Neurons Counted In Adults

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.

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.”