Damaged brain cells retrace their steps when they repair themselves, say US scientists.

A microfluidic device has enabled the American team to study single mammalian axons - projections sent out by nerve cells - as they regenerate after laser-induced injury. Digant Davé and colleagues at the University of Texas, Arlington, say their method could provide insights into the effects of injury on the nervous system including the events that occur after spinal trauma. 'So much of regenerative neurobiology remains largely unknown,' declares Davé.

After axons are cut, the nerve cell projections can regrow (left to right)

The new device has three components: a microfluidic chip to isolate single axons, a laser for highly localised injury and a custom-built incubator, all on an inverted microscope. Using these tools, Davé's group can cut hundreds of individual axons reproducibly and at precise locations. In this way the team can monitor the axons as they regenerate and observe their growth over distances of several millimetres, which until now has not been possible using conventional tools.

The researchers have made many surprising observations using the device. 'Very interestingly, we found that neurons [from the brain cortex] closely follow the same track they had before injury after regeneration,' says Davé. In contrast, neurons found in the spinal nerve follow a new path.

'Neuronal repair studies are important for improving our understanding of the mechanisms that enable functional recovery from traumatic injury to the nervous system,' explains Larry Millet, an expert in neurobiology and microfluidics at the University of Illinois at Urbana-Champaign, US. Microfluidic devices are indispensable to control and manipulate the microenvironment in these studies, he adds. 

Davé's team has also used the platform to create a neuronal circuit model to study how injury affects connections between axons and muscles. 'It's simply amazing to see how dynamic these neurons are,' says Davé. He describes how little outgrowths extend during axon regeneration. 'It looks like it's feeling its way out,' he says. 

Nicola Cogan

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