Axon guidance in cultured epithelial fragments of Drosophila wing.

Growing axons can be guided by a number of different cues: adhesive substrates, diffusible factors, electrical fields and even factors intrinsic to the neurone itself have all been shown to affect axon orientation and outgrowth in vitro. However, in most intact systems it has proved difficult to test directly the role played by these putative guidance cues. Here, we describe a system, the developing wing of the fruitfly, in which we have tested simultaneously two putative guidance mechanisms, physical constraints to axon growth (channels) and the position of neuronal somata (guideposts), using surgical techniques. We show that pioneer sensory axons can navigate correctly and form their normal stereotyped pattern of axon bundles in wing fragments that apparently lack both physical and neural cues. This technique allows access to the surface along which neuronal pathfinding takes place, making possible a wide range of experimental manipulations on the developing system.