BMP gradients: A paradigm for morphogen-mediated developmental patterning

Bone morphogenetic proteins (BMPs) act in dose-dependent fashion to regulate cell fate choices in a myriad of developmental contexts. In early vertebrate and invertebrate embryos, BMPs and their antagonists establish epidermal versus central nervous system domains. In this highly conserved system, BMP antagonists mediate the neural-inductive activities proposed by Hans Spemann and Hilde Mangold nearly a century ago. BMPs distributed in gradients subsequently function as morphogens to subdivide the three germ layers into distinct territories and act to organize body axes, regulate growth, maintain stem cell niches, or signal inductively across germ layers. In this Review, we summarize the variety of mechanisms that contribute to generating reliable developmental responses to BMP gradients and other morphogen systems.