A paravascular pathway facilitates CSF flow through the brain parenchyma and the clearance of interstitial solutes, including amyloid β

The founding paper of the glymphatic system — the brain's paravascular CSF-and-interstitial-fluid circulation that clears metabolic waste from the brain parenchyma during sleep. Lead author Jeffrey Iliff was at the University of Rochester Medical Center at the time; senior author Maiken Nedergaard is the Danish-American neuroscientist who directs the Center for Translational Neuromedicine at Rochester and a parallel programme at the University of Copenhagen. The paper is the first systematic characterisation of a brain-wide fluid-clearance mechanism that the body's lymphatic system handles in peripheral tissues but that the brain — long thought to lack a lymphatic system — was mysteriously without.

Published in Science Translational Medicine in August 2012, the paper is a primary empirical report combining two-photon imaging of fluorescent tracer flow in mice with aquaporin-4 knockout experiments. The finding: CSF enters the brain parenchyma along paravascular spaces surrounding penetrating arteries, mixes with brain interstitial fluid, and clears along paravenous drainage pathways. Astrocytes expressing the water channel aquaporin-4 are the couplers of this bulk flow; transgenic mice lacking aquaporin-4 showed a 70% reduction in clearance of large solutes including amyloid-β.

The piece sits as adjacent rather than directly central to the body-tissue interstitium literature: the glymphatic system is the brain's own variant of interstitial fluid circulation, distinct anatomically and physiologically from the Benias-described peripheral interstitium, but conceptually a sibling. The Nedergaard programme has continued through the 2010s and 2020s with substantial follow-on work on sleep-driven glymphatic clearance (Xie et al. 2013 in Science showed glymphatic flow increases 60% during sleep), on dysfunction in Alzheimer's, and on the clinical implications for neurodegeneration. The work is the brain-side analogue to what Wiig and Swartz survey in the periphery.

The stake is scientific and clinical. The glymphatic hypothesis has been contested — some authors argue the pressure gradients required for the bulk-flow model do not exist, others propose diffusion-based clearance instead — and the field is still working out the details. The clinical implications (sleep, ageing, neurodegeneration) have driven substantial NIH funding for the Nedergaard group and others. Read this paper for the founding empirical claim; the post-2012 debate is alive and the strong version of the bulk-flow model remains under empirical scrutiny.