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Ramya R Singireddy, Kenneth R Sloan, Jeff W. Lichtman, Christine A. Curcio, Dennis M. Dacey; Exploring Müller cell-cone interactions in human fovea using 3-dimensional volume electron microscopy (EM). Invest. Ophthalmol. Vis. Sci. 2018;59(9):1478.
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© ARVO (1962-2015); The Authors (2016-present)
Müller cells, the radial glial cells of the retina, play many roles in healthy and diseased eyes. Foveal Müller cells envelope cone pedicles (Burris et al., J. Comp. Neurol. 453:100-111, 2002). We wished to learn more about how they interact along the full length of individual foveal cones. We addressed this question by 3-dimensional electron microscopic (EM) reconstructions of Müller cells surrounding one cone in the center of a healthy human fovea.
From the eyes of a 21-year-old male organ donor, retinas were dissected and placed into oxygenated Ames medium for 2 hours and fixed in 4% glutaraldehyde. A tissue block 3.5 mm2 centered on the fovea was prepared for Automated Tape Ultramicrotomy (Kasthuri et al., Cell 162:648-661, 2015). From 1462 serial 65 nm sections in the horizontal plane, an area ~250 x 250 μm was imaged at 6 nm xy resolution. Images were stitched and aligned. TrackEM software on a pen display was used to trace, reconstruct, and display cone #5 (of 186) and its contacting Müller cells.
Cone 5 is ensheathed by two types of Müller cells, Outer and Inner (Dacey ARVO 2016). The Outer Cell is first seen at the external limiting membrane (ELM) between cones 5 and 17. Moving inward from the ELM, it tightly wraps around cone 5’s fiber in a C-shape profile for 78 µm. This Müller cell also intermittently projects to neighboring cones, two of which were close to cone 5 at the ELM. As cone 5’s axon approaches the pedicle, it contorts into a corkscrew. The Outer Cell fluidly molds to this changing shape. At this level, this Müller cell doubles in volume to encompass not only cone 5, but also cone 17 and another Müller cell. In the final 17 µm of the block the Müller cell’s volume quickly dissipates as it sends a small projection towards the internal limiting membrane, eventually encasing an OFF midget bipolar cell also associated with cone 5. In contrast to this Outer Cell, an Inner Müller Cell adjoining cone 5 spans only 19 µm, interacting directly with cone 5 and the Outer cell for 3.9 µm.
Neural-glial relationships in a human fovea are visible through 3-dimensional volume EM. The volume of Müller cells in the fovea was impressive, consistent with a pivotal role in the function and dysfunction of cones. It is possible that individual glia also ensheath the post-receptoral neurons in a cone-driven circuit.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
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