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L. L. Vidal, K. S. M. Vidal, M. L. V. Marceliano, L. R. G. Britto, B. L. S. Andrade-da-Costa, E. S. Yamada, J. P. Coimbra; Topography of Displaced Retinal Ganglion Cells in the Silver-Beaked Tanager Ramphocelus Carbo (Aves: Emberezidae). Invest. Ophthalmol. Vis. Sci. 2007;48(13):1222.
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Displaced ganglion cells in the retinas of birds are thought to play an important role in the control of the oculomotor reflexes of the eye and head. However, no information is available about the organization of these cells in passerines. Therefore, we aim to characterize the topographic distribution of displaced ganglion cells in the retina of a fruit-eating passerine bird Ramphocelus carbo using neurofilament immunohistochemistry.
We used retinas of adult specimens of R. carbo fixed by immersion in 4% paraformaldehyde in 0.1M phosphate buffer for two hours. Retinas were incubated with a pan-neurofilament antibody, a specific marker for the three neurofilament subunits, for 72 h. Neurofilaments are known to be specific retinal ganglion cell markers. Subsequently, retinas were incubated in a biotinylated secondary antibody and finally processed with avidin-biotin peroxidase complex. Displaced ganglion cells were counted using a systematic sampling method to map the distribution along the entire retina at 1 mm intervals. Densities were plotted on the map and isodensity contours were interpolated.
Neurofilament immunohistochemistry revealed the presence of displaced ganglion cells in the retina of R. carbo. The topographic maps showed two regions of high displaced ganglion cell densities in the retina of R. carbo. Displaced ganglion cells reached highest densities of 150 cells/mm2 around the foveola and a peak of 100 cells/ mm2 in the temporal area. Both retinal specializations were surrounded by an inconspicuous horizontal visual streak.
In birds, displaced ganglion cells are the main input to the nucleus of the basal optic root (nBOR) the cells of which present large receptive fields and are motion sensitive. Considering that the displaced ganglion cells of R. carbo exhibit a similar projection pattern, the topographic arrangement of these cells could afford the detection of moving stimuli across the visual fields. This would be important for the detection of approaching predators, but not for the detection of food because this species rely heavily on fruit.
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