Neuroscientists usesd to think that immune system cells were carried in the bloodstream from, say, big leg bones in order to address brain inflammation following a stroke, injury, or disorder. It turns out that the brain gets neutrophils from much closer to home, right under the scalp. That’s what one group discovered when looking for the cells in the tibia (the larger of the shinbones in the leg) in mice:

They found that the skull contributed significantly more neutrophils to the brain in the event of stroke and meningitis than the tibia. But that raised a new question – how were the neutrophils being delivered?

“We started examining the skull very carefully, looking at it from all angles, trying to figure out how neutrophils are getting to the brain,” said Matthias Nahrendorf of Harvard Medical School and Massachusetts General Hospital in Boston.

“Unexpectedly, we discovered tiny channels that connected the marrow directly with the outer lining of the brain.”

Michelle Starr, “There Are ‘Secret’ Tunnels Connecting Your Skull And The Brain” at ScienceAlert (March 17, 2022)

Pieces of skull from neurosurgery enabled the team to locate human tunnels as well, though the human tunnels are five times as big as the mouse ones:

Since the original discovery of these tiny tunnels, researchers have studied them more closely in mice, confirming in 2021 that the connection they form to bone marrow means the blood cells taking the trip aren’t derived from the bloodstream, but are indeed produced directly from nearby marrow, making them highly localized and specific.

Michelle Starr, “There Are ‘Secret’ Tunnels Connecting Your Skull And The Brain” at ScienceAlert (March 17, 2022)

It seems that our brains are specially protected by “highly localized and specific” systems, not just general ones. The researchers hope that this knowledge could help fight autoimmune disorders such as multiple sclerosis.

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