Does Your Brain Produce New Cells?

Neurogenesis refers to the production of new nerve cells. Everyone wants to believe the human brain continues to produce new cells throughout life, but as you'll see from the article, the evidence for this is thin on the ground, and several prominent researchers are very sceptical about it. ________________________________________________________

I'm sitting at a long lab bench in the MRC Centre for Developmental Neurobiology, peering down a microscope at the hindbrain of a three-day- old chicken embryo. Earlier, the egg had been injected with bromodeoxyuridine (BrdU), a compound whose structure resembles that of thymidine, one of the four main components of DNA, and which is incorporated into newly-synthesized DNA.

The embryo was then removed, the hindbrain dissected and treated with an antibody that binds BrdU. Now, split along the top and splayed onto a glass slide, it appears subdivided into eight compartments, each revealing its newborn cells with their DNA stained dark brown.

Andrew Lumsden, the centre's director, explains that each segment expresses a unique combination of patterning genes and that the segment boundaries restrict the movements of immature cells. Neurons in each segment acquire a unique identity – those born in the front segment coalesce to form the nucleus of the fifth cranial nerve, while those further back form other cranial nerves.

At this developmental stage, the nervous system is a hollow tube running along the embryo's back. Its walls contain wedge-shaped cells that divide near the inner surface to produce neurons that migrate outwards. This occurs at different rates along the tube, producing three bulges at one end, which eventually form the brain. Successive waves of migrating cells populate the developing brain to give the cortex its characteristic layered appearance. Upon arrival at their destination, they differentiate into the brain's three main cell types – neurons, astrocytes and oligodendrocytes – then sprout connecting branches to form functional tissue.

Fountain of eternal youth?

For much of the past century, it was thought that the production of new neurons – neurogenesis – was restricted to embryonic development. "Once development was ended," wrote Santiago Ramón y Cajal, the father of modern neuroscience, "the founts of growth… dried up irrevocably. In the adult, the nerve paths are… immutable. Everything may die, nothing may be regenerated."

In fact, Joseph Altman of the Massachusetts Institute of Technology had reported evidence of adult neurogenesis in the 1960s, in the hippocampus of adult rats and guinea pigs and cortex of cats, but his work was ignored and then ridiculed. "Altman started the idea of adult neurogenesis, but his data weren't convincing," says Nottebohm. "Our results showed, beyond reasonable doubt, that neurons are born in adulthood and incorporated into existing circuits. They brought to an end most resistance against the idea."

In all mammalian embryos, neurogenesis occurs along the entire length of the neural tube. In adults, the tube's hollow cavity has transformed into the brain ventricles, which are filled with cerebrospinal fluid, and neurogenesis is restricted to two brain regions, each containing a niche of different types of stem cells.

The larger niche, in the walls of the C-shaped lateral ventricles, produces immature neurons that migrate in chains within the rostral migratory stream (RMS) to the olfactory bulb. Some differentiate into mature neurons that integrate into local circuits and participate in the processing of smell information. The other produces cells that integrate into the dentate gyrus of the hippocampus and play important roles in learning and memory. Exactly how new cells participate in information processing remains unclear. They may replace dying cells, or could be added to existing circuits to provide additional information processing capabilities.

Other regions of the lateral ventricles contain dormant stem cells, which can be activated following brain injury to produce new cells that migrate to the injury site.

From mice to monkeys and men

In the late 1990s, Elizabeth Gould of Princeton University reported evidence of adult neurogenesis in the monkey hippocampus, and showed that stress decreases stem cell division in the dentate gyrus. The monkey brain is much bigger than that of rodents, however, and the process is protracted. Fewer cells are produced, they migrate larger distances and take longer to mature. According to one recent study by researchers from the University of Illinois, new cells in the macaque dentate gyrus take at least six months to mature fully. Adult neurogenesis is implicated in depression and Alzheimer's disease, both of which involve hippocampal shrinkage. The anti-depressants Prozac and imipramine stimulate hippocampal neurogenesis in adult mice and some of their effects depend on the new cells. They also make immature hippocampal cells derived from human embryos divide in the Petri dish.

It is now taken for granted that adult neurogenesis occurs in humans, and the idea has revolutionized the way we think about the brain. It is widely believed that physical and mental exercise can stimulate hippocampal neurogenesis that offsets age-related cognitive decline and may protect against depression and Alzheimer's. "Everyone wants to believe that functional neurogenesis happens in adult humans," says Lumsden. "Everyone wants to believe that we can repair damaged brains, but there's precious little evidence for it."

The biggest sceptic is Pasko Rakic, who revealed how newborn cells migrate in the developing brain in a series of classic experiments performed in the early 1970s. Rakic injected macaque monkey fetuses with radioactive thymidine and sliced their brains into hundreds of ultra-thin sections. He identified migrating neurons by their newly-synthesized, radioactive DNA and painstakingly reconstructed the sections, to show that the cells climb onto elongated cells called radial glia, which span the thickness of the tube to contact its inner and outer surfaces and they then crawl, amoeba-like, along the radial glial fibres to the outer surface. His hand-drawn diagrams depicting the process appear in textbooks to this day.

Now chairman of Yale's neurobiology department and director of the Kavli Institute for Neuroscience, Rakic casts a long shadow, and has been extremely critical of some of the adult neurogenesis research. He points out that BrdU can induce cell division, and also labels dying cells, which synthesize DNA just before they die, so cannot give accurate counts of newborn cells in adult brain tissue. This can be overcome by double staining with other antibodies, to verify that BrdU-labelled cells are indeed dividing.

"Rakic was reasonable in demanding higher levels of proof," says Nottebohm, "but he railed against adult neurogenesis so aggressively that to many it struck as a defence of the old dogma. As a participant in the battles, I found him too negative and not particularly perceptive. His own work used animals housed under conditions that inhibit the formation and survival of new neurons."

Nottebohm and others say that Rakic has held back adult neurogenesis research, but according to Gage, he has been "an important driver for making the field more rigorous. He challenges the weakness in their work and it's up to researchers in the field to address them." But Gage notes that immature neurons derived from mouse stem cells are more active than their mature counterparts, so an extended maturation period may actually be beneficial. "I'm not surprised that maturation would take longer in humans, but the other way to look at it is that newborn cells have an extended period of plasticity."

Rakic's scepticism is, however, supported by the scientific evidence – or rather, lack of it.

In 1998, Gage and the late Peter Eriksson examined the brains of five cancer patients who had been injected with BrdU for diagnostic purposes. They treated the hippocampal tissue with antibodies against BrdU and proteins synthesized by immature neurons, and found some staining in the dentate gyrus. This was the first evidence that the adult human brain contains newborn neurons, but the researchers emphasized that it did not show that the cells are functional.

Others have isolated stem cells from various regions of the adult human brain. These cells have a limited capacity for self-renewal when grown in the lab, but can generate mature astrocytes, oligodendrocytes and neurons with normal electrical properties.

In 2006, Jonas Frisén of the Karolinska Institute and colleagues examined the cortex in autopsied brains of seven adults. They looked for radioactive carbon from Cold War nuclear bomb tests, which accumulates in newly-synthesized DNA, but detected only atmospheric levels, and concluded that neurogenesis does not occur in the cortex.

More recently, Gerd Kempermann of the Center for Regenerative Therapies in Dresden and colleagues examined brains from 54 individuals aged up to 100, using antibodies for multiple proteins, and found small numbers of newborn hippocampal cells in all of them. "It appears to be the same as in rodents," says Kempermann. "There's very steep decline in early life but you end up with a very low level that is maintained. We saw small numbers of cells, but we saw them up to very old age."

But Arturo Alvarez-Buylla, a professor in the Department of Neurological Surgery at the University of California, San Francisco, isn't entirely convinced. "Gage and Erikkson provided evidence that some proliferation occurs in the adult hippocampus," he says, "but this has to be treated with caution, because some of the labelled cells might have been dying."

They identified a 'ribbon' of astrocytes in the walls of the lateral ventricles which produce immature neurons, astrocytes and oligodendrocytes and which has not been seen in other species. They also identified the RMS in infants, and found that it contains small numbers of migrating cells, as well as a previously unidentified migratory pathway, which branches off from the RMS to enter the prefrontal cortex.

According to their data, migration occurs in both streams postnatally, but declines steeply by 18 months of age and has almost completely disappeared by early adulthood. "We concluded that if migration occurs then it is very scarce," says Alvarez-Buylla, "and that cells are not forming large bundles that migrate to the olfactory bulb." The data conflict with those of a 2007 study by Erikkson and Maurice Curtis, who saw a robust RMS containing large numbers of migrating cells, but were confirmed last year by Chinese researchers, who found small numbers of migrating neurons in the adult RMS, but no new cells in the olfactory bulb itself.

"How much neurogenesis occurs in older people, and how much it contributes to local plasticity, are still open questions," says Alvarez-Buylla. "There is controversy over how much cell renewal there is in the hippocampus and how persistent the stem cells are throughout life. If they decline with age they're not really self-renewing."

Overall, the few available studies suggest that the fountain of youth is reduced to a mere trickle in adults. There is no evidence whatsoever for adult neurogenesis in the human cortex; the existence of the RMS in adults is still disputed, and evidence for hippocampal neurogenesis is very thin on the ground. If the hippocampus does produce new cells, are there enough to be any significance?

Kempermann believes there are: "The network requires very few cells to be added and still be functionally relevant," he says. Other adult neurogenesis researchers also believe that small numbers of cells could be relevant to the function of the hippocampus. But this question remains unanswered, and the possibility that the number of cells produced is not large enough to be functionally significant has serious implications for popular claims, such as that exercise can improve memory, and also for the new view of the brain that has been adopted so quickly.

"One side-effect of having a large and complex brain is that you wouldn't want naïve newcomers barging in," says Lumsden. "How would new neurons usefully integrate into complex neural networks? If anything, evolution would have made damn sure that mechanisms exist to eliminate these party-crashers. Lack of neurogenesis after the connectional plan of the brain is complete would be a selective advantage."

The brain may, therefore, favour stability over plasticity. Human adult neurogenesis may be an evolutionary relic, and one that comes at a very high cost, as stem cells in the adult human brain likely contribute to brain tumour formation.

There's still hope

"Rakic was mostly correct," says Nottebohm. "Until now, the overwhelming evidence is that most neurons are formed early in development, including a short while after birth." But even if functional adult neurogenesis does not occur in the human brain, or if the numbers of cells produced are too small to be of any significance, there is still hope that neural stem cells could be of therapeutic value.

"Rakic missed what was central about the argument," Nottebohm continues. "There is a rich collection of neural stem cells that continue to generate new neurons in adulthood. This is of the greatest importance. It shows, in principle, that this reservoir might be exploited for purposes of brain repair."

To this end, researchers are exploring two approaches to develop neural stem cell-based therapies for neurological conditions, although any such treatments are still a long way off. One approach is to coax the brain's stem cells to generate neurons that migrate to injured or diseased sites. The other is to transplant lab-grown neurons of specified types directly into the brain. Indeed, neurons derived from human neural stem cells can differentiate into fully functional neurons when transplanted into foetal rat brain, and can now be tracked in live animals using magnetic resonance imaging.

"We found the first evidence for replaceable neurons," says Nottebohm, "and I have no doubt that a whole new field will emerge around this concept. I'm sure this will have a profound effect sooner or later. This is just the beginning."

  • Neuroscience
  • Neurophilosophy

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