Cell signaling pathways controlling an axis organizing center in the zebrafish

Body axis formation in vertebrate development entails the remarkable feat of patterning a myriad of specialized cell types and organ progenitors from a field of unpatterned, multipotent cells. This feat is achieved largely by secreted cell-cell signaling molecules, enabling cells at different positions within the embryo to adopt distinct fates. During patterning of the vertebrate embryonic axes, a multitude of cell fates is induced by a surprisingly small set of signaling pathways: Wnt, Nodal, Bone Morphogenetic Protein (BMP), and Fibroblast Growth Factor (FGF) signaling. These signals function as morphogens, specifying multiple cell fates in a concentration-dependent mechanism, and must therefore be distributed non-uniformly throughout the embryo. A primary signaling center that sets up spatial asymmetries in these signaling pathways to break the symmetry of the vertebrate embryo is known as the dorsal organizer. Discovered nearly a century ago by Hilde Mangold and Hans Spemann in the newt, the organizer has the remarkable ability to induce a secondary body axis when grafted ectopically into a host embryo. Here, we review the cell-cell signaling pathways that control the establishment of the dorsal organizer and its inductive functions in the zebrafish Danio rerio, a vertebrate model highly amenable to genetic manipulation. The organizer's remarkable inductive abilities continue to provide a fascinating source of scientific inquiry in the field of developmental biology.