Biologists at Harvard have discovered unusual channels connecting the bone marrow in the thickness of the skull to the brain tissue, which may explain how immune cells enter the brain during inflammation. Their findings were presented in the journal Nature Neuroscience.
“We plan to study how these channels affect the development of Alzheimer's disease, strokes, hypertension and other ailments associated with inflammatory processes. In addition, they can be used to directly deliver drugs to the brain,”notes Mathias Nahrendorf of Harvard University (USA).
Impregnable fortress
Our brain is isolated from the rest of the body by the so-called blood-brain barrier (BBB) - a dense layer of special astrocyte cells that surround all blood vessels of the brain and do not let anything inside it except oxygen and nutrients.
As the scientists explain, this barrier is not a monolithic wall, but a set of peculiar "brick" cells, between which there are small ducts, the so-called tight contacts. Recently, biologists have discovered that these contacts can expand under certain conditions and help the brain clear itself of debris.
Narendorf and his colleagues accidentally discovered another possible "back door" for infections or immune cells to enter the human brain, studying how the immune system of mice responded to various forms of strokes caused by surgery or chemistry.
His team, as the biologist notes, was interested in where the little bodies come from, the first to react to vascular damage and cerebral hemorrhage. To answer this question, they injected into different parts of the bone marrow, where neutrophils are formed, immune cells of "quick response", a special paint that made them glow in different colors, and followed their migrations.
Promotional video:
Much to Narendorf's surprise, it was not immune bodies that formed in the tibia, the main "factory" of such cells, but their "cousins" that arose inside the bones of the skull that penetrated the damaged vessels of the brain of mice first.
"Service entrance" of the brain
Something like this was previously considered impossible, since biologists believed that cells that arise inside the skull have no way to seep through the BBB and get into the brain. Trying to understand how this happened, the authors of the article studied in detail the structure of the border between the skull and its insides.
“We looked at the bones of the skull from all possible angles, trying to understand how neutrophils could enter the brain. Surprisingly, we found many microscopic canals directly connecting the bone marrow to the brain,”Narendorf continues.
Further observations of these channels showed that cells move through them only when an infection appears in the brain or its vessels are damaged during strokes. In other cases, neutrophils do not enter the brain, but into the vessels that feed the bones of the skull, and spread throughout the rest of the body.
Similar structures, which are approximately five times larger, are also present in the human skull. They, as scientists suggest, can pass not only neutrophils, but also other types of cells that have yet to be discovered and studied.
Observing them, Harvard biologists hope, will help us understand how various inflammations occur in the brain and how they may be associated with neurodegenerative and autoimmune diseases.
