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IT IS being hailed as one of the most significant advances in nerve
regeneration in a decade. After severing an optic nerve in rats, neurologists
have found a way to reconnect it to the brain so that it once again transmits
normal electrical signals.

The achievement is a first in mammals, and may hint at ways of reversing some
types of blindness in people. Scientists also hope to use a version of the
technique to treat people with spinal cord injuries.

In many simple creatures, damaged nerves mend themselves. But mammals, with
their large brains, have traded this flexibility for stability. With such a
complex nervous system, rewiring damaged nerves the wrong way could do more harm
than not rewiring at all. So mammals keep a lid on nerve cell growth by
producing proteins that inhibit axons鈥攖he part of a nerve cell that
conducts signals鈥攇rowing in the scar tissue that forms after injury.

Now a team led by Solon Thanos at the University of M眉nster in Germany
has got severed nerves to regrow up to 14 millimetres鈥攎ore than three
times as far as anyone has managed before. They first anaesthetised rats and
severed their optic nerves. Then they sutured the two cut ends back together to
repair the connective tissue sheath surrounding the nerve. Finally they
punctured the lenses in the rats鈥� eyes which releases proteins called
crystallins.

The proteins are known to inhibit apoptosis鈥攖he mechanism by which
cells self-destruct鈥攂ut Thanos thinks they also promote regrowth in the
axon by blocking receptors for growth-inhibiting proteins at the growing tip of
the nerve.

Previous studies by Larry Benowitz at Harvard Medical School have shown that
injury to the lens can stimulate optic nerves to grow by up to 4 millimetres. He
says the German researchers have achieved 鈥渁stounding levels of axon growth鈥�
with their technique. But he disagrees with their explanation of how it works.
Benowitz thinks that the axons are stimulated by the inflammatory immune
response triggered by the surgery, rather than by any inhibiting effect of
crystallins.

Whatever the mechanism, three months after the surgery, about 30 per cent of
the nerve fibres had regenerated. This included all major cell types, including
those carrying colour and contrast information from the eye to the brain. 鈥淓ven
10 per cent is sufficient for residual sight,鈥� says Thanos.

The regenerated nerves also carried normal electrical signals, suggesting
that they had rewired themselves into the brain, although the connections were a
bit scrambled. The team is currently studying the rats鈥� behaviour to assess how
good their eyesight is.

Jerry Silver, a nerve regeneration expert at Case Western Reserve University
in Ohio, says the crucial question now is whether the findings can be applied to
other parts of the nervous system.