Dendrites hit the spot

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Researchers have now found evidence that dendrites actively seek out specific targets in the developing brain when making functional circuits.

During development of the nervous system, countless connections must be formed between neurons. How axons and dendrites come together to make the functional circuits of the brain is a subject of endless fascination. A new insight comes from Mumm et al., who have found evidence that dendrites actively seek out specific targets in the developing brain.

Developing axons use various strategies to ensure that they reach the correct target, but dendrites have generally been thought to be more promiscuous. Instead of forming selective synapses, they are believed to generate widespread arborizations and connections that are then refined through removal, or 'pruning', of the inappropriate synapses and branches.

Mumm et al. used in vivo time-lapse imaging to investigate how the dendrites of retinal ganglion cells in the zebrafish form lamina-specific dendritic arborizations in the inner plexiform layer of the retina. Zebrafish embryos develop from an egg to a larva with a functioning visual system in just 5 days, and because they are transparent they are perfect for studying this kind of problem. By using transgenic zebrafish that expressed fluorescent dyes in specific neurons, the authors could watch as the dendritic arbor developed.

Remarkably, they found that many of the retinal ganglion cells formed dendritic arborizations in a sequential pattern, targeting specific laminae and working from the inner to the outer retina. This indicates that, rather than relying solely on dendritic pruning to establish accurate connections, some neurons can project dendrites towards specific targets.

The authors also found that the retinal ganglion cells formed stratified arborizations in laminae in which amacrine cell neurites had already formed laminated plexuses. The neurites of amacrine cells might therefore 'prepattern' the inner plexiform layer and provide cues that guide the dendrites of ganglion cells to the correct laminae.

These findings add to the view of dendrites as complex and sophisticated structures that are gradually emerging from the shadow of the axon. Further work will lead to a better understanding of how dendrites can seek out appropriate targets during development.

Rachel Jones

References

Mumm, J. S. et al. In vivo imaging reveals dendritic targeting of laminated afferents by zebrafish retinal ganglion cells. Neuron 52, 609–621 (2006) | Article | PubMed |