THE BRAIN’S SENTINELS: Microglia (stained green in this rat brain culture) fight infection in the central nervous system. Neuronal processes stained in red.© GERRY SHAW/WIKIMEDIA COMMONS

P. Réu et al., “The lifespan and turnover of microglia in the human brain,” Cell Rep, 20:779-84, 2017.

Evidence has emerged that some of the brain’s cells can be renewed in adulthood, but it is difficult to study the turnover of cells in the human brain. When it comes to microglia, immune cells that ward off infection in the central nervous system, it’s been unclear how “the maintenance of their numbers is controlled and to what extent they are exchanged,” says stem cell researcher Jonas Frisén of the Karolinska Institute in Sweden.

Frisén and colleagues used brain tissue from autopsies, together with the known changes in concentrations of carbon-14 in the atmosphere over time, to estimate how frequently microglia are renewed. They also analyzed microglia from the donated brains of two patients who had received a labeled nucleoside as part of a cancer treatment trial in the 1990s.

Microglia, which populate the brain as blood cell progenitors during fetal development, were replaced at a median rate of 28 percent per year; on average, the cells were 4.2 years old. For Marie-Ève Tremblay, a neuroscientist at the Université Laval in Québec City who was not involved in the study, what stands out is the range of microglia ages found—from brand-new to more than 20 years old. “That’s quite striking!” she writes in an email to The Scientist.

Tremblay notes that this variation in age meshes with the heterogeneity of microglia types that has begun to emerge in other studies. “With electron microscopy, we find a variety of immune cells in the brain, especially in contexts of disease,” she writes.