Korva is a scroll of the internet’s best ideas.
Brain immune cells fade after 50
What the tissue showed
Researchers studied postmortem hippocampal tissue from 40 neurologically healthy adults between the ages of 20 and 95, one cell at a time. The hippocampus plays a central role in learning and memory.
Microglia first develop during embryonic growth, and scientists have long believed they stay in the brain and keep renewing themselves for a person's whole life. In the tissue, their numbers fall from about 50 to 75, and the cells that appear to replace them carry traits similar to immune cells that originate in peripheral blood. That shift may help explain the long-lasting brain inflammation seen in neurodegenerative diseases.
Tracing where cells came from
Gene activity only shows what a cell is doing today, so the team added maps of the genome's 3D structure and of its chemical modifications, known as the epigenome, which preserve where a cell came from. Those maps exposed a change in the identity and lineage of the immune cells that gene activity data alone would not have revealed.
Other cells change too
Other cells showed age-related changes too. Cells that help maintain the blood-brain barrier, the protective boundary controlling what passes from the bloodstream into the brain, showed signs of decline. Across many kinds of brain cells, the genome's physical organization changed in widespread, coordinated ways, and those structural disruptions were closely linked to shifts in gene regulation and cell identity.
Next, the researchers will look into why resident microglia are lost with age and whether the newly identified immune cell transition directly contributes to Alzheimer's disease.
Why is the hippocampus so central to memory?
The hippocampus is a curved structure deep in each temporal lobe that helps turn new experiences into lasting memories. The best-known evidence comes from a patient called H.M., whose hippocampi were surgically removed in 1953 to treat severe epilepsy. Afterward he could recall his earlier life but could not form new memories of events. Older memories appear to be stored across the cortex, which is why they survived the surgery.
How can scientists tell where a cell came from?
Chemical tags on DNA and on the proteins that package it, together called the epigenome, are laid down as a cell develops and are copied when it divides. A cell that began as a brain-resident microglia and one that began in the bone marrow carry different patterns of these tags. Researchers read them with methods that detect which stretches of DNA are open or methylated. Because the tags persist after gene activity changes, they work like a record of a cell's lineage.
How do microglia protect the brain day to day?
Microglia constantly extend and retract thin branches to sample their surroundings for debris, dying cells, and signs of infection. When they find something, they engulf it and release signaling molecules that call in other defenses. During development they also prune excess connections between neurons, which shapes how circuits wire up. The Spanish neuroscientist Pío del Río-Hortega first described them as a distinct cell type in the 1910s and 1920s.
What makes the blood-brain barrier so selective?
The barrier is built from endothelial cells lining the brain's capillaries, joined by tight junctions that leave almost no gaps between them. Astrocytes and pericytes wrap around these vessels and help keep the junctions sealed. Small fat-soluble molecules such as oxygen and carbon dioxide slip through, while most larger molecules and many pathogens are blocked. Nutrients like glucose cross through dedicated transporter proteins in the vessel wall.
Why does brain inflammation matter in Alzheimer's disease?
Alzheimer's disease is marked by clumps of amyloid-beta protein outside neurons and tangles of tau protein inside them. Microglia gather around amyloid plaques in the brain, and their inflammatory signals are thought to influence how the disease progresses. Large genetic studies have found that many gene variants linked to higher Alzheimer's risk are active mainly in microglia. That finding put immune cells at the center of research that once focused almost entirely on neurons.