What, if anything, is the function of adult neurogenesis in humans? Does neurogenesis even exist in our adult brains, or does it shut down during childhood?
The debate over human neurogenesis has been one of the most prominent disputes in 21st century neuroscience. Just last year, two opposing papers appeared in leading journals, one claiming firm evidence of ongoing neurogenesis in the adult human dentate gyrus, while the other study came to the opposite conclusion. The fact that adult neurogenesis is reliably seen in rodents only adds to the confusion. If rats and mice have it, and we don’t, what does that mean?
Now, in a new article in Trends in Neurosciences, neurogenesis researcher Jason S. Snyder attempts to make sense of the mess.
Snyder argues that there is no mystery about humans vs. rodent patterns of neurogenesis – the only real difference is in the timing:
This graph, a summary of published data, shows that dentate gyrus neurogenesis follows the same timeline in all species studied: it initially peaks, and then declines to very low levels. The difference is that in humans, the peak occurs before birth, whereas in rodents, neurogenesis peaks much later (relative to their lifespan), at birth or shortly afterwards.
In other words, Snyder says, it’s no surprise that adult neurogenesis has been found to be low in human adults. Neurogenesis is low throughout most of the lifespan of all species.
The timing of DG neurogenesis is therefore consistent with the broader comparative pattern of neurodevelopment, where humans and nonhuman primates are born with a mature nervous system (at least in terms of cell production) compared to that in rodents [64,65].
Still, even if neurogenesis only occurs at a very low rate in human adults, Snyder points out that it might still be functionally important:
Spalding et al. estimated that only 0.004% of neurons are added each day in adult humans [10]. While this would appear negligible under the microscope (1 cell in 25,000), it translates to ∼15% over a decade; a sizable fraction…
Even if we believe that neurogenesis absolutely ceases during human adolescence, the importance of neurogenesis might linger on, as there is evidence that the youngest neurons (the ones ‘born’ during childhood) retain distinct properties that make them especially important for learning and plasticity in adulthood:
Newborn dentate gyrus neurons might be expected to have enhanced synaptic plasticity for over a year in primates and even longer in humans. Similarly, if the enhanced capacity for morphological plasticity, which lasts at least 4 months in rats [100], is scaled according to human lifespan (30×), dentate gyrus neurons in humans would be expected to retain this heightened plasticity for at least a decade.
