The Stem Cell Model for PGC Specification in Mice

Specification of germ cells in mice occurs relatively late in embryonic development. It is initiated by signals that induce expression of Blimp1, a key regulator of the germ cell, in a few epiblast cells of early postimplantation embryos. Blimp1 represses the incipient somatic program in these cells and promotes progression toward the germ cell fate. Blimp1 may also have a role in the maintenance of early germ cell characteristics by ensuring their escape from the somatic fate as well as possible reversion to pluripotent stem cells. Primordial germ cells (PGCs), the foundercells of the germ cell lineage, are usuallyestablished early during embryonic de-velopment. Specification of PGCs can occur either through the inheritance of germ cell determinants already present in the egg, as in Caenorhabditis elegans and Drosophila, or in response to inductive signals, as in mice and probably all mammals. In all instances howev-er, germ cells are maintained by mechanisms that prevent them from differentiating into somatic cells