New cache of fresh neurons found in human brains

Brain cell regeneration has been discovered in a new location in human brains. The finding raises hopes that these cells could be used to help people recover after a stroke, or to treat other brain diseases.

For years it was unclear whether or not we could generate new brain cells during our lifetime, as the process – neurogenesis – had only been seen in animals. Instead, it was thought that humans, with our large and complex brains, are born with all the required neurons.

Then last year Jonas Frisén of the Karolinska Institute in Stockholm, Sweden, and his colleagues found that neurogenesis occurs in the hippocampi of the human brain. These structures are crucial for memory formation

Now they have found more new brain cells in a second location – golf-ball-sized structures called the striata. These seem to be involved in many different functions, including in learning and memory. These particular aspects, related as they are to the hippocampi, lead Frisén to speculate that these new brain cells may also be involved with learning. "New neurons may convey some sort of plasticity," he says, which might help people learn and adapt to new situations.

Radioactive clue

To reveal the new brain cells, the team exploited the fact that there have been varying levels of a radioactive isotope of carbon – carbon-14 – in the atmosphere since nuclear bomb tests during the cold war. This means that the year of creation of many cells in the body can be found by measuring the ratio of carbon-14 to carbon-12 in its DNA. Analysis of 30 donated brains revealed which brain cells had been born during the lifetimes of the donors.

The finding of new brain cells in the striata solves a long-standing mystery. In rodents, neurogenesis is seen in the hippocampi, as well as another area called the lateral ventricle wall. After they are created, the cells made in this second location migrate to the part of the brain that controls the sense of smell. Hints of neurogenesis had already been seen in the lateral ventricle walls of human brains. But when Frisén looked for new brain cells in human smell centres, he couldn't find any. Now it looks like we know where they ended up.

Arnold Kriegstein at the University of California, San Francisco, agrees that the latest carbon-14 work is confirmation that in humans, the striatum is their destination. "It's nicely demonstrated," he says.

Fresh hope

It is too early to know what these new brain cells are doing in the striata, but any evidence of neurogenesis in the human brain provides fresh hope for the development of treatments for neurodegenerative brain diseases.

Frisén's team also found that the donor brains of 11 people who had had Huntington's disease – a rare, degenerative brain disorder – had fewer new neurons in their striata than the donor brains of formerly healthy people. This lack of new neurons may have contributed to the characteristic problems of Huntington's disease, which include movement issues and cognitive deficits.

And immature neurons have in the past been spotted in the striata of people who had had a stroke. "It's very tempting to think that it would be possible to promote the generation of more striatal neurons," says Frisén.