Topographic maps - arrangements of synaptic connections that mirror the relationships between neighbouring neurons - ensure the spatially ordered flow of information through the nervous system. Many insights into how these maps are established have come from studying developing visual systems. Now, on , Umetsu and colleagues report that Hedgehog (Hh) and the transcription factor Single-minded (Sim) regulate the ordered connection of retinal axons with their synaptic partners in the optic lamina of the Drosophila visual system. As this system develops, retinal axons extend from the eye into the lamina ganglion layer, where they stimulate proliferation and differentiation of the lamina neurons by secreting Hh. The researchers show that Sim is induced by Hh in the lamina neurons and that this expression is required for the association of retinal axons with lamina neurons - the first step in forming a topographic map. Thus, postsynaptic cells may interact dynamically with presynaptic cells to establish topographic maps rather than waiting passively for axons to arrive, as previously thought.
