Atomic bomb tests provide evidence of adult brain neuron growth

Science : In 1998 researchers showed, contrary to expectations, that some neurons in adult human brains did divide to produce new cells. The test was never duplicated because one of the substances used was found to be toxic. A new study, led by Kirsty Spalding of the Karolinska Institute in Stockholm, Sweden, has provided the first support for the original study, using an unusual source. The testing of nuclear weapons in the 1950s and 60s doubled the levels of radioactive carbon-14 in the atmosphere. Because the levels stopped increasing after the 1963 Limited Test Ban Treaty, ¹⁴C has steadily decayed to ¹²C. The ratio of those isotopes in cells can serve as a time stamp for the cells’ ages. Spalding and her colleagues spent 10 years developing techniques to isolate specific neural tissues and to then extract and identify the very small amounts of carbon isotopes present. They found that more than one third of the hippocampal neurons were regularly replaced during adulthood, with roughly 1400 new neurons added daily. Whether that growth is actually significant in how the brain functions is far from clear, but the question of whether neuron growth occurs at all appears to be settled.