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K. W. Headington, S. Choi, D. Nickla, N. Doble; Single Cell, in-vivo Imaging of Chick Retina With Adaptive Optics. Invest. Ophthalmol. Vis. Sci. 2010;51(13):2321.
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© ARVO (1962-2015); The Authors (2016-present)
Previous studies have shown disagreement in photoreceptor density at different retinal locations in the chick eye. These results are based on in vitro measurements on excised retina, with possible artifacts caused by tissue drying and/or shrinkage during histological preparation. Through the use of adaptive optics (AO) technology, it is now possible to image and directly count cells in living eyes. Our study implemented high resolution AO retinal imaging to determine if there are regional variations in cell density in the chick eye.
The current New England College of Optometry AO fundus camera was modified to allow high resolution imaging of the chick retina. The AO system consisted of a Hartmann-Shack wavefront sensor and a 37 element Iris AO Inc MEMS deformable mirror. A wavelength of 550 ± 20 nm was used for imaging over a 2.5o field of view. Relay optics were added to de-magnify the 6.2mm pupil used for human imaging to a 2mm pupil for the chick. The bird was anesthetized and mounted in a prone position on a goniometer stage to allow for tip / tilt adjustment. Retinal locations were determined with direct ophthalmoscopy and fundus photographyTwo 6 week old chicks were used in this experiment. The pecten served as the retinal landmark for retinal imaging over a range of 45o. Multiple, adjacent images were taken at one retinal location and subsequently montaged together to form a 10o image. This was repeated at three further locations spanning horizontally from 25o nasal of the pecten tip to 20o temporal. Automated software was used to determine the cell density at each location. Images were scaled using axial lengths determined from high-frequency A-scan ultrasound measurements.
Upon closing the AO control loop after a trial lens correction, the rms. wavefront error was reduced from 0.40 ± 0.05 to 0.06 ± 0.01 microns over the 2mm pupil. Analysis of the images gave a center-to-center cell spacing of 5 microns which was constant throughout the image. Photoreceptor density measurements were found to be uniform over the 4 imaged areas with an average density of 43,821 ± 767 [cells/mm2].
The initial results of this study indicate that there is no change in cell density with retinal eccentricity in the chick eye. In-vivo retinal studies of the chick photoreceptor density are now possible with AO imaging, which has important implications for studies of myopia development as the chick is a frequently used animal model.
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