A newly published study suggests adult human brains can produce as many new brain cells as younger brains. The research comes only a month after a controversial study claimed the human brain most likely does not produce new neurons beyond childhood.
In early March, a study from scientists at UC San Francisco concluded that human brains most likely do not produce new neurons past childhood. The research examined brain specimens from 59 subjects, focusing on the hippocampus, an area previously suggested to be a key location for neurogenesis. It found no evidence of immature, or young, neurons in brains older than around 13 years of age.
Now a new study, led by scientists at Columbia University, has fired back with research suggesting the exact opposite. Hippocampi from 28 healthy subjects ranging from 14 years of age to 79 were studied. The subjects had all died suddenly and were not cognitively impaired or suffering from depression.
"We found that older people have similar ability to make thousands of hippocampal new neurons from progenitor cells as younger people do," says lead author on the study, Maura Boldrini. "We also found equivalent volumes of the hippocampus (a brain structure used for emotion and cognition) across ages."
The primary difference between older and younger brains identified in this new study was a decline in vascularization. The older brains did display a reduced ability to generate new blood vessels and it is hypothesized that this decline could affect the ability of new neurons to make connections.
The team behind last month's contrary study quickly hit back, suggesting that while some parts of Boldrini's study were interesting, particularly the observations regarding a decline in vascularization, the ultimate conclusion regarding adult neurogenesis was far from convincing.
"Based on the representative images they present, the cells they call new neurons in the adult hippocampus are very different in shape and appearance from what would be considered a young neuron in other species, or what we have observed in humans in young children," the authors of last month's study wrote in a response published by the Los Angeles Times.
Boldrini countered by claiming her team's study utilized better maintained and collected brain samples, as well as more effective analysis techniques. Speaking to Gizmodo, Boldrini says, "They had brains from four different places around the world, like Asia, Europe, the US. And they said they analyzed three to five sections of the hippocampus. So they didn't have the whole hippocampus, of course. These brains were also treated with different chemicals, and they were comparing them to each other. And each of these chemicals that are used to preserve the brain can affect the cells that you see – it's like comparing a roasted chicken with a chicken that's been fried."
The debate is surely set to continue, and it's not just an academic argument. Discovering whether or not adult neurogenesis occurs will fundamentally inform future research into treating conditions such as Alzheimer's disease. If we know that adult brains do effectively produce new neurons then we can better work to understand, and manipulate, the mechanisms behind that process. But, if adult neurogenesis does not take place then we need to work out how adult neurons adapt and continually learn in order to battle neurodegenerative diseases.
The new study was published in the journal Cell Stem Cell.
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