The brain, textbooks tell us, is the only organ without a lymphatic system to help clear waste products from the rest of the body. However, the discovery of lymphatic cells in the brains of zebrafish may force a rewrite, revealing how much we still don't know about the insides of our own heads. If humans possess something similar, we may be able to supercharge it to protect against neurodegenerative conditions like Alzheimer's Disease.
Zebrafish make great study animals because they are transparent, allowing advanced microscopes to observe under their skin while they are still alive. In studying these useful fish two teams almost simultaneously discovered they possess a type of brain cell we were not previously familiar with, which both suspect also exists in humans. The cells appear to form part of the blood-brain barrier, scavenging wastes that leak out of the bloodstream before they can damage the brain.
The existence of scavenger cells responsible for removing damaging molecules has been known previously, and many medical researchers blame neurodegenerative diseases on their failure to keep up with the build-up of damage. However, these have been found surrounding the brain, rather than growing around the blood vessels within it.
“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,” said the University of Queensland's Dr Ben Hogan in a statement. Hogan is senior author of a paper in Nature Neuroscience reporting one team's findings.
The cells may be equivalent to human mato cells, which are known to play a role in clearing proteins and lipids such as cholesterol from the brain, but have not currently been identified as being lymphatic in origin. Should the connection turn out to be correct, it could be very important for fighting diseases. “We already have lots of ways to manipulate lymphatic cells, as a result of cancer research,” Hogan told IFLScience. “So we might be able to re-purpose these to increase [the cells] effectiveness.”
A possible explanation for the failure to notice these cells before, assuming they exist in humans, or at least to identify their nature, is that lymphatic cells in other organs form tubes, which are used to extract waste products from where they could be damaging. The newly identified zebrafish cells don't do this, instead existing as single isolated cells. Nevertheless, they appear to fulfill a similar role, metabolizing harmful molecules.
The other team who found themselves working in parallel with Hogan published their study in eLife. Having learned at conferences how similar their findings were, the two teams agreed to publish their papers at the same time.