Transplanted mouse stem cells giving rise to neurons

Are we born with all the brain cells we’ll ever have?

We help you separate scientific fact from brainy baloney

Neuroscientists had always believed that the adult brain could not produce new cells and that we are born with all the neurons we will ever have. This viewpoint slowly began to change in the 1970s, when scientists discovered that the brains of rats and songbirds produce new cells throughout life. This is now widely accepted as being correct, but whether the same is true in humans has long been a divisive subject.

Up until recently, there was very little evidence for human neurogenesis (the production of new neurons). According to the latest estimates, the brain contains about 86 billion neurons and roughly the same number of glial cells. As far as we know, the vast majority of these cells are produced in the womb – during early pregnancy, about 250,000 brain cells are produced every minute. The majority of the remaining neurons are produced during a short period of time after birth – maybe as little as a few months, or as much as a year. As a result, the brain produces at least twice as many cells than it actually needs to work properly.

Neurons begin to die before we have even been born and continue to die every day of our lives. Researchers have estimated that about 85,000 neurons die every day in the cerebral cortex. That’s equivalent to one every second.

However, new evidence suggests that in a specific area of the human hippocampus, known as the dentate gyrus, new neurons are produced continuously into adulthood – around 1,400 a day. With age, the number of new neurons being produced decreases, but at a much less dramatic rate in humans than in mice.

The most interesting difference, though, is that in mice, new neurons survive for a long time, meaning that the hippocampus grows as they are produced. However, in humans new neurons do not live for long and, with age, the hippocampus shrinks as more neurons die than are produced. The reasons for the differences in neurogenesis between humans and other mammals are not known; studying neurogenesis is difficult and requires ingenious methods of measurement, so it may be a while before more conclusive facts are established. 

Lead image:

Confocal micrograph image of transplanted mouse neural stem cells giving rise to neurons.

Yirui Sun/Wellcome Collection CC BY

References

About this resource

This resource was first published in ‘Inside the Brain’ in January 2013 and reviewed and updated in November 2017.

Topics:
Cell biology, Neuroscience
Issues:
Inside the Brain, Thinking
Education levels:
16–19, Continuing professional development