These are the titles of a review in PNAS and another in Science of a PNAS paper by Bhardwaj et al. that unequivocally settles a hotly contested issue. It had been reported that a large number of neurons stream daily from proliferative layers near the cerebral ventricle to the overlaying neocortex in adult nonhuman primates, raising speculation that new neurons are continuously added to the adult human cerebral cortex. However, this finding could not be confirmed in either the primate or rodent cortex. Bhardwaj et al. took advantage of the integration of 14C, generated by nuclear bomb tests during the Cold War, in DNA to establish the age of neurons in the major areas of the human cerebral neocortex. Together with the analysis of the neocortex from patients who received BrdU, which integrates in the DNA of dividing cells, their results demonstrate that, whereas nonneuronal cells turn over, neurons in the human cerebral neocortex are not generated in adulthood at detectable levels but are generated before birth.
Thus new neurons are not born in the adult human brain, and changes required for memory, learning, and injury repair must involve alterations or growth of connections between existing nerve cells.
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