The lymphatic system is a network of vessels that helps regulate fluid homeostasis, waste clearance, and immune response. Most tissues in the body have some type of lymphatic drainage. The main holdouts have been the brain, the eyes, and our bones.
Ten years ago, the brain lymphatic system (called glymphatics since they are associated with neuroglial cells) was discovered. This system is critically important to brain health. During deep sleep, our neurons shrink in size, allowing CSF to flush through the glymphatic system. This sweeps the accumulated debris (including tau and beta-amyloid) out through the glymphatics to be disposed of.
In 2014, Schlemm’s canals in the eye were also identified as functioning as a lymphatic system. These collect the aqueous humor absorbed by the trabecular meshwork on the surface of the iris.
And now, the Institute of Molecular Medicine at Oxford has discovered a lymphatic network in our bones! As anybody who has played with a microscope can attest, it’s easy and fascinating to view soft tissues. But working with calcified tissue is very challenging. For this reason, the organization of blood vessels in bone has been difficult to observe.
The authors used light sheet fluorescence microscopy to image intact bone specimens. This technique shines a sheet of laser light through a labeled specimen. A fluorescence detector perpendicular to the sheet of light records light output from the tagged items of interest in one two-dimensional layer. This technique can actually be used in living specimens, although in this study the bones were prepped and the calcium was carefully removed.
The authors identified lymphatics in mouse and human bone specimens. They also found that these lymphatics expanded in response to stress, which resulted in formation of more lymphatics. This, in turn, induced regeneration of the bone itself and hemopoietic cells in the bone marrow. However, as the animals aged, their lymphangiogenesis lessened, which may explain why bones in the elderly do not heal as well or as quickly.
Bottom line: This is an exciting discovery using a novel imaging technique. It showed not only the structure of these lymphatics but also their role in healing from injury. It raises the interesting possibility that manipulating the lymphatic endothelial cells might allow us to accelerate healing after injury.
First, we had lymphatics. Then, when they were found in the brain, we called them “glymphatics.” So now I will take the prerogative to name the ones discovered in bone as “blymphatics.” Not very sexy, but you get the idea!
Reference: Lymphatic vessels in bone support regeneration after injury. Cell. 2023 Jan 19;186(2):382-397.e24. doi: 10.1016/j.cell.2022.12.031. PMID: 36669473.