X-CHROMOSOME ACTIVITY OF THE MOUSE PRIMORDIAL GERM-CELLS REVEALED BY THE EXPRESSION OF AN X-LINKED LACZ TRANSGENE

We have determined the timing of the inactivation and reactivation of the X chromosome in the mouse primordial germ cells (PGCs) by monitori ng the expression of an X-linked HMG-lacZ reporter gene. PGCs were ide ntified by their distinct alkaline phosphatase activity and they were first localised in the primitive streak and allantoic bud of the 7.5-d ay gastrulating embryo. Although inactivation of the transgene was fou nd in some PGCs at these sites, at least 85% of the population were st ill expressing the lacZ gene. This suggests that, although X-inactivat ion has commenced during gastrulation, the majority of PGCs still poss ess two active X chromosomes. Transgene activity remained unchanged du ring the relocation of PGCs to the hindgut endoderm, but decreased abr uptly when PGCs left the hindgut and migrated through the mesentery. X -inactivation was completed during the migration of PGCs, but not simu ltaneously for the whole population. The first wave of PGCs entering t he genital ridge at 9.5 days did not immediately re-activate the silen t transgene until about 24 hours later. Re-activation of the transgene took place in over 80% of PGCs entering the genital ridge at 10.5-13. 5 days p.c,, preceding the entry into meiosis. About 90% of the meioti c germ cells in the 14.5-15.5 day fetal ovary expressed the transgene. Similar profiles of transgene activity were observed in PGCs of embry os that have inherited the lacZ transgene from different parents, show ing unequivocally that X-inactivation in the germ cell lineage is not related to parental legacy. In contrast to those germ cells in the gen ital ridges, a small population of PGCs that was left outside the geni tal ridges at 13.5-15.5 days did not re-activate the silent X. This st rongly suggests that reactivation of the silent X chromosome in the fe male germ cells is a response to local signals in the genital ridge.