Morphogen gradients, positional information, and Xenopus: Interplay of theory and experiment

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Abstract
The idea of morphogen gradients has long been an important one in developmental biology. Studies with amphibians and with Xenopus in particular have made significant contributions to demonstrating the existence, identity, and mechanisms of action of morphogens. Mesoderm induction and patterning by activin, nodals, bone morphogenetic proteins, and fibroblast growth factors have been analyzed thoroughly and reveal recurrent and combinatorial roles for these protein growth factor morphogens and their antagonists. The dynamics of nodal‐type signaling and the intersection of VegT and β‐catenin intracellular gradients reveal detailed steps in early long‐range patterning. Interpretation of gradients requires sophisticated mechanisms for sharpening thresholds, and the activin‐Xbra‐Gsc system provides an example of this. The understanding of growth factor signal transduction has elucidated growth factor morphogen action and provided tools for dissecting their direct long‐range action and distribution. The physical mechanisms of morphogen gradient establishment are the focus of new interest at both the experimental and theoretical level. General themes and emerging trends in morphogen gradient studies are discussed.