Adult Neurogenesis: Do We Grow New Neurons?

For a century, textbooks said adults never make new brain cells. Then rodents, songbirds and radiocarbon from nuclear tests suggested otherwise — and the human question is still fiercely debated.

Santiago Ramón y Cajal wrote that in the adult brain "nerve paths are something fixed, ended, immutable. Everything may die, nothing may be regenerated." For most of the 20th century, that was the consensus.

Challenges to dogma

In the 1960s Joseph Altman used radioactive labels to show signs of new neurons in adult rats, but his work was largely ignored. In the 1980s Fernando Nottebohm showed that adult canaries grow new neurons in song-control regions each year as they learn new songs. By the 1990s, it was clear that adult rodents produce new neurons in two places: the dentate gyrus of the hippocampus and the zone lining the lateral ventricles (whose neurons migrate to the olfactory bulb). In rodents, running, enriched environments and learning increase neurogenesis; stress decreases it.

Evidence in humans

In 1998 Peter Eriksson, Fred Gage and colleagues studied the brains of cancer patients who had been given BrdU, a chemical that labels dividing cells, for diagnostic purposes. After death, labelled neurons were found in the hippocampus. In 2013 Jonas Frisén's group used an ingenious clock: atmospheric nuclear weapons testing in the 1950s and 60s raised carbon-14 levels, which then fell steadily, so the carbon-14 in a cell's DNA dates when it was born. They estimated that about 700 new neurons are added to each human hippocampus per day.

The 2018 clash

In 2018 two studies in prominent journals reached opposite conclusions. Shawn Sorrells, Arturo Alvarez-Buylla and colleagues found hippocampal neurogenesis dropping to undetectable levels in adults. Weeks later, Maura Boldrini and colleagues reported continued neurogenesis into old age. Differences in tissue handling, post-mortem delay and markers may explain the conflict. Later studies using single-cell RNA sequencing have also disagreed. The question remains open.

Why it matters

If adult human neurogenesis is substantial, it could contribute to memory, mood and recovery — and might be boosted to treat depression or dementia. If it's minimal, plasticity in adults must rely mainly on changing connections.