Fig. 1.

Determination of the age of neocortical neurons. (A) Neuronal (NeuN-positive) and nonneuronal (NeuN-negative) cell nuclei from the adult human cerebral necortex were separated and isolated by flow cytometry. (B) The levels of ¹⁴C in the atmosphere have been stable over long time periods, with the exception of a large addition of ¹⁴C in 1955–1963 as a result of nuclear weapons tests (blue line, data from ref. 26), making it possible to infer the time of birth of cell populations by relating the level of ¹⁴C in DNA to that in the atmosphere (horizontal arrows) and reading the age off the x axis (vertical arrows). The average age of all cells in the prefrontal cortex is younger than the individual (black arrows), indicating cell turnover. Dating of nonneuronal cells demonstrates they are younger, whereas neurons are approximately as old as the individual. The vertical bar indicates the year of birth of the individual. ¹⁴C levels from modern samples are, by convention, given in relation to a universal standard and corrected for radioactive decay, giving the Δ¹⁴C value (50).