XIST EXPRESSION IN HUMAN OOCYTES AND PREIMPLANTATION EMBRYOS

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 extraembryonic cell lineages of the developing female embryo. Rec ently, inactivation of the paternally inherited X chromosome has also been shown to occur in the extraembryonic cell lineages of the human f emale conceptus. In this paper, we determine whether the pattern of XI ST expression in human preimplantation embryos is similarly correlated with paternal X inactivation. We developed procedures sensitive to th e single cell, for the simultaneous analysis of XIST and HPRT expressi on and of sexing, initially using human fibroblast cells. Application of these procedures to human cleavage-stage embryos derived by in vitr o fertilization revealed a pattern of XIST expression different from t hat 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 o f XIST were detected in both male (hence from the maternally inherited allele) and female preimplantation embryos. This pattern of expressio n is not consistent with a role for the early expression of the XIST g ene in the choice of paternal X inactivation in the extraembryonic cel l lineages of the developing human embryo.