Bmp4 is required for the generation of primordial germ cells in the mouse embryo

In many organisms the allocation of primordial germ cells (PGCs) is determi ned by the inheritance of maternal factors deposited in the egg. However, i n mammals, inductive cell interactions are required around gastrulation to establish the germ line. Here, we show that Bmp4 homozygous null embryos co ntain no PGCs. They also lack an allantois, an extraembryonic mesodermal ti ssue derived, like the PGCs, from precursors in the proximal epiblast. Hete rozygotes have fewer PGCs than normal, due to a reduction in the size of th e founding population and not to an effect on its subsequent expansion. Ana lysis of beta-galactosidase activity in Bmp4(lacZneo) embryos reveals that prior to gastrulation, Bmp4 is expressed in the extraembryonic ectoderm. La ter, Bmp4 is expressed in the extraembryonic mesoderm, but not in PGCs. Chi mera analysis indicates that it is the Bmp4 expression in the extraembryoni c ectoderm that regulates the formation of allantois and primordial germ ce ll precursors, and the size of the founding population of PGCs. The initiat ion of the germ line in the mouse therefore depends on a secreted signal fr om the previously segregated, extraembryonic, trophectoderm lineage.