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Andréas Reichenbach"Roles of Müller glial cells in retinal functioning"

Abstract :

Müller cells are the principal glial cells of the retina.
Similar as the astrocytes in the brain, they constitute an anatomical and functional link between retinal neurons and blood vessels, and support the neurons by providing neurotrophic factors and blood-derived nutrients and by removing metabolic waste. Müller cells are responsible for the maintenance of the homeostasis of the retinal extracellular milieu (ions, water, neuro¬transmitter molecules, and pH). They are involved in the control of angiogenesis, and in retinal blood flow regulation. In addition to these "housekeeping" functions (which might be considered as important but boring) recent data suggest that Müller cells may also more directly contribute to information processing in the retina. This starts at the very beginning - i. e., during image transport to the photoreceptor cells.
One should keep in mind that the vertebrate retina is "inverted" - which means that the photoreceptor cells are directed away from light entering the eye, such that the optical image has to pass the entire thickness of the retina before it can be detected. As the retinal tissue contains a wealth of light-scattering elements (such as the synapses in the two plexiform layers) this situation would be equivalent to placing a thin diffusing screen directly over the film in your camera... if not the Müller cell scaffold would act as a fiber optic plate, consisting of many parallel light-guiding fibers, to transport the image across the thickness of the tissue. For this purpose, the Müller cell stem processes display a higher refractive index than the surrounding neurons, and their endfeet have the properties of ideal light collectors.
Another recent finding is that Müller cells can "sense" neuronal activity in the retina, elicited by physiological light stimulation. Müller cells respond to such activity by (at least) two different types of intracellular Ca2+ rises, depending on the intensity and duration of retinal illumination. This neuro-to-glial signal transmission is then "translated" into glio-to-neuronal signaling - demonstrable, for instance, as a modulation of the light-driven activity of ganglion cells.
Thus, Müller cells might be involved in control mechanisms of retinal information processing, such as adaptation to a wide variety of light intensities. This rises the picture of Müller cells as fascinating partners of the retinal neurons, rather than as their housekeeping servants (which has been the conventional view).

Selected publications

Iandiev I, Uckermann O, Pannicke T, Wurm A, Tenckhoff S, Pietsch UC, Reichenbach A, Wiedemann P, Bringmann A, Uhlmann S.
Glial cell reactivity in a porcine model of retinal detachment.
Invest Ophthalmol Vis Sci. 2006 May;47(5):2161-71.
Pannicke T, Iandiev I, Wurm A, Uckermann O, vom Hagen F, Reichenbach A, Wiedemann P, Hammes HP, Bringmann A.
Diabetes alters osmotic swelling characteristics and membrane conductance of glial cells in rat retina.
Diabetes. 2006 Mar;55(3):633-9.
Iandiev I, Biedermann B, Reichenbach A, Wiedemann P, Bringmann A.
Expression of aquaporin-9 immunoreactivity by catecholaminergic amacrine cells in the rat retina.
Neurosci Lett. 2006 May 8;398(3):264-7.

Stéphane Oliet