A network of channels within the long bones of mice has been observed for the first time, reports a paper published online this week in Nature Metabolism. These tiny blood capillaries pass through the bone itself, connecting the bone marrow to the wider circulatory system.
Blood has been assumed to enter long bones (such as shinbones) through arteries at the bones’ ends or through the few blood vessels along the bones’ shafts, before passing through the marrow and leaving at the other end. However, this path does not explain how emergency drug infusions into the bone marrow rapidly spread to the rest of the body.
Matthias Gunzer and colleagues imaged mouse shinbones and found hundreds of tiny capillaries crossing the hard outer shell of the bone. These channels, named ‘trans-cortical vessels’ (TCVs), appear to be a central component of the circulatory system in mouse bones. The authors calculated that most of the blood passing through bones flows through TCVs. In addition, the authors found similar, if thicker, canals in human long bones, although whether they are also TCVs remains to be confirmed.
The authors also directly observed immune cells migrating through TCVs, which may provide a shortcut between the bone marrow and inflammation sites. In a mouse model of chronic arthritic bone inflammation, they found that new TCVs develop within weeks. If direct links are established between TCVs and inflammatory diseases in future studies, these findings may help develop new treatments for bone inflammation and tissue injuries by regulating blood flow or cell migration through TCVs.