Brain doesn't age suddenly; key changes occur after 50, US study reveals

Contrary to the notion of a sudden decline, a new study indicates that the human brain undergoes significant, coordinated changes rather than simply accumulating damage as it ages. Researchers from the University of California, San Diego, have identified critical shifts in the brain's cellular composition, particularly within the hippocampus, a region vital for learning and memory.

The study, published in the journal *Science*, analyzed single cells from human hippocampal tissue across different adult age groups. Using advanced single-cell analysis techniques, the team mapped the genetic states and structures of these cells to understand the aging process. They discovered that the composition of hippocampal cells begins to change notably between the ages of 50 and 75.

A key finding was the significant decrease in microglia, immune cells responsible for maintaining the brain's environment. Simultaneously, a new population of cells emerged, exhibiting genetic characteristics similar to microglia but also possessing features of circulating immune cells. These have been termed 'microglia-like cells.' This discovery challenges the long-held belief that microglia, formed before birth, persist throughout life in the brain.

Researchers noted that microglia-like cells show stronger inflammatory characteristics, suggesting they may contribute to chronic neuroinflammation associated with brain aging. The decline in microglia's ability to perform their essential cleaning and maintenance functions could lead to the accumulation of toxic substances, triggering inflammatory responses that may promote neurodegenerative diseases. Furthermore, the study observed a reduction in cell populations responsible for maintaining the blood-brain barrier, a crucial protective shield for brain tissue.

These findings suggest that brain aging is not merely a passive process of damage accumulation. Instead, the immune, vascular, and nervous systems appear to undergo coordinated changes throughout life. The research opens avenues for identifying new therapeutic targets to maintain brain function and neural circuit integrity across the lifespan.