Note: Single-source report; awaiting corroboration.
The brain contains specialized immune cells called microglia, which protect neurons from pathogens and other threats. While in mice these cells originate during embryonic development and are replenished internally, new research suggests that in humans, some microglia or their precursors may derive from bone marrow cells crossing the blood-brain barrier, especially during aging.
Researchers from Stanford University, funded by the NIH and including Drs. Julia Belk, Howard Chang, and Siddhartha Jaiswal, used genetic tracing methods to investigate the origins of microglia. They utilized clonal hematopoiesis, a process where bone marrow stem cells acquire mutations over time that are inherited by their blood and immune cell progeny, to identify cells born outside the brain.
Examining postmortem brain tissue from twenty older adults, the team found cells sharing mutations with blood cells, indicating a bone marrow origin. These marrow-derived cells resembled microglia and made up a significant portion of the microglia population in brain tissue from older individuals. Notably, brain tissue from older adults contained more marrow-derived microglia than that from younger individuals, suggesting migration and replacement of microglia as part of aging.
These findings indicate that bone marrow-derived immune cells may replenish some microglia populations in the aging human brain, which could have implications for delivering therapies for brain disorders.