The microglia in healthy and diseased retina

•Microglial cells are the immune cells of the retina and originate from the yolk sack.•They monitor the state of the retina and help to keep homoeostasis.•Retinal microglia becomes activated in case of an injury or disease.•Activated microglia phagocytoses injured neurons and promotes inflammation.•Inhibition of microglia may reduce inflammation and helps injured neurons to survive.

The microglia are the immune cells of the central nervous system and, also the retina. They fulfil several tasks of surveillance in the healthy retina. In case of an injury or disease, microglia become activated and tries to repair the damage. However, in a lot of cases it does not work, and microglia deteriorate the situation by releasing toxic and pro-inflammatory compounds. Moreover, they further promote degenerative processes by attacking and phagocytosing damaged neurones and photoreceptors that otherwise would possibly have the chance to survive. Such deleterious action of the microglia has been observed in degeneration of retinal ganglion cells and photoreceptors, and it takes place in hereditary diseases, infections as well as in case of traumatic or light injuries. Therefore, a number of attempts has been undertaken so far to inhibit the microglia, with varying success. The task remains to study behaviour of the microglia and their interaction with other retinal cell populations in more detail with respect to released factors and expressed receptors including the time points of the corresponding events. The goal has to be to find a better balance between helpful and detrimental actions of the microglia.