Drosophila hedgehog acts as a morphogen in cellular patterning.

The patterning of cell types in embryogenesis is specified by signals emanating from specialized organizer regions. We demonstrate that engrailed-expressing cells in the Drosophila epidermis have organizer properties. These cells influence the pattern of cell type differentiation across the segment. We show that this function is mediated by the hedgehog (hh) gene. The results of modulating the levels of hh in the embryo suggest that hh acts as a morphogen, specifying distinct cell fates by a concentration-dependent mechanism. We present a model that integrates the role of hh with that of the wingless signal in establishing the segmental array of cell type diversity.