Characterizing Shear-Induced Migration of Particles in the Murine Glymphatic System

Cerebrospinal fluid (CSF) flow is important for the brain because it aids in metabolic waste clearance. The brain accounts for only 2% of the body's total mass yet consumes ~25% of its total energy each day. Cellular debris generated from brain activity has been shown to be cleared by flow of CSF through annular pathways surrounding the cerebral vasculature. The clearance of this metabolic waste is critical for chronically maintaining healthy brain physiology to help mitigate development of neurodegenerative diseases. To quantify CSF flow speeds, some experimentalists inject fluorescent microspheres into the CSF of mice, then image their transport to quantify CSF flow. Through further characterization of particle flows in the PVS, we seek to better understand how glymphatic experiments might be affected by choice of microspheres. Our goal is to answer two questions: (1) Do smaller particles flow more deeply into the brain? and (2) Do larger particles exhibit shear induced migration? We injected 0.2 µm, 1 µm, and 4 µm fluorescent microspheres into the CSF of mice and imaged the resulting flow at high spatial and temporal resolution using two-photon microscopy. CSF flow speeds and trajectories were quantified using custom particle tracking velocimetry codes. Insights from our experiments provide guidance for future experiments that seek to probe glymphatic transport in the brain to understand – and hopefully prevent – neurodegenerative diseases.