Comparative immunochemistry of elasmobranch retina muller cells and horizontal cells

The neural retinas of all vertebrates have certain common cellular and structural features. I have used immunochemical methods to investigate the biochemical constancy of specific types of retina cells. Elasmobranchs such as the skate, ray, and shark exhibit retinal characteristics which are shared by all vertebrates as well as characteristics which seem to be limited to the more primitive retinas of fish. The Muller glial cells of all vertebrates possess high concentrations of the enzyme glutamine synthetase. Antibodies directed against chick glutamine synthetase specifically label Muller cells in all elasmobranch retinas examined thus far. Antibodies to carbonic anhydrase also label Muller cells but to varying degrees of intensity depending on species and, surprisingly, the location within the retina (i.e., peripheral vs. central). Antibodies to carbonic anhydrase also label the horizontal neurons of elasmobranch retina. In some examples, carbonic anhydrase labelling of horizontal cells is intense enough to allow visualization of the overall pattern of horizontal cell networking in wholemount preparations. A monoclonal antibody to carbonic anhydrase shows horizontal cell densities and distribution which may be predictive of the retinal areas of highest visual acuity. Horizontal cell compartmentalization of carbonic anhydrase is not seen in terrestrial vertebrates such as birds and mammals suggesting a unique and perhaps primitive character for these cells in the elasmobranch retina. Other immunochemical markers have also been explored such as specific carbohydrate moieties found on mammalian neural adhesion molecules. Again elasmobranch horizontal cells show unique labelling patterns with such antibodies as HNK-1. Results show the potential utility of immunochemical markers developed for terrestrial vertebrate retina systems in the analyses of the cell biology of elasmobranch retina. Furthermore, these findings underscore certain fundamental characteristics of the vertebrate retina which have remained constant for vast evolutionary periods.