XIST EXPRESSION FROM THE MATERNAL X-CHROMOSOME IN HUMAN MALE PREIMPLANTATION EMBRYOS AT THE BLASTOCYST STAGE

In the somatic cells of female mammals, either the maternally or pater nally derived X chromosome (X-M or X-P) is randomly inactivated to ach ieve dosage compensation for X-linked genes, In early mouse developmen t, however, selective inactivation of X-P occurs first in extraembryon ic lineages at the blastocyst stage around the time of implantation be fore later random inactivation in the embryonic ectoderm from which th e fetus is derived, Xist, a gene mapping to the X-inactivation centre (Xic), is exclusively expressed from the inactive X-chromosome and is thought to be involved in the initiation of X-inactivation. Consistent with this, Xist is first expressed at the 4-to 8-cell stages, prior t o functional inactivation at the blastocyst stage, exclusively from X- P in female embryos, This also suggests that genomic imprinting may in fluence the earliest expression of Xist resulting in selective inactiv ation of X-P and a candidate methylation site in the promoter region h as recently been described. Here we report the expression of the human homologue, XIST, in human preimplantation embryos from the 5- to 10-c ell stage onwards consistent with its role in the initiation of inacti vation, In contrast to the mouse, however, transcripts were detected i n both male and female embryos demonstrating XIST expression from the X-M in male embryos ((XY)-Y-M).