Neuroscientists May Finally Know Whether the Adult Brain Can Grow New Cells

"In some respects, it's just one of the things that humanity has always hoped for." 

Given all the things we do to damage our brain cells, it’s comforting to believe that the brain grows new cells throughout our lives. The idea, however, has proved controversial. A paper published Thursday in Trends in Neurosciences makes the case that maybe it’s time to face facts: We probably don’t continue to churn out new brain cells as we get old. But letting go of that old idea frees us to answer far more important questions.

There’s evidence for and against adult neurogenesis — the idea that we continue to grow brain cells into adulthood. For instance, a study released 2018 indicated that we stop producing new neurons at age 13, but it immediately faced backlash from the scientific community.

In the new paper, Jason Snyder, Ph.D., a University of British Columbia behavioral neuroscientist, argues that if you take a close look at all the studies on animals from mice to humans, the facts are quite clear: Animals probably don’t develop significant amounts of new brain cells as we enter adulthood. There’s still hope for some neurogenesis, but not a huge amount.

“In some respects, it’s just one of the things that humanity has always hoped for — staying young,” he tells Inverse. “So I think it’s been disconcerting that there might not be as many of these young cells that are malleable, that are adaptive, that are capable of learning earlier in life. Of course we want those things to be there, but I think that introduces some bias.”

aging
It would be nice to believe that we continue to develop new brain cells as we age, but Snyder argues that it might not happen. 

To be clear, Snyder doesn’t argue that the field is biased. Instead, his argument is based on the analysis of past studies that have looked into this topic in humans, primates, and mice. There he admits that there’s been some confusion — some studies seem to show that the brain can continue to develop new cells later in life, while others show that it can’t. Specifically, he says that it’s been hard to let go of the idea of neurogenesis because of the results of animal studies (many on mice) “demonstrating persistent neurogenesis throughout life.”

In the paper, Snyder suggests that if we account for the differences in the ways human and rodent brains develop, it becomes clear that neurogenesis trails off into adulthood in mice too. He argues that humans tend to see the bulk of their brain cell formation before birth — though it continues after for a period of time as well — before trailing off upon reaching adulthood.

“The idea is that, because most of the neurogenesis happens earlier in primates and humans, the stem cells that are producing all these new neurons might have a limited number of times they can divide. By starting earlier in life they might fizzle out earlier,” Snyder explains. By comparison, Rodents tend to see the bulk of that new cell formation after birth, which means that as this high rate of cell formation trails off it persists longer into their adult lives, making it easier to spot new neurons.

“That trailing off is still measurable and meaningful, but it will extend later in life in rodents,” Snyder says. “It might explain why it’s easier to find [new cells] in rodents because that tail extends later in life than in primates and humans.”

neurogenesis
Snyder argues that it may be easier to find evidence of neurogenesis in rodents due to the way their brains develop. When you account for this difference, he believes that there's not a ton of evidence that neurogenesis happens in adulthood in humans or other animals. 

Importantly, there have been studies also done on humans that seem to provide evidence of some neurogenesis into adulthood, which is why Snyder’s paper adds the caveat that it’s not that neurogenesis totally ceases — it’s just likely happening at low rates, or maybe in specific brain regions. “While it remains unresolved whether neurogenesis drops to zero in adult humans, our analysis suggests that it falls to low rates for much of adult life in all species,” he writes.

But the point of the paper is actually to make a far larger observation point about what we miss when neuroscientists are so hung up on whether the brain grows new cells throughout life: For decades we’ve been so focused on whether new cells are there, when we should be focusing on what the cells that are already there can do.

“Even these negative reports that may be disconcerting can stimulate us to ask better questions,” he says. “If there aren’t many of these new cells, well, how might they be working differently? That may still make them important in terms of cognition.”