杏吧原创

STRETCHING neurons on the rack might seem like torture, but it could be the
key to repairing spinal cords.

By gradually pulling apart bunches of neurons, Douglas Smith and his
colleagues at the University of Pennsylvania in Philadelphia have persuaded the
cells鈥� processes, or axons, to grow up to a centimetre in just 10 days. 鈥淔or an
axon just a micron wide, that鈥檚 an enormous distance,鈥� Smith says.

The researchers say these nerve cells could be used to bridge the gap between
damaged nerves in the spinal cords of people who are paralysed. They have
already begun trials in animals. 鈥淵ou can think about what we have as jumper
cables,鈥� Smith says.

While it鈥檚 too early to know if the approach will work for people with spinal
cord injuries, Smith thinks it is a viable alternative to other strategies. For
example, many researchers are trying to encourage nerve cells to regrow in the
spine by, say, implanting an artificial scaffold seeded with appropriate
chemicals.

鈥淏ut everything about the spinal cord is screaming 鈥榮top growing鈥�,鈥� Smith
says, so it鈥檚 hard to get axons to grow long distances. 鈥淭he difference with
growing cells outside the body is that you don鈥檛 have to worry about this
inhibitory environment.鈥�

Smith鈥檚 team placed groups of human neurons on adjacent membranes and grew
them for three days to allow the axons from the groups to form connections. The
membranes were then pulled apart 3.5 micrometres every 5 minutes over 10 days,
until the axons connecting the two groups of cells had grown a centimetre. Any
faster and the axons were torn apart. They ended up with long bundles containing
tens of thousands of axons.

It鈥檚 an interesting approach, says Paul Reier, a pioneering researcher into
spinal cord repair at the University of Florida. The big problem will be
implanting the cells in the right place and keeping them alive, he says. 鈥淎dult
neurons usually die when transplanted.鈥�

But Smith says his team is using a cell line that managed to survive when
implanted into stroke patients. He also speculates that transplanting integrated
bundles of cells will boost their survival. 鈥淐ells may be less likely to die if
they stay with their 鈥榝riends鈥�,鈥� he says.

Reier thinks it will someday be possible to persuade cells to grow long
distances in the spinal cord, though. 鈥淎xon growth is becoming less of a
challenge than we thought.鈥�

Whatever approach is used, many questions remain. It鈥檚 not even clear that
bridging a damaged part of the spinal cord will restore nerve activity. In some
injuries, some or all of the nerve fibres remain intact, yet still don鈥檛 work,
Reier points out.

Smith鈥檚 work may also help us understand other disorders, however. He thinks
that stretch-induced growth plays an important role in embryos and children.
Some degenerative disorders in young children may be caused by nerve fibres that
can鈥檛 grow fast enough to keep up. Smith鈥檚 team is now trying to understand
exactly how the process works. 鈥淣obody鈥檚 ever studied this type of growth
before,鈥� he says.