Like many neural circuits, the vertebrate retina is organised into laminae. But how are neurites targeted to the correct synaptic layer during retinal development? Kay et al. use a powerful combination of in vivo confocal imaging and genetic manipulation to study the inner plexiform layer (IPL) of the zebrafish retina (see ). The IPL contains post-synaptic retinal ganglion cells (RGCs)and their pre-synaptic partners, the amacrine cells and bipolar cells, and is subdivided into ON and OFF sublaminae that respond differently to illumination. The researchers study IPL development in real time by labelling amacrine cell subsets with GFP in a zebrafish mutant in which RGCs never form. IPL formation was delayed and disorganised in the absence of RGCs, but the early neurite projection errors were later corrected to produce a near-normal IPL. Thus, while RGCs have a transient role in organising the earliest amacrine cell projections, interactions between amacrine cells are sufficient to form the IPL and its sublayers.
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