Ultrastructure of Mugil brasiliensis Teleost Retina I: Cones, Rods, Horizontal, Bipolar, Piriform Amacrines, Tubular Cells, Undulate Amacrine Cells, Outer and Inner Plexiform Layers

Aims: To analyze the ultrastructure of cones, rods, horizontal, bipolar and amacrines cells from Mugil brasiliensis teleost retina as well as all the connections found between the cells.

Methodology: The retina was fixed with glutaraldehyde and osmium tetroxide and radial and tangential sections stained with lead citrate and uranyl acetate to be analyzed in a Hitachi 11B electron microscope at 75 Kv.

Results: Horizontal cells had finger like process reaching the outer plexiform layer ending in cone and rod invaginations, and also lateral unions between plasma membranes, forming a net, with a functional contact between them. Bipolar cells were radially oriented with dendrites branching in the outer plexiform layer contacting cones and rods and lower terminals branching in the inner plexiform layer. Stellate amacrine cells send ascendant process to the outer plexiform layer and form a continuous net below horizontal cells. A new layer named tubular cells was described by the first time below internal horizontal cells forming a net. Undulate amacrine cells around bipolar cells were also described by the first time. Cones and rods terminals had ascendant expansions of horizontal and bipolar cells in a triad structure around the synaptic band. Piriform amacrine cells were described with dark cytoplasm due to numerous glycogen granules and with clear cytoplasm with scarce glycogen granules, both sending prolongations to the inner plexiform layer.

Conclusion: This paper is a comprehensive analysis of ultrastructure of seven retinal cells, finding two new cells in teleost retina, 1- tubular cells forming an extensive tangential net below internal horizontal cells, with characteristic structures not seen in the literature on retina before and, 2- undulate amacrine cells that surrounds many bipolar cells across the retina. Functional contacts structured by bilaminar membranes were found between digital terminals of horizontal cells and bipolar cells in rods and cones, between bipolar dendrites, tubular cells, stellate amacrine cells, piriform amacrine cells, as well as between lateral expansions of horizontal cells.