Summary: A new paper examines the controversies and differing results surrounding adult neurogenesis.
Source: Cell Press
For decades scientists have debated whether adult humans generate significant numbers of new neurons. From early studies in the 1960s through more recent high-profile papers, published evidence has swung between affirmative and negative conclusions. In a review published January 24 in Trends in Neurosciences, University of British Columbia professor Jason Snyder examines that debate, arguing that apparently conflicting findings can be reconciled once differences in methodology, species development, and age at study are taken into account. He also cautions that focusing only on whether adult neurogenesis exists distracts from more practical questions about its function and therapeutic potential.
Snyder notes that the binary question “Does adult neurogenesis occur?” is overly simplistic and diverts attention from deeper issues. He says it is important to determine whether methodological variation fully explains why some teams fail to detect new neurons, or whether there is a genuine decline in neurogenesis with advancing age in humans. To answer that, he took a quantitative look across the literature to identify patterns that might clarify the field.
One key factor Snyder emphasizes is age: many rodent studies that report robust levels of neurogenesis examine young mice, whereas human tissue studies tend to use samples from middle-aged or older adults. Additionally, developmental timing differs between species. In humans, most neuronal populations in some brain regions are generated during the first half of gestation, while in mice significant neurogenesis continues through birth and into the early postnatal period. These timing differences mean that higher apparent neurogenesis in rodents may reflect later developmental schedules rather than an intrinsic species disparity in adult neuron production.
Snyder acknowledges that the evidence for ongoing neurogenesis in adult humans is still uncertain. If adult neurogenesis does occur, he argues the best current evidence points to low rates restricted to particular brain regions—most notably the hippocampus, a structure central to memory formation. Even sparse neurogenesis in adults could still be functionally meaningful, however, because newly generated neurons may retain heightened plasticity for long periods and might serve different roles depending on when they are born during the lifespan.
Another important observation is that older rodents also show reduced neurogenesis; when animals are compared at equivalent life stages, the apparent differences between species become less dramatic. In other words, striking discrepancies often reflect mismatched ages and developmental timing between animal models and human samples. Snyder suggests the field will benefit from more careful alignment of developmental stages when translating findings from animals to people.
Beyond resolving the existence question, Snyder urges the research community to shift emphasis toward understanding the functional relevance of adult-born neurons and exploring whether enhancing neurogenesis could have therapeutic benefits. He points out that phases in which adult neurogenesis was dismissed led to lost opportunities, since laboratories scaled back research and potential clinical applications were not pursued. Rather than treating the issue as a debate to be won, Snyder advocates collaboration, rigorous methods, and asking targeted questions about function and relevance for human health.
Funding: Research in the Snyder laboratory is supported by the Canadian Institutes of Health Research, the Natural Sciences and Engineering Research Council of Canada, and the Michael Smith Foundation for Health Research.
Source: Carly Britton, Cell Press
Publisher: Organized by NeuroscienceNews.com
Image source: NeuroscienceNews.com image is in the public domain
Original research: Jason S. Snyder, “Recalibrating the Relevance of Adult Neurogenesis,” Trends in Neurosciences, published January 24, 2019
DOI: 10.1016/j.tins.2018.12.001
Suggested citation: Cell Press. “Lessons Learned From the Adult Neurogenesis Debate.” NeuroscienceNews. January 25, 2019.
Abstract
Recalibrating the Relevance of Adult Neurogenesis
Conflicting reports about whether adult hippocampal neurogenesis occurs in humans raise questions about its significance for health and the translational relevance of animal models. Drawing upon published data, Snyder reviews species-specific neurogenesis rates across the lifespan and proposes that accelerated neurodevelopmental timing in primates is consistent with lower adult neurogenesis rates in primates and humans. Nonetheless, extended developmental windows may produce neuron populations that retain plastic properties for long intervals and could perform distinct functions depending on when they were generated. With conceptual recalibration and careful cross-species comparisons, the field can reconcile disparate findings and better address how animal studies of adult neurogenesis relate to human health.