Never-before-seen microscopic tunnels connect the bone marrow in the skull to the lining of the brain called the meninges, researchers reported Monday (August 27) in Nature Neuroscience. The finding could offer insights to how the brain responds to injury and disease.
“We always thought that immune cells from our arms and legs traveled via blood to damaged brain tissue. These findings suggest that immune cells may instead be taking a shortcut to rapidly arrive at areas of inflammation,” Francesca Bosetti, program director at the US National Institute of Neurological Disorders and Stroke, which funded the study, says in a statement. “Inflammation plays a critical role in many brain disorders and it is possible that the newly described channels may be important in a number of conditions.”
Matthias Nahrendorf, a molecular biologist at Harvard Medical School and Massachusetts General Hospital in Boston (MGH), and his colleagues discovered the channels while trying to determine where immune cells come from after a stroke or heart attack. In mice having a stroke, immune cells called neutrophils are more likely to come from the skull, they found. But in mice that have had a heart attack, a similar number of neutrophils come from the skull and the tibia.
In the case of stroke, advanced imaging showed the neutrophils leaving the skull and traveling through the previously undiscovered tunnels to the damaged tissue. An analysis of human skull samples after surgery revealed the channels are not unique to mice.
“While we’ve still got a lot to learn about these channels, I think their very special role as conduits for inflammatory cross-talk between the marrow and the central nervous system is quite different from any other vasculature,” Nahrendorf says in an MGH statement. “In addition to these channels carrying immune cells from the skull marrow to the brain, we think inflammatory substances that derive from the brain may alert the skull marrow to an injury faster than marrow from the rest of the body.”
Because inflammation is part of many brain diseases, “it would be great to learn how the channels contribute to those diseases and whether modulating their contributions could change outcomes,” Nahrendorf says. “Another idea is that the channels could serve as a route of drug delivery, allowing transport to the meninges of drugs delivered into the skull marrow.”