Ganglion cell density and oil droplet distribution in the retina of brown-eared bulbul (Hysipetes amaurotis)

Abstract

This study was intended to determine the number and density of both retinal ganglion cells and the oil droplets of cone photoreceptor cells in brown-eared bulbul (Hypsipetes amaurotis). For this study birds were killed with proper dose of anesthetic (pentobarbital, 30 mg/kg), and the eyes were removed from the orbital cavity to isolate the retina. For the ganglion cell study retinal whole-mount specimens were prepared and stained with 0.1% cresyl violet. The different types of oil droplets were counted from color microphotographs of freshly prepared retinal samples. The mean total number of ganglion cells was estimated at approximately 2.5 x 10(6); with an average density of 16 523 cells/mm(2). Two high-density areas, namely the central area (CA) and the dorso-temporal area (DTA), are located in the central and dorso-temporal retinas, respectively, in bulbuls (24 032 cells/mm(2) in the CA; 23 113 cells/mm(2) in the DTA). 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, orange, green and clear - were identified with an average density of 29 062/mm(2). Among the different colors, the green oil droplets had a significantly higher population (13 083/mm(2)) than the others across the retina. The central retina had a significantly higher number of all types of oil droplets, at a density of 60 552/mm(2). The density and size of the different colored oil droplets were inversely related across the regions of the retina. Taken together, it is concluded that the CA of the retina is an excellent quality area for visual perception due to peak density of ganglion cells and oil droplets. Moreover, each specific oil droplet makes a distinct contribution to visual perception, thereby ensuring that the bird has a retina that best matches its natural environment and feeding behavior.