News | Mind & Brain

A unique form of carbon dating, made possible by the Cold War, suggests that new neurons rarely survive in the human olfactory bulb after birth

By Ferris Jabr | June 7, 2012

BOMBSHELL FINDINGS: A new study relying on radioactive carbon from Cold War nuclear tests argues that the adult human brain rarely weaves new neurons into the olfactory bulb, but not everyone is convinced. Image: Adapted from Wikimedia Commons images

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The human body is a tireless gardener, growing new cells throughout life in many organsin the skin, blood, bones and intestines. Until the 1980s most scientists thought that brain cells were the exception: the neurons you are born with are the neurons you have for life. In the past three decades, however, researchers have discovered hints that the human brain produces new neurons after birth in two places: the hippocampusa region important for memoryand the walls of fluid-filled cavities called ventricles, from which stem cells migrate to the olfactory bulb, a knob of brain tissue behind the eyes that processes smell. Studies have clearly demonstrated that such migration happens in mice long after birth and that human infants generate new neurons. But the evidence that similar neurogenesis persists in the adult human brain is mixed and highly contested.

A new study relying on a unique form of carbon dating suggests that neurons born during adulthood rarely if ever weave themselves into the olfactory bulb's circuitry. In other words, peopleunlike other mammalsdo not replenish their olfactory bulb neurons, which might be explained by how little most of us rely on our sense of smell. Although the new research casts doubt on the renewal of olfactory bulb neurons in the adult human brain, many neuroscientists are far from ready to end the debate.

In preparation for the new study, Olaf Bergmann and Jonas Frisn of the Karolinska Institute in Stockholm and their colleagues acquired 14 frozen olfactory bulbs from autopsies performed between 2005 and 2011 at the institute's Department of Forensic Medicine. To determine whether the neurons were younger than the people they came fromwhich would mean the cells were generated after birththe researchers needed to isolate the cells' DNA. First, they dissolved the brain tissue into a kind of soup, which they spun at high speeds so that the dense cell bodies and nuclei containing DNA sank to the bottom of the flasks. Using Y-shaped proteins called antibodies, which were hitched to fluorescent markers, the researchers tagged nuclei from both neurons and from glia, non-neuronal brain cells. After a laser-equipped cell-sorting machine identified and separated the nuclei, the researchers isolated and purified the DNA within.

Excerpt from:
How Nuclear Fallout Casts Doubt on Renewal of Some Adult Brain Cells

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