gms | German Medical Science

The glial cell types of vertebrate retinas are microglial cells, Müller glial cells and in vascular retinas additionally astrocytes. Microglial cells are the immune competent cells in retinal tissue, have surveillance function for neurons and are activated by all pathological changes. The Müller glial cell is the dominating macroglia cell type and plays a pivotal role in the metabolic interactions of neurons and glial cells. Each single Müller cell extends from the outer to the inner surface of the retina and forms intimate contacts with blood vessels, with somata of all types of retinal neurons, as well as with synapses in the two plexiform layers. That makes the Müller cells suitable to fulfill a lot of physiological tasks. For instance, Müller cells are involved in spatial buffering of potassium ions, in water transport, in transmitter recycling such as glutamate-glutamine-cycle and in the defense against oxidative stress, respectively. Ion and water channels such as inwardly rectifying potassium channels (Kir4.1) and aquaporin 4 water channels are expressed by Müller cells. Glutamate released from active neurons is taken up by the glial specific glutamate transporter EAAT1, is converted into glutamine by the glial specific enzyme glutamine synthetase and subsequently, glutamine is released by Müller cells as a recycling component for the neuronal transmitter re-synthesis. Additionally, large amounts of glutathione are synthesized by Müller cells and will be transferred to retinal neurons after experimental application of hydrogen peroxides. Beyond that, recent findings showed that Müller cells have even light guidance capabilities. Therefore, the physiological activities of Müller glial cells are essential for normal retinal function and pathophysiological disturbances of these functions might have deleterious consequences for neurons.