Researchers at Columbia University's Zuckerman Institute found that microglia, immune cells that play an important role in protecting and developing the brain, take several years to mature in humans.
The findings suggest that the unusually slow development of these cells could be one of the factors behind the complexity and cognitive abilities of the human brain.
Microglia are the brain's most abundant immune cells. Besides protecting the brain from harmful substances and helping remove damaged cells, they also play an important role in shaping developing brain circuits.
Researchers found that human microglia can take approximately four to eight years to mature. By comparison, microglia in mice reach maturity in about three weeks.
The study focused on SRGAP2, a gene with copies that emerged specifically in humans.
Earlier research had shown that these human-specific copies influence neurons by increasing the number of connections between them while slowing the process through which those connections mature.
In the latest study, scientists made an unexpected discovery. The human-specific copies of SRGAP2 were found to be nearly 10 times more abundant in microglia than in neurons.
Experiments involving mice and human cells showed that the gene significantly slows the development of human microglia.
Scientists believe this slow developmental process may allow microglia and neurons to develop in coordination, helping shape the sophisticated networks found in the human brain.
The researchers linked the finding to a process known as neoteny, in which human development occurs over a much longer period compared with many other mammals.
The extended development of the human brain is believed to contribute to the emergence of advanced cognitive abilities.
The researchers now want to understand more precisely how SRGAP2 controls the development of microglia and other brain cells.
They also hope the findings could provide new insight into conditions involving brain development and degeneration, since microglia have been linked to neurodevelopmental disorders and neurodegenerative diseases.
The study, published in the journal Neuron, provides new evidence that the distinctive development of the human brain may depend not only on neurons but also on the immune cells that help organize and maintain brain circuits.
