Songbird study suggests risks of making new brain cells in adulthood

Newly minted brain cells may jostle more than they help. In a study published today in Current Biology, researchers report that adult-born neurons in zebra finches tunnel through established brain tissue and deform surrounding cells as they migrate—behavior that could disrupt existing neural circuits and memories.

The finding adds support to the idea that mammals’ limited capacity to make new neurons in adulthood may be an evolved safeguard. “Birds, reptiles, fish: they all have widespread neurogenesis throughout their forebrains throughout life,” said Benjamin Scott, the study’s senior author and an assistant professor at Boston University.

“It’s really in mammals where we see this restricted.” Scientists have often hoped that switching on neurogenesis in adult brains could counter cognitive decline. But the new work suggests that building new neurons in an already crowded organ may carry costs.

Using electron microscopy, Scott and his colleagues analyzed how new neurons form and move in the forebrain of zebra finches, a species that continues to generate neurons throughout life. Contrary to the common assumption that young neurons might follow glial scaffolds—as they do during early development—the team observed the cells burrowing directly through older neural pathways.

The nascent neurons appeared more rigid than the “squishy” mature cells around them and were embedded throughout the tissue, closely contacting many established neurons. Because adult brains have little room for new structures, some tunneling was expected.

Even so, the researchers say the mechanical push of migrating neurons highlights a destructive side to neurogenesis: clearing space for new connections may deform tissue and potentially disrupt circuits that store long-term memories. Scott said the process could break existing connections as the cells move.

That interpretation may help explain why humans and other mammals generate relatively few neurons as adults. Still, the authors and outside experts urge caution in extrapolating from birds to mammals.

“The human and bird forebrains have different organization patterns, so some caution is called for in extending parallels to the level of brain circuits and cells,” said Eliot Brenowitz, a neurobiologist at the University of Washington who was not involved in the study.

Whether a similar tunneling process occurs in mammalian brains remains an open question. For now, the findings underscore that turning up neurogenesis in adults may not be an unalloyed benefit—and that evolution’s apparent restraint could be protective.