Image: neurons in the infant brain. Image source: snyderlab.com.
Does neurogenesis occur in human adults? In mature animals? Is it a yes or no question, or should it be? A new editorial published today in the journal Trends in Neurosciences contextualizes fifty years of controversy and conflicting reports, and invites researchers to consider new perspectives in how the brain changes across the lifespan.
Neurogenesis is the process by which new neurons are formed in the brain. The process is generally associated with neurodevelopment; brain cell formation occurs during childhood and adolescence and slows as an individual ages to maturity. There is no scientific consensus as to whether it continues into adulthood or if it occurs in some areas of the brain but not others or at what rate.
Dr. Jason Snyder argues that whether or not neurogenesis occurs in adult humans is not the right question.
“It’s clear that there is a lot of controversy, which to me seems unwarranted because a yes or no for ‘is there adult neurogenesis’ is a little too simplistic and distracts us from other important questions,” says Dr. Snyder. “It’s worth asking if methodological differences are the only reason that some people aren’t finding new neurons or if there is some truth to the observations that neurogenesis may be limited with age in humans. I wanted to take a quantitative look at the research and see where it all leads.”
One major flaw in neurogenesis research is that we are using young mice to answer questions affecting older people. Once age is accounted for, neurogenesis occurs earlier in the lifespan in humans than in rodents which suggests there could indeed be less opportunity for adult neurogenesis in humans.
“However, in animals with long lifespans, such as humans, we know that cells are plastic for a lot longer,” explains Dr. Snyder. “This means that even cells born in childhood could remain malleable for many years after they are born. In terms of development, we also know that cells born at different stages of life serve different purposes. That the issue is controversial shows that we have a lot invested in it, and we still have a lot of unanswered questions.”
The neurogenesis controversy is a great example of scientific progress in action.
“We’re nowhere near consensus, but I don’t think that’s such a bad thing,” says Dr. Snyder. “We’ve gone through phases where people didn’t even study the topic because a paper concluded adult neurogenesis didn’t occur, which meant labs weren’t investigating its potential for human health.”
“We need to appreciate the work of others—everyone is producing a lot of really sound and solid stuff. But rather than focus on winning the debate, we need to work together, and to ask better questions,” says Dr. Snyder. “There are other angles to consider, and a lot of possibility ahead of us. The field is still finding its way.”