Extracellular sheets and tunnels modulate glutamate diffusion in hippocampal neuropil

• Published in: Journal of comparative neurology (1911) Vol. 521; no. 2; pp. 448 - 464
• Main Authors: Kinney, Justin P., Spacek, Josef, Bartol, Thomas M., Bajaj, Chandrajit L., Harris, Kristen M., Sejnowski, Terrence J.
• Format: Journal Article
• Language: English
• Published: Hoboken Wiley Subscription Services, Inc., A Wiley Company 01-02-2013; Wiley Subscription Services, Inc
• Subjects: Algorithms; Animals; Cell Membrane - ultrastructure; Diffusion; electron microscopy; extracellular space; Extracellular Space - physiology; Glutamic Acid - metabolism; Hippocampus - metabolism; Image Processing, Computer-Assisted; MCell; Microscopy, Electron; Monte Carlo Method; Nerve Net - physiology; Nerve Net - ultrastructure; Neuropil - metabolism; Neurotransmitter Agents - metabolism; Rats; reconstruction; shrinkage; Software; Studies; Subcellular Fractions - metabolism
• ISSN: 0021-9967; 1096-9861
• DOI: 10.1002/cne.23181

Although the extracellular space in the neuropil of the brain is an important channel for volume communication between cells and has other important functions, its morphology on the micron scale has not been analyzed quantitatively owing to experimental limitations. We used manual and computational techniques to reconstruct the 3D geometry of 180 μm3 of rat CA1 hippocampal neuropil from serial electron microscopy and corrected for tissue shrinkage to reflect the in vivo state. The reconstruction revealed an interconnected network of 40–80 nm diameter tunnels, formed at the junction of three or more cellular processes, spanned by sheets between pairs of cell surfaces with 10–40 nm width. The tunnels tended to occur around synapses and axons, and the sheets were enriched around astrocytes. Monte Carlo simulations of diffusion within the reconstructed neuropil demonstrate that the rate of diffusion of neurotransmitter and other small molecules was slower in sheets than in tunnels. Thus, the non‐uniformity found in the extracellular space may have specialized functions for signaling (sheets) and volume transmission (tunnels). J. Comp. Neurol. 521:448–464, 2013. © 2012 Wiley Periodicals, Inc.
We reconstructed the 3D geometry of 180 cubic microns of rat CA1 hippocampal neuropil from serial electron microscopy and corrected for tissue shrinkage. The reconstruction revealed an interconnected network of tunnels, formed at the junction of three or more cellular processes, spanned by sheets between pairs of cell surfaces. Tunnels tended to occur around synapses and axons and sheets were enriched around astrocytes. Simulations suggested that the rate of diffusion of neurotransmitter was slower in sheets than in tunnels.