Researchers have found a new type of cell that removes waste from around the brain.
The researchers - from the University of Queensland (UQ) in Australia - report the discovery of the new type of lymphatic cell in the journal Nature Neuroscience.
They found the cell in freshwater zebrafish, which provide a useful model for studying human biology because many of their cells and organs are similar to ours.
Another advantage of using zebrafish is that they are transparent, and the researchers were able to use advanced light microscopes to see what was happening in their brains.
Senior author Ben Hogan, an associate professor in the Institute for Molecular Bioscience at UQ, says, "It is rare to discover a cell type in the brain that we didn't know about previously, and particularly a cell type that we didn't expect to be there."
The lymphatic system has three main functions: it is part of the immune system; it returns fluid from tissues to the blood; and it absorbs fat-soluble nutrients and fats from the digestive tract and carries them to the bloodstream.
Zebrafish version of human 'mato' cells
The lymphatic system comprises: a fluid called lymph that is similar to blood plasma; vessels that carry lymph; and organs - such as the lymph nodes, tonsils, thymus, and spleen.
- Unlike blood vessels, lymphatic vessels only carry fluid away from tissues.
- The lymphatic system produces white blood cells in response to infection.
- It helps to keep blood volume and blood pressure normal and prevents edema.
It is similar to the cardiovascular system in that it carries fluid through a network of vessels that permeates nearly every type of tissue in the body.
However, whereas the cardiovascular system has a pump - that is, the heart - to move blood through the vessels, the lymphatic system relies on contraction of smooth muscle in its vessel walls, together with movement of skeletal muscle and breathing to push the lymph along.
For a long time, it was thought that the brain did not have any lymphatic vessels, but a recent landmark discovery showed that it does and that the reason it took so long to find them is because they are "very well hidden."
Now, in the new study, the UQ researchers make another surprising discovery in finding that the brain may also contain isolated lymphatic cells that help to clean up waste that leaks from the bloodstream.
Prof. Hogan says that "the cells appear to be [the] zebrafish version of cells described in humans called 'mato' or lipid-laden cells."
He adds that in humans, mato cells "clear fats and lipids from the system but were not known to be lymphatic in nature."
The new lymphatic cells exist independently of vessels
When excess fats and other waste leaks from the bloodstream into surrounding tissue, the lymphatic system clears it away to prevent it damaging organs, Prof. Hogan explains.
Using the advanced light microscopes, the researchers were able to closely examine the zebrafish brain, see the cells, and observe in detail how they develop and work.
Prof. Hogan says that normally, lymphatic cells develop into lymphatic vessels that carry the lymph fluid.
Consequently, they were surprised to find that "in the adult zebrafish brain these cells exist individually, independent of vessels and collect waste that enter the brain from the bloodstream." He concludes:
"Our focus now is to investigate how these cells function in humans and see if we can control them with existing drugs to promote brain health, and improve our understanding of neurological diseases such as stroke and dementia."
The following video from UQ summarizes the study's findings.