Muller glia and phagocytosis of cell debris in retinal tissue

Muller cells are the predominant glial cell type in the retina of vertebrates. They play a wide variety of roles in both the developing and the mature retina that have been widely reported in the literature. However, less attention has been paid to their role in phagocytosis of cell debris under physiological, pathological or experimental conditions. Muller glia have been shown to phagocytose apoptotic cell bodies originated during development of the visual system. They also engulf foreign molecules that are injected into the eye, cone outer segments and injured photoreceptors. Phagocytosis of photoreceptor cell debris in the light-damaged teleost retina is primarily carried out by Muller cells. Once the microglial cells become activated and migrate to the photoreceptor cell layer, the phagocytic activity of Muller cells progressively decreases, suggesting a possible mechanism of communication between Muller cells and neighbouring microglia and photoreceptors. Additionally, it has been shown that phagocytic Muller cells acquire proliferating activity in the damaged teleost retina, suggesting that engulfment of apoptotic photoreceptor debris might stimulate the Muller glia to proliferate during the regenerative response. These findings highlight Muller glia phagocytosis as an underlying mechanism contributing to degeneration and regeneration under pathological conditions.