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The newly-discovered ‘trans-cortical vessels’ connect the bone marrow with the periosteal circulation, according to new research published in the journal Nature Metabolism.
Closed circulatory systems underlie the function of vertebrate organs, but in long bones their structure is unclear although they constitute the exit route for bone marrow leukocytes. To understand neutrophil migration from bone marrow, Grüneboom et al studied the vascular system of murine long bones. In a mouse model, the team shows that hundreds of capillaries originate in bone marrow, traverse cortical bone perpendicularly along the shaft and connect to the periosteal circulation. Structures similar to these trans-cortical vessels also exist in human limb bones. Image credit: Grüneboom et al, doi: 10.1038/s42255-018-0016-5.
Bones are very hard organs. Still they do possess a tight meshwork of blood vessels in their inner cavity, where the bone marrow is located, as well as on the bone surface, that is covered by the highly vascularized periosteum. This is why bone fractures can bleed extensively.
The same blood vessel system is also essential to transport blood- and immune cells from their place of origin, the bone marrow, to the outside.
“Like any other organ also bones need a closed circulatory loop (CCL) to function properly,” said Dr. Anika Grüneboom, from the University Hospital in Erlangen, Germany.
“This delivers fresh blood via arteries into the bone and transports used blood out via veins.”
“How exactly the CCL of long bones functions was not totally clear until now.”
In the long bones of mice, Dr. Grüneboom and colleagues observed and characterized ‘trans cortical vessels’ (TCVs) — a new type of capillary that perpendicularly to the long axis crosses the entire hard bone, which is termed corticalis.
The vessels could either be of arterial or venous origin. Surprisingly, the researchers could demonstrate, that the overwhelming majority of arterial and venous blood in long bones flows through the TCV system.
“Previous concepts described just a handful of arterial inlets and two venous exits on long bones of mice,” said Professor Matthias Gunzer, from the Institute for Experimental Immunology and Imaging at the University Hospital of the University Duisburg-Essen, Germany.
“This is obviously too oversimplified and does not correctly reflect the true natural situation.”
“It is really unexpected being able to find a new and central anatomical structure that has not been described in any textbook in the 21st century.”
Using state-of-the-art imaging techniques, the researchers were able to demonstrate that structures similar to TCVs also exist in human long bones.
“Future work shall now investigate which role is played by TCVs in normal bone physiology as well as under disease conditions such as osteoporosis, rheumatoid arthritis or tumors, that metastasize into bones,” they said.
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Anika Grüneboom et al. A network of trans-cortical capillaries as mainstay for blood circulation in long bones. Nature Metabolism, published online January 21, 2019; doi: 10.1038/s42255-018-0016-5
