Adult neurogenesis · Grey Matter

Adult neurogenesis is the birth of new neurons in the mature brain, and its interest is that it would add whole new cells to circuits, beyond changing synapses; in rodents it clearly happens in the hippocampus, while in adult humans how much of it happens is still disputed.

Adult neurogenesis. Adult neurogenesis is the birth of new neurons in the mature brain, and its interest is that it would add whole new cells to circuits, beyond changing synapses; in rodents it clearly happens in the hippocampus, while in adult humans how much of it happens is still disputed.

In mice and rats, stem cells in the dentate gyrus of the hippocampus keep producing granule neurons throughout life. Each new neuron takes several weeks to grow its dendrites and axon and connect, and for a while it is more excitable and more plastic than its mature neighbours, which is thought to help distinguish similar memories. Rodents also make new neurons in the wall of the lateral ventricles that migrate to the olfactory bulb. Most of the newborn cells die within weeks unless activity and learning keep them.

The human evidence conflicts. Carbon-14 dating of DNA (Spalding and colleagues, 2013) suggested that about a third of hippocampal neurons are replaced in adulthood, about 700 per day in each hippocampus, a turnover of about 1.75 % per year within the renewing population. Two 2018 studies then disagreed: one found immature neurons in older adults, the other found almost none after childhood.

Method explains much of the disagreement. The markers for immature neurons fade with long fixation of post-mortem tissue, and the patient groups differed in age and disease, so the size of human neurogenesis is unsettled.

Even if it is real, it is small. It concerns one part of the hippocampus, so it adds little to the brain's 86 billion neurons, whose identity is otherwise fixed; most change in the adult brain is synaptic.

The adult brain mostly rewires the neurons it already has.

New neurons, where they exist, are a narrow additional mechanism, and whether humans keep them is an open question.

Questions: Does the adult human brain make new neurons? Possibly, in one place, and how much is disputed. Rodents keep making new granule neurons in the dentate gyrus of the hippocampus throughout life, and carbon-14 dating of human hippocampal DNA suggested about 700 new neurons per day in each hippocampus in adults. In 2018 two studies looking for immature neurons in post-mortem human tissue reached opposite conclusions, one finding them into old age and the other almost none after childhood, and differences in tissue handling and patient groups explain part of the conflict. Either way, the number is tiny next to the brain's 86 billion neurons, and almost all adult change happens in synapses.