The nervous system has 10,000 different types of neuron (Image: Jean Livet)

YOU were born with all the brain cells you’ll ever have, so the saying goes.

So much for sayings. In the 1990s, decades of dogma were overturned by the discovery that mammals, including people, make new neurons throughout their lives. In humans, such “neurogenesis” has been seen in two places: neurons formed in the olfactory bulb seem to be involved in learning new smells, while those born in the hippocampus are involved in learning and memory.

The discovery that new neurons can integrate into the adult brain raises intriguing possibilities. Could the process be harnessed to treat diseases of the brain, such as Parkinson’s and Huntington’s?

The trick will be in replacing diseased cells with just the right kind of neuron, says Jeffrey Macklis, who studies cortical development and neurogenesis at Harvard University. By some estimates, the nervous system is made up of 10,000 different kinds of neuron.

This complexity means you can’t just hijack any old cell produced by natural neurogenesis. However, there may be other ways of growing new neurons to order.

Olle Lindvall at Lund University in Sweden has shown what might be possible. He transplanted dopamine-producing neurons taken from aborted fetuses into the brains of people with Parkinson’s, and showed the new neurons can improve brain function, although the treatment didn’t work for everyone. Lindvall is now looking for ways to make these specialised neurons from embryonic stem cells or stem cells made by reprogramming adult skin cells.

Even outside the two well-known neurogenic parts of the brain, there are small numbers …