Regionally specific induction by the Spemann-Mangold organizer

Eighty years ago, Spemann and Mangold discovered the extraordinary inductive potency of the dorsal blastopore lip in amphibian embryos. Many inducers released by this organizer have now been identified and they typically encode antagonists of bone morphogenetic protein, Nodal or Wnt growth factors. The different expression domains of these growth factors and their antagonists create signalling gradients, which pattern the early embryo in a combinatorial fashion and explain the regional specificities of head, trunk and tail organizers. New findings indicate that both quantitative and qualitative mechanisms account for regionally specific organizer function.