In vivo particle tracking to parameterize simulations: insight into mechanisms of reduced glymphatic flow with age

The average human brain weighs only about 3 pounds, but accounts for approximately 20% of the human body’s metabolism. Despite this incredible metabolic rate, there are no lymphatic vessels in the brain, which is how the rest of the body removes waste. Rather, in vivo experiments performed in mice a decade ago demonstrated that cerebrospinal fluid (CSF) flows through the brain to remove metabolic wastes, such as amyloid-β, which form the plaque deposits in the brain that are a hallmark signature of neurological disorders like Alzheimer’s. Perivascular spaces (PVS) in the brain, which are annular channels surrounding blood vessels, provide the pathway for CSF to move through the brain and clear out these metabolic wastes. This pathway is known as the glymphatic system. Quantification of glymphatic flow has proven to be difficult due to the small length scales, need for adding a flow visualization tracer, challenges with gaining optical access to the skull’s interior, and the delicate nature of the brain’s environment. To visualize glymphatic flow in mice, we place a cranial window above the middle cerebral artery on the dorsal aspect of the skull and inject 1 µm fluorescent microspheres into the mouse’s CSF. We then perform particle tracking to quantify glymphatic flow velocities in the PVS of a live mouse. In this study, we report and compare preliminary measurements of PVS velocities of young and old mice (C57BL/6J). While it is known that CSF flow decreases with age, our measurements obtained from particle tracking enable parameterization and validation of simulations that provide insights into the specific mechanisms that reduce CSF flow with age.