Gradient Scaling and Growth

Tissue growth is controlled by the temporal variation in signaling by a morphogen along its concentration gradient. What determines the final size of an animal? Secreted molecules called morphogens control tissue and organ growth during development ( 1 ). As morphogens diffuse away from their source, a concentration gradient forms. Target cells “read” the local concentration and activate genes involved in differentiation. One such morphogen is Decapentaplegic (Dpp), a member of the transforming growth factor–β family that controls fly ( Drosophila melanogaster ) development ( 2 – 6 ). Paradoxically, although Dpp forms a concentration gradient, it promotes seemingly uniform growth across its target tissue. Earlier studies argued that the slope rather than the concentration of the Dpp gradient may control growth ( 7 ), or that cell proliferation is modulated by mechanical constraints ( 8 , 9 ). On page 1154 of this issue, Wartlick et al. ( 10 ) propose instead that cells control growth by computing the relative temporal variation in Dpp activity.