Human Growth and Development / Unit 15 HGD 2118 review, 2 to 3 minutes
Adult Neurogenesis
The idea
Neurogenesis, the formation of new neurons, is largely complete before birth, with one possible exception: the hippocampus, a structure central to memory. Whether older humans continue to make hippocampal neurons is contested. In twenty eighteen, Sorrells and colleagues examined human brain tissue and reported that new hippocampal neurons fall to undetectable levels in adults. The next year, Moreno-Jiménez and colleagues, using different tissue-handling methods, reported many immature neurons in the hippocampus of adults into their nineties, declining in Alzheimer's disease. The two teams disagreed partly over method: how tissue is preserved and which markers identify a new neuron. Animal work adds a further strand. In older mice, physical activity and enriched environments produced new neurons. But those are animal findings, and mice are not people. The human question remains open.
In practice
Two laboratories examining brain tissue from older adults, one finding almost no new neurons and the other finding many, with the difference traced partly to how the tissue was preserved.
Where people slip
The misconception to avoid is treating either result as settled. The honest summary is that adult human hippocampal neurogenesis is contested, that methods drive much of the disagreement, and that the animal evidence, though encouraging, cannot settle a human question. Claims that a product or habit "grows new brain cells" go beyond this evidence. What is better established is that the aging brain compensates: some older adults recruit additional regions and perform better for it, and neuroplasticity declines but does not vanish. For the exam, know that neuroplasticity generally declines with age, that frontal compensation occurs, and that human adult neurogenesis is debated. Neuroscience advances through exactly this kind of argument over method and replication.
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