Stunning discovery links the brain and the immune system
— June 4, 2015
Most of us don’t give our lymphatic systems much thought. In fact many folks have their lymphatic headquarters, the spleen, removed altogether with little immediate ill effects. Yet without our lymphatic system, our circulatory system isn’t complete. Although most of our blood cells stay within the confines of our vasculature, around 20 liters per day of liquid plasma pass out of our capillaries and into the surrounding tissue. The lymphatic collection system returns it.
Whenever this system is shown on anatomy websites, it seems the artists have invariably forgotten to draw in a lymphatic channel to drain the brain. That’s because nobody has ever found one. Researchers from the University of Virginia have just reported in Nature that the books need to be changed. In fact, the brain has had one all along. That’s fantastic news, because in recent years researchers have learned a lot about the critical role the lymphatic system plays in immune function and disease.
Since the lymphatic system also provides immune cells a way to network, its direct connections to the brain will undoubtedly have immediate implications for many neurological diseases that have an immune component, such as Alzheimer’s and multiple sclerosis. Immune cells have been regularly spotted in the brain. But until now, their presence had to be accounted for by assuming some structural breach secondary to infection or other damage.
The researchers explored the various fluid compartments of the brain using two main methods: Antibody labeling for molecular markers expressed by different kinds of cells in membranous linings, and injecting dyes into the fluid compartments to see where it ends up. They found that the cerebral lymphatic passages followed the major sinuses that collected and drained blood from the brain.
Understanding the larger hydraulic operation of the brain is crucial to knowing how to repair it when things go wrong. It will also be essential for ensuring its continued smooth operation when healthy folks begin to add internal hardware to augment its natural operation. We wrote yesterday about a so-called ‘Gauss gun‘ which magnetically delivers and guides devices through the brain. Such a device would have use in relieving the swelling in hydrocephalus, or potentially opening and stitching together new channels for drugs or immune cells to gain access.
It’s been known for a few years now that flux of fluid and nutrient across the brain results in significant volume changes in various cerebral structures each day. Collectively known as the “glymphatic system,” these changes would likely stress the tissue in the vicinity of any fixed components (like the electrode arrays now used in brain-computer-interfaces) and eventually compromise the integrity of their bond. Furthermore, any small and free-floating devices, like the intriguing ultrasonic smart dust devices now under development, would be at the whim of the larger ebb and flow of neural hydraulic influence.
Knowing where such devices tend to accumulate over time will be important in determining not only where to extract them after their useful lifecycle, but also where to introduce them. The natural circulation of the brain’s CSF through its many foramen (holes) is fairly complicated in its own right. The direction of its flow changes over time according to various conditions or positions of the body. Adding new knowledge about additional pressure heads or sinks that bear on the overall flow, like this new lymphatic accessory system, will reduce the uncertainties we now face in many areas when making alterations to the brain to restore or improve its function.
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