New Evidence for Adult Neurogenesis to Occur Even in Late Old Age in Humans

You might have missed it, but the past year has seen quite the debate in scientific circles over whether or not humans exhibit the same processes of neurogenesis in adult life that are observed in mice. Neurogenesis is the process by which new neurons are created in the brain and integrated into neural circuits. It is a part of the plasticity that allows for cognitive function and maintenance of that function in the face of damage. Obviously, this is a very important topic for those groups seeking ways to apply regenerative medicine to the brain.

Prior to the discovery of adult neurogenesis in the 1990s, it was thought that no such process took place after early development, and that the tissues of the adult brain were not maintained in this way. Since the 1990s an enormous amount of investigative work on this topic has taken place in mice, and comparatively little in human brains and brain tissue. In this context, much upheaval occurred in the wake of a study published last year that found no evidence of adult human neurogenesis. Shortly thereafter, another study was published with contradictory results, showing that there were signs of neurogenesis. The debate continues, and we will see more, ever more careful studies on this topic. Currently the weight of evidence leans in the direction of neurogenesis in adults, thankfully - if the process exists, then there is a path to enhance it in older individuals, in order to find ways to postpone and reverse portions of age-related neurodegeneration.

People keep making new brain cells throughout their lives (at least until the age of 97), according to a study on human brains. The idea has been fiercely debated, and it used to be thought we were born with all the brain cells we will ever have. The researchers also showed that the number of new brain cells tailed off with age. And it falls dramatically in the early stages of Alzheimer's disease - giving new ideas for treating the dementia.

Most of our neurons - brain cells that send electrical signals - are indeed in place by the time we are born. Studies on other mammals have found new brains cells forming later in life, but the extent of "neurogenesis" in the human brain is still a source of debate. The study looked at the brains of 58 deceased people who were aged between 43 and 97. The focus was on the hippocampus - a part of the brain involved in memory and emotion.

Neurons do not emerge in the brain fully formed, but have to go through a process of growing and maturing. The researchers were able to spot immature or "new" neurons in the brains that they examined. In healthy brains there was a "slight decrease" in the amount of this neurogenesis with age. "I believe we would be generating new neurons as long as we need to learn new things. And that occurs during every single second of our life."

But there was a different story in the brains from Alzheimer's patients. The number of new neurons forming fell from 30,000 per millimetre to 20,000 per millimetre in people at the beginning of Alzheimer's. "That's a 30% reduction in the very first stage of the disease. It's very surprising for us, it's even before the accumulation of amyloid beta, a hallmark of Alzheimer's, and probably before symptoms, it's very early. Larger studies will need to confirm these findings and explore whether they could pave the way for an early test to flag those most at risk of the disease."


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