A technique used to date artifacts is settling a debate about neurogenesis. Bhardwaj et al. show that few (if any) new neurons are added to the human neocortex after development by using a carbon-14 (1C)-dating technique, as reported in a recent article in Proceedings of the National Academy of Sciences.
During the Cold War, tests of nuclear weapons nearly doubled the atmospheric levels of 1C from 1955 until 1963, when the Test Ban Treaty was signed. Since then, 1 C levels have declined exponentially as the oceans have equilibrated 1 C in the atmosphere. 1 C incorporates into the DNA of dividing cells, marking their birth dates.
The authors acquired brains from autopsy patients who were born in Sweden between 1933 and 1973. They isolated nuclei from the neocortex and sorted them into NeuN-positive neuronal and NeuN-negative non-neuronal populations. Then they extracted DNA from the nuclei and used accelerator mass spectrometry to measure the amount of 1 C in each population. The concentration of 1 C in the cell matched the atmospheric concentration of 1 C in the year that it stopped dividing.
The authors first examined brains from individuals born after 1963. NeuN-negative cortical cells had less 1 C than the amount present in the atmosphere at the person's time of birth. On average, these cells were 4.9 years younger than the individual, suggesting that most non-neuronal cells are generated in childhood. The concentration of 1 C in NeuN-positive cortical cells corresponded to the birth date of the individual, suggesting that new neurons are not generated in the human neocortex after development.
Is it possible that new neocortical neurons exist in such small numbers that they would not be detected? According to the authors, their technique would detect neurogenesis in less than one percent of all cortical neurons in people born before 1955 because the resolution in 1 C levels is so great during and just after the Cold War nuclear tests. NeuN-positive neocortical cells were the same age as people born before 1955, consistent with a lack of new neocortical neurons after development.
The 1 C assay would not detect rapid turnover of neurons. Therefore, the authors examined the brains of people with cancer who died 4.2 months to 4.3 years after they were injected with the proliferating-cell marker bromodeoxyuridine (BrdU) for diagnostic purposes. Unlike neurons in the granular layer of the dentate gyrus that are known to undergo neurogenesis in the adult, none of the 515 BrdU-labeled cells in the neocortex had nuclear morphology or colocalized with NeuN, as shown by confocal microscopy. Therefore, the authors concluded that continual renewal of neurons in the neocortex is unlikely and would require a cellular lifespan of less than 4.2 months.
Together, these data are consistent with a growing body of research showing that new neurons are not added to the mammalian neocortex after development. This 1 C-dating technique offers a unique opportunity to study neurogenesis in discrete regions and populations of the human brain and its potential alteration by disease or injury.