Muller glial cells in anuran retina

Whereas in the brain, the activity of the neurons is supported by several t ypes of glial cells such as astrocytes, oligodendrocytes, and ependymal cel ls, the retina (evolving from the brain during ontogenesis) contains only o ne type of macroglial cell, the Muller (radial glial) cells, in most verteb rates including the anurans. These cells span the entire thickness of the t issue, and thereby contact and ensheath virtually every type of neuronal ce ll body and process. This intimate topographical relationship is reflected by a multitude of functional interactions between retinal neurons and Mulle r glial cells. Muller cells are the principal stores of retinal glycogen, a nd are thought to fuel retinal neurons with substrate (lactate/pyruvate) fo r their oxidative metabolism. Furthermore, Muller cells are involved in the control and homeostasis of many constituents of the extracellular space, s uch as potassium and perhaps other ions, signaling molecules, and of the ex tracellular pH. They also seem to play important roles in recycling mechani sms of photopigment molecules and neurotransmitter molecules such as glutam ate and GABA. By containing the main retinal stores of glutathione, Muller cells may protect retinal neurons against free radicals. Moreover, Muller c ells express receptors for many neuroactive substances, and may also releas e such substances to their neighbouring neurons. Thus, Muller cells exert m any functions crucial for signal processing in the normal retina. Moreover, Muller cells change their properties in cases of retinal disease and injur y, and may either support the survival of neuronal cells or accelerate the progress of neuronal degeneration. (C) 2000 Wiley-Liss, Inc.