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Cover Signaling proteins traverse tissue to form morphogen gradients during animal development. Shown here is a Drosophila wing imaginal disc that will eventually give rise to adult wing blade and thorax. The wing imaginal disc is divided into anterior (A) and posterior (P) compartments. Two morphogens, Hedgehog (Hh) and Decapentaplegic (Dpp), determine adult wing development. Hh protein produced by posterior disc cells (shown in red) reaches across the A-P boundary to activate Hh-responsive genes, including ptc (shown in green) in the A compartment. These target genes are essential for the proper patterning of the adult wing blade. The vertebrate neural tube is another model system used for studying morphogen gradients. Shown here as insets are immunofluorescence analyses of three mouse neural tubes at embryonic day 10.5, using antibodies directed against Sonic hedgehog (Shh) protein (green), Foxa2 (red), and Isl1/2 (blue). (For details, see Zhu and Scott, p. 2985.)

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