The aging brain may not decline gradually — it may switch to a new biological phase around midlife. A landmark single-cell study of the human hippocampus reveals that between roughly ages 50 and 75, the brain's cellular landscape undergoes coordinated remodeling: immune cells are replaced, the genome's physical organization weakens, and gene regulation is reshaped across multiple cell types.

Researchers analyzed individual cells from human hippocampal tissue spanning the adult lifespan, mapping gene regulation and three-dimensional genome architecture in unprecedented detail. The hippocampus is a brain region essential for learning and memory, and advancing age is the strongest known risk factor for Alzheimer's disease and other dementias.

The clearest transition appeared in microglia, the brain's resident immune cells. Between approximately ages 50 and 75, microglia formed during embryonic development declined sharply, replaced by cells whose molecular profiles resemble immune cells circulating in the blood. This challenges the long-held assumption that microglia established before birth stay in the brain for life. The replacement cells also carried stronger inflammatory signatures, potentially contributing to the persistent neuroinflammation associated with brain aging.

The analysis also found a substantial decline in cell populations that maintain the blood-brain barrier, the protective boundary that keeps harmful substances in the bloodstream from entering brain tissue.

Across several brain cell types, the genome's three-dimensional structure became progressively less organized. DNA is not stored as a loose strand; it folds into a carefully arranged architecture that determines which genes a cell can use. The widespread erosion of that organization suggests declining genome architecture is a basic feature of brain aging.

"Importantly, this study reveals that aging is not simply a gradual decline, but involves coordinated and dynamic remodeling of immune, vascular, and neuronal systems," said Xiangmin Xu of UC Irvine, a co-corresponding author. "These findings open the door to identifying new therapeutic targets aimed at preserving circuit integrity and brain function across the lifespan."

The research, led by scientists including Bing Ren of Columbia University and the New York Genome Center, is one of six studies published in Science through the National Institutes of Health's 4D Nucleome program — a decade-long initiative mapping how the genome's architecture changes across space and time.

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