There is a secret passage between your skull and your brain

Image source: Herrison et al.
, Nature Neuroscience, 2018 Written | Translated by Michelle Starr | Reviewed by Xiong Yin | Shi Yunlei Did you know there are tiny passages in your head? In 2018, a medical team showed that in mouse and human brains, tiny channels connect the bone marrow of the skull to the meninges
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The study shows that when the brain is damaged, this channel can provide a direct pathway for immune cells to quickly enter the brain from the bone marrow
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Previously, scientists had assumed that immune cells would only reach the brain from the rest of the body via the bloodstream in response to brain inflammation triggered by stroke, injury or brain disease
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The finding suggests that these immune cells have always had a shortcut
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The researchers discovered these tiny channels when they set out to investigate whether immune cells from them were transported to the brain after a stroke or meningitis
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In the study, they tracked a specific type of immune cell called neutrophils, also known as the "sentinels" of the immune team
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When something goes wrong in the body, they are also the first cells to reach the site of inflammation, helping to clear the factors that cause it
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The team developed a technique that allows cells to be tracked by labeling them with membrane fluorescent dyes
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They treated the cells with dye and injected them into the bone marrow of mice
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They injected red fluorescently labeled cells into the skulls of mice and green fluorescently labeled cells into the tibias of mice
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 When cells survived in these areas, the researchers induced several types of acute inflammation in mice, including stroke and chemically induced meningoencephalitis
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They found that when mice developed stroke and meningitis, their skulls contributed significantly more neutrophils than their tibias
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But it also raises a new question: How are these neutrophils transported? "We started looking at the skull very carefully, looking at it from every angle, trying to figure out how the neutrophils got in," said Matthias Nahrendorf of Harvard Medical School and Massachusetts General Hospital in Boston.
Brain
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Unexpectedly, we found tiny channels connecting the bone marrow to the dura, the protective membrane that wraps the brain
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 Image credit: Herrison et al.
, Nature Neuroscience, 2018 The team imaged the inner surface of mouse skulls using organ-bath microscopy, which contains a small container filled with a solution that maintains The integrity of the isolated tissue is tested
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Here, they discovered tiny blood vessel channels that connect the bone marrow and dura mater of the skull
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Normally, red blood cells travel from inside the skull to the bone marrow through these channels
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But in a stroke, these channels can be used to back-transport neutrophils from the bone marrow to the brain
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But this was the case in the mouse brain, and to see if the same phenomenon occurs in the human brain, the researchers analyzed slices of human skull bone and imaged it in detail
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They noticed the presence of channels in the inner and outer layers of the human skull bone slices, which were five times the diameter of the mouse skull channels
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After discovering these tiny channels for the first time, the researchers conducted further studies in mice and confirmed in 2021 that the connection of these tiny channels to the bone marrow means that these blood cells are not derived from blood, but are directly produced by neighboring produced by the bone marrow, so they are highly concentrated and specific
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This is an astonishing finding, because inflammation in many brain diseases could help scientists learn more about the pathology involved
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It could advance understanding of diseases such as multiple sclerosis, caused by the immune system attacking the brain
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A paper corresponding to the study was published in the journal Nature Neuroscience
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