Big brain evolution: adult neurogenesis? (Evolution)

by David Turell @, Thursday, March 08, 2018, 00:59 (110 days ago) @ David Turell

New research casts doubt on much adult development of new neurons in one area:

"Pick up any article on neuronal development in adulthood, and there is a good chance you will read that the birth of new neurons has been observed in the hippocampal region of the brain in every mammalian species examined, including humans. This idea underlies the view — widespread among neuroscientists — that analysis of such neurogenesis in animals can benefit our understanding of learning, emotional disorders and neurodegenerative disease in humans. But in a paper in Nature, Sorrells et al.1 report that, unlike in other mammals, the last new neurons in the human hippocampus are generated in childhood. These findings are certain to stir up controversy.


"Although the scope and function of neurogenesis remain debatable, there has been a general consensus that the hippocampus is one region in which adult neurogenesis exists in humans as it does in animals. This is based on several studies. For example, one study in patients given a synthetic nucleoside molecule called bromodeoxyuridine (BrdU) showed that it had been incorporated into the DNA of dividing cells in the dentate gyrus4. Another found that protein markers of neurogenesis in animals were present in post-mortem human brain tissue5, and a third used radiocarbon dating to identify hippocampal-neuron turnover6. However, methodological challenges make human studies difficult to interpret, and more are required to make definitive conclusions.

Sorrells et al. set out to address this need using classic immunohistochemical techniques in which specific antibodies are bound to proteins of interest, revealing their locations in tissue. The authors used this strategy to count neural precursor cells, proliferating cells and immature neurons in samples from 59 human subjects, spanning fetal development through to old age. They found streams of all three cell types migrating from an embryonic ‘germinal zone’ to the developing dentate gyrus at 14 weeks of gestation. By 22 weeks, migration was reduced, and immature neurons were largely restricted to the dentate gyrus. And there were many fewer immature neurons at one year of life than at earlier stages. The oldest sample containing immature neurons was taken from a 13-year-old individual. These findings are in stark contrast to the prevailing view that human hippocampal neurogenesis extends throughout adult life.


"How do the authors’ findings fit with the animal literature? With a bit of conceptual recalibration, they might fit quite well. Rodents are born with relatively immature nervous systems, so adult rodent neurogenesis could be a decent model of neurogenesis in children or adolescents. Given that depression, schizophrenia and Alzheimer’s disease are rooted in early hippocampal defects, even neurons generated in childhood could have a key role in the aetiology of disease in humans. In addition, primate data10 suggest that new neurons in humans could go through an extended period of maturation (years or even decades) relative to what occurs in rodents, during which time they might have enhanced plasticity and important functional properties. Thus, whereas the continual addition of new neurons might provide plasticity in adult rodents, the prolonged development of neurons could provide a similar plasticity in adult humans.

"At the other end of the developmental spectrum, even in rodents, neurogenesis is very low by middle age2. Thus, Sorrells and colleagues’ human data again are not wholly inconsistent with the animal literature. "

Comment: the hippocampus is a deep structure in the brain in the so-called limbic system which is an area of earlier cortical structures which are also in lesser animals, but not a part of pre-cortical development as in humans.

From Wikipedia: The structures of the limbic system are involved in motivation, emotion, learning, and memory. The limbic system is where the subcortical structures meet the cerebral cortex

The hippocampus deals in part with new memories. This study does not preclude pre-frontal neuronal development, but does not advance in any way the theory that adult use of the brain forces enlargement.

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