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A. M. Querubin, B. J. O'Brien, K. M. Bumsted O'Brien; Morphological Classification of Retinal Ganglion Cell Types in the Pigeon Retina. Invest. Ophthalmol. Vis. Sci. 2010;51(13):890.
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The pigeon has two areas of high acuity vision, the fovea and the area dorsalis. There is some evidence of a midget-like retinal ganglion cell (RGC) in these regions; however, a large scale morphological classification of pigeon RGC types using modern techniques is not yet available. The aim of this study was to characterize RGC types to determine the array of RGCs and the prevalence of midget-like ganglion cells in all areas of the pigeon retina (the yellow field, the red field containing the area dorsalis, and the fovea).
RGCs (n = 376) were labeled with DiI or DiO in pigeon retinal wholemount preparations (n=20) using a Di-olistics approach with coated tungsten beads (PDS-1000/He System). Labeled RGCs were imaged using a confocal microscope. Soma size, dendritic field size and branching pattern, inner plexiform layer (IPL) stratification and eccentricity were measured and then RGCs were classified based on these parameters.
Our data demonstrate that the pigeon retina contains 17 identifiable types of RGC. The dendritic field diameter varied from the smallest (14 µm or 7 min of arc) found in the area dorsalis in the red field, to the largest (394 µm or ~3 deg) found in the yellow field. Seven RGC types were similar to previously identified mammalian RGC types with horizontally oriented dendrites. Four pigeon RGC types were completely different from any reported mammalian RGC types in that the cells projected only vertically oriented dendrites. Stratification varied from clearly monostratified (6 RGC types) to quadruple stratified (1 RGC type). Within these classes, stratification occurred at different IPL depths. Some RGC types with mainly horizontally oriented dendrites also had vertically protruding dendrites lacking further horizontal branching. RGC dendrites varied from very "loosely" organized to very densely packed and were classified as "radiate" or "recursive". Near the fovea, four RGC types were identified that had a small dendritic field diameter (35 µm or 17.5 min of arc).
Of the 17 different types of pigeon RGCs, the smallest dendritic field diameter observed in our sample (14 µm or 7 min of arc in the area dorsalis) was approximately twice the size required to resolve 12 cpd (the behaviorally measured acuity for this region, Rounsley & McFadden, 2005). None of the RGC types corresponded exactly to the primate midget type, though one pigeon RGC type in the area dorsalis shared similarities with the Golgi-impregnated midget-like RGC previously reported by Lockhart (1979).
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