Songbirds reveal the dark side of creating new brain cells as adults

A new study of songbirds could help explain why humans don’t generate many new brain cells, called neurons, as adults.

By KR Callaway edited by Claire Cameron

Every day, the human body replaces billions of cellseliminating the old ones and generating new, healthy ones. The average lifespan of a red blood cell is just under four months, while skin cells last about a month and those in the intestinal lining only last a few days. This renewal is the default, but there is one part of the body in which humans and other mammals do not seem adapted to generating new cells: the brain.

Aging and damaged brain cells, or neurons, can cause memory problems and limit the brain’s ability to recover from illness. Some scientists have argued that if we could simply activate the ability to create new neurons in the brain – a process called neurogenesis – some of these deleterious changes could be reversed. But a new study suggests that neurogenesis may be more destructive than we thought, adding weight to a counter-theory that our brains’ apparent limitation is actually an evolved protection.

“Birds, reptiles, fish: they all have widespread neurogenesis in their forebrains throughout their lives,” says Benjamin Scott, lead author of the study and assistant professor at Boston University. “It’s really in mammals that we see restrictions.”

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In the new article, published today in Current biology, Scott and his colleagues analyzed the brains of zebra finchessmall songbirds that undergo neurogenesis throughout their lives. The researchers wanted to know how adult neurogenesis affected surrounding brain tissue. So they used an electron microscope to observe how new neurons reach their destination in the brain. Researchers previously thought that neurons might follow structures in the brain called glial scaffolds, which guide neurons to the right place during development. But Scott and his team observed that the new neurons passed directly through older neuronal pathways and that the new brain cells were stiffer than the mature “squishy” neurons.

“They’re all over the tissue,” Scott says of the new neurons. “They hit all the mature cells. They’re right in the middle of all the action.”

Because adult brains are finished growing, they don’t have room for new structures, so the tunnel wasn’t a total surprise to the researchers. Still, understanding the destructive side of neurogenesis—removing old pathways through the brain to make new connections—could help researchers understand why mammals limit this ability in adults.

“One of the things this study told us is that as new neurons move through the brain, they appear to push or deform the tissue,” Scott says. “One could imagine that they could modify the circuitry, severing the connections that underlie stored memories.”

Humans and other mammals may have evolved to limit adult neurogenesis to preserve important long-term memories, he and his colleagues speculate. But because mammals and birds are so different, it’s unclear whether the same tunneling process also occurs in mammalian brains.

“The forebrains of humans and birds have different organizational patterns…so some caution needs to be exercised before extending parallels to the level of brain circuits and cells,” says Eliot Brenowitz, a neurobiologist at the University of Washington, who was not involved in the new study.