.

We have examined the expression of the myotonin protein kinase gene (MPK) in individual human preimplantation embryos. An allele-specific, reverse transcriptase polymerase chain reaction assay was developed to the required sensitivity using human buccal cells. Human cleavage stage embryos, derived by in vitro fertilisation, were assayed for MPK transcription directly, or after biopsy of a single blastomere used to determine the genotype of the embryo. The maternal and paternal genotypes were determined from cumulus cells and sperm, respectively. MPK gene transcripts were detected in embryos at the 1-cell--4-cell stage of development. Detection of transcripts from the paternally-inherited allele confirmed onset of embryonic MPK gene expression at this very early preimplantation stage.