Estimates of neurogenesis rate in human hippocampus and it's implication for memory formation?
I'd like to know some rates of neurogenesis, particularly in the human hippocampus. How many new neurons per day are generated, or how long does it take to add, say, 1% new neurons to the hippocampus?
I think the fact (assuming it is correct) that the hippocampus is the primary place in the brain where neurogenesis occurs (aside also from the olfactory bulb) is a very big clue towards a model of memory formation and consolidation in the brain.
Perhaps the hippocampus is like a notepad where new, temporary information is stored before being permanently copied to the neocortex. The only way to re-use the notepad is to generate a new clean sheet (neurogenesis) that can be written on.
A recent article in Neuron (vol 54, p559) by Hongjun Song at Johns Hopkins University School of Medicine in Baltimore, Maryland, and his colleagues states that they have found that "(hippocampal) cells aged between 1 and 1.5 months were easier to activate and displayed a greater degree of activation than younger or older cells ( Neuron, vol 54, p 559)." (quoted from From issue 2605 of New Scientist magazine, 24 May 2007, page 14-15).
Goncalves 02:46, 6 June 2007 (EDT)
The number of new neurons born into the dentate gyrus remains somewhat vague. The paradigms for delivering BrdU (the proliferation marker most often used for quantitation) vary considerably in the number of injections and concentration. Furthermore, proliferation levels are highly regulated by other factors, including strain and the animal's behavior.
Regardless, in rats the total number of cells born appears to be somewhere betweem 1,000 and 10,000 per day (total DG size ~ 1 million neurons).
Corresponding with their much smaller hippocampus, mouse-levels appear considerably lower than rats, with estimates between 500 and a 1000 per day (total DG size ~ 250,000 neurons).
Of these, only a fraction differentiate into neurons and survive (estimates range from 20% to 50%). This too is dependent on numerous factors, with enrichment approaching full survival in some cases (van Praag et al., Nature Neuroscience 1999).
Human neurogenesis levels are especially hard to measure. In the Eriksson et al., 1998 study, where BrdU was administered to terminal aged cancer patients, neurogenesis levels appear on the order of ~400/mm^3 soon after injection and ~100/mm^3 over a year after injection. These densities appear similar to those seen in young adult rats, though a direct comparison really cannot be made due to significant technical differences. Regardless, it appears that neurogenesis levels in adult humans is at levels near those seen in rodents.