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Regional specialization of the Tree Sparrow Passer montanus retina: ganglion cell density and oil droplet distribution

ABSTRACT The purpose of this study was to define the number and density of both retinal ganglion cells and the oil droplets of cone photoreceptor cells in the Tree Sparrow Passer montanus. Retinal whole-mount specimens were prepared and stained with 0.1% cresyl violet for the ganglion cell study, and fresh retinal samples were subjected to color microphotography for the oil droplet study. The mean total number of ganglion cells was estimated at approximately 1.6×5106; with an average density of 18,539/mm2. The density of the ganglion cells peaked in the area centralis, at 26,000 cells/mm2, and gradually decreased with retinal eccentricity. Small ganglion cells persisted in the highest density areas, whereas the largest soma sizes were found in the lowest density areas of the retina. Four types of different colored oil droplets—red, yellow, green and clear—were identified with an average density of 25,680/mm2. Among the different colors, the green oil droplets showed a significantly higher density (12,247/mm2). The central retina contained a significantly higher number of oil droplets, with a density of 50,731/mm2, similar to the specialization of this area for ganglion cells. The density and size of the different colored oil droplets were inversely related across the regions of the retina, also similar to the pattern shown by the ganglion cells. Thus, the characteristic specializations of ganglion cells, especially peak density of central retina may contain the retinal fovea and the highest population of green oil droplets may be important to adjust the suitable light wavelength.