The mystery of the missing brain cells

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http://www.newscientist.com/article/mg21328521.600-the-mystery-of-the-missing-brain-cells.html

The idea that we can grow new neurons has brought tantalising hope of repairing the brain after injury and disease. But could it be based on wishful thinking?

I AM sitting at a lab bench peering down the microscope at the brain of a chicken embryo. Dense networks of delicate young nerve fibres surround patches of newborn cells with their DNA stained dark brown.

I am witnessing the end products of neurogenesis, the birth of new brain cells. It is one of the hottest topics in neuroscience, and the idea that we can boost the growth of new brain cells with various kinds of physical or mental exercise seems to have equally taken hold of the public imagination. On top of this is the exciting prospect that we could one day use new neurons to repair the brain after injury or disease.

But does it really happen? While there is good evidence that adult neurogenesis takes place in animals, there is reason to believe that does not necessarily apply to our own species. "Everyone wants to believe that functional neurogenesis happens in adult humans, everyone wants to believe that we can repair damaged brains," says Andrew Lumsden, head of the MRC Centre for Developmental Neurobiology at King's College London, where I saw the chicken brain. "But there's precious little evidence for it."

The current faith in our brains' regenerative abilities is in fact something of a reversal. For most of the last century, it was thought that neurogenesis was restricted to our time in the womb. "Once development was ended the founts of growth dried up irrevocably," wrote Santiago Ramón y Cajal, the 19th-century Spanish anatomist seen as the founder of modern neuroscience. "In the adult, the nerve paths are immutable."

One basis of this belief was our limited capacity to recover after a stroke or a blow to the head. Such injuries can have long-lasting effects on abilities like speech and movement. Plus the brain is so much more complex than organs with proven regenerative powers, such as the skin and liver. "How would new neurons usefully integrate into complex neural networks after the connectional plan of the brain is complete?" asks Lumsden. "One side effect of having a large and complex brain is that you wouldn't want naive newcomers barging in."