Morphogen gradients provide cells in developing organisms with essential positional information. But do gradients form through extracellular morphogen diffusion alone or is intracellular trafficking also involved? On p. , Kruse and colleagues address this controversial issue and conclude that in the Drosophila developing wing epithelium, extracellular diffusion is not sufficient to establish the Decapentaplegic (Dpp) morphogen gradient. Transient `shadows' of Dpp signalling are known to occur behind clones of endocytosis-defective cells, and the researchers calculate that if Dpp gradients were established through diffusion alone, the endocytosis-defective cells would have to upregulate Dpp receptor cell surface expression to cause these shadows. Yet, when Kruse et al. use specific antibodies to measure Dpp receptors on such cells, there is no receptor upregulation. Further experiments are now needed to determine the relative importance of different transport methods for Dpp spreading, say the researchers.
