Neurons Counted In Adults | June 17, 2013 Issue - Vol. 91 Issue 24 | Chemical & Engineering News
Volume 91 Issue 24 | p. 8 | News of The Week
Issue Date: June 17, 2013

Neurons Counted In Adults

Neuroscience: Carbon dating measures birth of nerve cells in human brains
Department: Science & Technology | Collection: Life Sciences
News Channels: Analytical SCENE, Biological SCENE
Keywords: neurogenesis, nuclear bomb testing, carbon dating, neuroscience, neurons, carbon-14
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Neurogenesis takes place in a two-compartment region of the adult human brain called the hippocampus (orange).
Credit: Roger Harris/Science Source
This is an image of the human brain showing the hippocampus (orange).
 
Neurogenesis takes place in a two-compartment region of the adult human brain called the hippocampus (orange).
Credit: Roger Harris/Science Source

The amount of nerve cell birth, or neurogenesis, in the adult human brain has been a big unknown.

Now an international team of scientists has measured the number of newly created neurons in the hippocampus region of adults—an area associated with memory and learning.

Most nerve cells in the human brain develop in the womb. According to the new study, however, some 1,400 neurons are created in the brain each day during adulthood (Cell 2013, DOI: 10.1016/j.cell.2013.05.002). That’s a lot more than scientists expected.

Neuroscientists first observed neurogenesis in the brains of adult humans more than a decade ago. But the process was never measured quantitatively. And the molecular marker used at that time to assign birth dates to cells, bromodeoxyuridine, has since been recognized as unsafe, so the experiments could not be repeated.

“So it’s been a lingering question, whether there’s sufficient neurogenesis in the adult human brain to have any role” in how the brain works, says Jonas Frisén, a molecular biologist at the Karolinska Institute, in Sweden.

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Researchers at Lawrence Livermore National Laboratory, who were also members of Frisén’s team, used the facility’s high-energy mass spectrometer (shown here) to count carbon-14 atoms in the nerve cells of deceased subjects.
Credit: LLNL
This is a photo of a lab at LLNL.
 
Researchers at Lawrence Livermore National Laboratory, who were also members of Frisén’s team, used the facility’s high-energy mass spectrometer (shown here) to count carbon-14 atoms in the nerve cells of deceased subjects.
Credit: LLNL

To put doubts to rest, Frisén and coworkers used carbon-14 dating, a technique typically applied to archaeological finds, to quantify human neurogenesis.

Aboveground nuclear bomb testing during the Cold War, from 1955 to 1963, caused a spike in the normally low concentration of 14C, a radioactive isotope, in Earth’s atmosphere. People who were alive at the time incorporated the excess 14C into the DNA of cells being generated in their bodies.

Frisén’s team took advantage of this fact, using accelerator mass spectrometry to count 14C atoms in DNA extracted from the hippocampal nerve cells of 55 autopsied human brains. The researchers compared atom counts with present-day atmospheric levels of 14C to determine the ages of the neurons. Via data modeling, the team determined that about 35% of the nerve cells in the hippocampus get renewed by neurogenesis over the course of adulthood.

According to Gerd Kempermann, a neurogenesis expert at the German Center for Neurodegenerative Diseases, in Dresden, the report “is an important confirmation” of adult human neurogenesis and “an interesting attempt to understand its dynamics.”

 
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