Summarizing the Recent Debate Over Adult Neurogenesis in Humans

Does the adult brain produce and integrate new neurons into its neural circuits, in a process known as neurogenesis? Near all of the evidence for this process to take place in adults has been established in mice, and over the past year a few new studies have suggested that this process doesn't in fact occur in humans. This is something of shock to the research community, as a fair number of regenerative medicine projects are progressing under the hope that existing neurogenesis can be increased in scope and pace, in order to repair and restore the aged brain to greater degrees than presently occurs naturally. If the neurogenesis so well characterized in mice doesn't exist in humans, then those projects will all fail. It is an important topic, but we shouldn't expect resolution of this debate to arrive in the near term. Conflicting data that is so carefully produced and so directly opposed tends to require years of work to resolve, particularly when human tissues are vital to the end goal.

Just a generation ago, common wisdom held that once a person reaches adulthood, the brain stops producing new nerve cells. Scientists countered that depressing prospect 20 years ago with signs that a grown-up brain can in fact replenish itself. The implications were huge: Maybe that process would offer a way to fight disorders such as depression and Alzheimer's disease. This year, though, several pieces of contradictory evidence surfaced and a heated debate once again flared up. Today, we still don't know whether the fully grown brain churns out new nerve cells.

In March of this year, contradicting several landmark findings that had convinced the scientific community that adults can make new nerve cells, researchers described an utter lack of dividing nerve cells, or neurons, in adult postmortem brain tissue. A return volley came a month later, when a different research group described loads of newborn neurons in postmortem brains, in an April paper. Scientific whiplash ensued when a third group found no new neurons in postmortem brains, describing the results in July. Still more neuroscientists jumped into the fray with commentaries and perspective articles.

This ping-ponging over the rejuvenating powers of the brain is the most recent iteration of a question on neurogenesis that still hasn't been answered. Despite the more recent negative results, many scientists still hold on to the notion that new growth happens. "The negative findings were very controversial. It's always very difficult to put aside a phenomenon just by not finding it. Let's face it: it's not easy to label and detect adult neurogenesis in human postmortem tissue. This year's studies provide a push to the field to develop more advanced tools and models."