Signaling reaches to new dimensions in Drosophila imaginal discs.

Finding that peripodial cells in wing and eye imaginal discs are essential for the growth and patterning of the separate layer of disc cells now opens the study of interacting cell layers to the powerful developmental genetic techniques with which the Drosophila system is blessed. We can anticipate that future work will identify how such interactions contribute to patterning and how the mechanisms and processes that are involved are conserved in vertebrates. We can also look forward to contributions that this work will make to understand-ing the role of interconnecting cell extensions in such signaling processes. In this minireview, we have noted numerous types of signaling cells in which cellular extensions have been observed. At present, neither the functional nor structural relationship of these related structures is known. It is certainly tempting to suggest that these structures are conduits for signals or that they function as sensors. There is, as yet, no direct experimental evidence for such roles.