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Do humans have functionally important neurogenesis throughout their life?

For roughly twenty years, since the work of Rusty Gage's group at UCSD circa 1998, neuroscientists have believed that a small amount of functionally significant neurogenesis (NG) occurs in both mammal (mice) and adult primate brains (monkeys). Adult NG was then found in a region called the hippocampus (HC) (and its subregion, the dentate gyrus, or DG). The HC is involved in short-term memory formation, and links to both our emotional centers of our brain (the amygdala) and our cerebral cortex, where our long term memories are stored.

This finding was later found for human brains by various studies, and it contradicted the previous longstanding "dogma" that adult brains don't form new neurons. The current leading theory of why NG occurs in the adult HC (if it does) is that it isn't some kind of regulatory failure (cancer, etc.) but that plays some functional role, perhaps in short-term memory storage.

In some neuroscience models, we are thought to store massive amounts of info in our HC over the last day or two of our lives, in synaptic connections, and we are also thought to flush this store out regularly, with only a subset of those memories being "written to the cortex" for long-term storage, usually while we dream and sleep. Adult NG is presumed by some to help this somehow, or play some other functional role.

But a March 2018 Nature paper by Sorrells and Paredes at UCSF recently found sharply decining NG after the age of 1 year in human brains, and no NG in humans after the age of 13 yrs.

The Sorrells paper used a more stringent set of surface markers to search for new neurons than previous papers, and it argues previous studies weren't sufficiently rigorous in their neural classification approaches. It has a lot of neuroscientists confused again, as it comes from a respected group using some very careful work, and it concludes that adult humans do not do functionally important neurogenesis over their lifetimes.

Then in April 2018 a careful stereology-based study by Boldrini at Columbia, also using postmortem hippocampi, contradicted the Nature paper. Boldrini's paper again argues the 20 year old view that adult human hippocampi continually does NG. They found about 1,000 neural progenitor cells in each of the front, middle, and back regions of the DG at any time, throughout the human lifespan. This is plenty enough, in some models, to be functionally important to human thinking and memory.

So which is it?

Either:

  1. Adult human NG exists and is functionally important to us throughout our lifespan (birth to death), or

  2. NG doesn't exist in significant numbers in older humans, or if it does occur it isn't functionally important.

Assuming we find out by 2028, which will it be? Resolution is positive for option 1.

Resolves positive if a definitive study or set of studies best accords with option 1, negative if it best accords with option 2. We'll define "definitive" as at least one study published in a top-tier journal (top 10 in the field by impact factor) with strong evidence for 1 or 2, along with the absence of a competitively compelling publication giving evidence for the other possibility, as of Jan 1 2028. Resolves ambiguous if not definitive.

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