Dubbed 'brainbow', an exciting new imaging technique is challenging our understanding of how the brain develops. A well-established principle in development is activity-dependent pruning of neural connections: neurons initially make lots of contacts with neighbouring cells, then many are lost, leaving only the functionally most important links. In the visual system, this was thought to lead to an almost one-on-one connection between intermediate cells of the retina, retinal ganglion cells (RCGs) and relay cells of the thalamus, in the forebrain. When researchers labelled RGCs with different fluorescent proteins, they expected to find clusters of the same colour, showing that the connections on each relay cell all came from a single RCG. Instead, their images revealed a kaleidoscope of intertwined colours, as shown in this mouse brain. This suggests that multiple RCGs contact each relay cell, and hints that neural development is yet more complex than we imagined.
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