How does the mouse get its trunk?

The body plan of the mouse embryo is established by signaling molecules produced by extra-embryonic tissues that do not contribute to the fetus itself. The local availability of these potent secreted factors is tightly regulated by agonists and antagonists, but precisely how and where the different components interact is hotly debated. The identification of the gene mutated in the amnionless mouse, which develops with a head and a tail, but no trunk, provides more grist for the mill. The gene (Amn) encodes a putative transmembrane Bmp antagonist that is unexpectedly localized on the surface of extra-embryonic cells facing away from the embryonic cells being patterned. This finding raises important questions about how information is relayed between closely apposed cell layers, a problem also faced by embryologists working with other model systems.