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16:07 17 August 2011 by Wendy Zukerman and Andrew Purcell
Magazine issue 2826. Subscribe and save
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The synaptic pruning that helps sculpt the adolescent brain into its adult form continues to weed out weak neural connections throughout our 20s. The surprise finding could have implications for our understanding of schizophrenia, a psychological disorder which often appears in early adulthood.
As children, we overproduce the connections – synapses – between brain cells. During puberty the body carries out a kind of topiary, snipping away some synapses while allowing others to strengthen. Over a few years, the number of synapses roughly halves, and the adult brain emerges.
Or so we thought. Pasko Rakic at Yale University and colleagues at the University of Zagreb, Croatia, and the VU University Medical Center in Amsterdam, the Netherlands, have now found that the brains of adults in their 20s are still subject to synaptic pruning.
Rakic's team analysed post-mortem tissue from a brain region called the prefrontal cortex (PFC) in 32 people aged between 1 week old and 91 years. Specifically, they calculated the density of dendritic spines – the tiny projections that protrude from the neuron's long dendrites, each of which facilitates communication with other neurons through a synapse.
As expected, Rakic's team found that spine density increased rapidly during infancy, reaching a peak before the 9th birthday. It then began to fall away as pruning began. Intriguingly, though, spine density did not plateau after adolescence, as might have been expected, but continued to fall gradually until the late 20s.
Rakic says the result could be good news for those hoping to gain new skills in their third decade. The period of pruning is associated with a heightened ability to learn – whether that is in picking up language skills or understanding new concepts, he says. "You should not give up learning just because you're in your 20s – it isn't too late," he says.
The finding also has implications for our understanding of some psychiatric disorders. The PFC is thought to be particularly relevant to late-onset disorders such as schizophrenia, says Rakic, but it is unclear whether such disorders are triggered by developmental or degenerative processes. The new finding is likely to give weight to the idea that schizophrenia emerges as a result of late brain development.
"I'm sure that for many people schizophrenia has a strong developmental component," says Sabine Bahn, who researches schizophrenia at the University of Cambridge – although she adds that some cases will likely have a degenerative component.
Elena Bagley at the University of Sydney, Australia, agrees with the conclusion. It is possible that the prefrontal cortex "is susceptible for longer to disorders and disease that result from abnormal pruning", she says. Such pruning may also contribute to memory loss and dementia, she adds.
Journal reference: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1105108108
