MULLER (GLIAL) CELLS IN THE TELEOST RETINA - CONSEQUENCES OF CONTINUOUS GROWTH

In the continuously enlarging eye of teleost fishes retinal growth is achieved by the generation of new cells and by stretching the existing tissue. As a consequence of stretching, the density for most neurons decreases except for rod photoreceptors, which are produced by stem ce lls in the outer nuclear layer (ONL). We investigated retinal Muller g lial cells with immunocytochemical markers against vimentin and glutam ine synthetase in animals of various sizes. In addition, we used Weste rn immunoblot analysis to investigate the changes in the glia-specific enzyme glutamine synthetase in the enlarging retina. We found that in the cichlid fish Haplochromis burtoni the density of Muller cells dec reases from about 14 cells/mm(2) to 4 cells/mm(2) with increasing body size. Since it is known that the density of rod photoreceptors remain s constant, it follows that the neuron to Muller cell ratio increases in the growing eye. In our estimates, this ratio ranges from around 54 :1 in small fish to more than 67:1 in larger animals. Quantified Weste rn blots revealed that the amount of glutamine synthetase per retinal area does not change in the growing eye, which means that the amount o f this enzyme in each Muller cell must increase. Staining isolated cel ls and retinal sections from small and large fish with an antibody aga inst glutamine synthetase showed stronger immunoreactivity in larger a nimals, especially in the areas of the photoreceptor cell bodies and o uter limiting membrane and a more extensive branching of Muller cell p rocesses. Thus, Muller cells in growing fish appear to compensate for the increasing metabolic challenge and are able to maintain their func tion. (C) 1998 Wiley-Liss, Inc.