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Our brains contain billions of neurons connected by trillions of synapses. Despite this enormous complexity the pattern of connectivity in the brain is highly ordered and stereotyped across individuals of the same species. A common organisational feature of the brain is the arrangement of synaptic connections into layers. Why the brain organises itself this way is not known. A clue to the significance of layers in the brain comes from the observation that synapses are not randomly distributed amongst different layers. Instead a particular layer will contain synapses between specific neuronal subtypes resulting in the formation of layers that contain synapses with similar functional properties. Thus, an attractive idea is that lamination of the brain during development is a necessary step in the development of synapses between specific neuronal subtypes, i.e. layers ensure that cell type A will only connect to cell type B and not to cell types C and D, for example. However, there is little experimental evidence demonstrating a role for layers in establishing cell-type specific connections between neurons.

Researchers from the Department of Developmental Neurobiology, Nikolas Nikolaou and Martin Meyer, used the visual system of zebrafish as a model system to provide novel insights into the role of layers in the brain. The main visual area of the zebrafish brain, the optic tectum, receives input from the eye via the axons of retinal ganglion cells (RGCs). These axons are organised as a highly precise laminar array within the tectum. In zebrafish astray mutants, which carry a mutation in the robo2 gene, the layered arrangement of RGCs in the tectum is lost. The authors used the astray mutant to study how loss of a layered neural architecture impacts the functional development of connections between RGC axons and their target neurons in the tectum. The study focuses on the development of direction-selective circuits, which enable the animal to tell the direction of movement of an object.