But guess what? They do grow back—by the thousands. Not until scientists became handy with advanced imaging tools that enabled them to peer into the brain did they find conclusive evidence, which was published in a seminal 1998 paper. It came from an unlikely source. Cancer patients are sometimes injected with a dye that shows up in proliferating cells so that the spread of the disease can be tracked. Researchers looked at the brains of terminally ill patients who had donated their bodies to science and found that their hippocampi were packed with the dye marker, proof that neurons were
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