Signaling for germ cells.

Shakespeare advises us that some men are born great, some achieve greatness, and some have greatness thrust upon them. So it is with germ cells. In Drosophila, the very first cells to be formed in the embryo, the pole cells, are germ cells whose descendants have no fate other than to give rise to gametes. In Caenorhabditis and in zebra fish, germ cell determinants present in the egg are asymmetrically segregated at each subsequent cell division until a definitive germ cell lineage is achieved. But in mice, and by extrapolation in all mammals, it seems that cells have germ cell status thrust upon them.