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M. L. Kisilak, K. Bunghardt, A. K. Ball, E. L. Irving, M. C. W. Campbell; In vivo Imaging of Photoreceptors in the Alert Chick During Development. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4010.
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In vivo measurements of photoreceptors in the chick eye, an animal model of myopia, are desirable given in vitro measurements in the literature, which suggest that differentiation of the oil droplets and possibly cones is not complete on day 0. In vivo images allow longitudinal measurements of the angular photoreceptor spacing in the growing chick eye. This data will then be compared to various models of eye growth. Methods developed could be extended to eyes with induced refractive errors.
Both eyes of 6 chicks (Ross Ross) were imaged in a confocal scanning laser ophthalmoscope in a 6 degree field of view (633nm light) on day 0 and day 14 and axial length measurements were taken using A-scan ultrasound. These measurements were taken close to the optical axis. Angular cone densities were measured directly. Using focal length to eye length ratios from published schematic eye models, linear cone spacings on the retina were calculated.
Cones were successfully imaged in all eyes of all birds for both days. Paired t-tests showed no significant difference in angular cone density between day 0 and day 14 for left or right eyes. Results for left and right eyes on each day were not significantly different. The calculated linear distance between cones for left eyes increased from 7.4 µm to 9.9 µm between day 0 and 14, in a proportion similar to that of the eye length increase. This cone spacing would imply that we are imaging more than one cone class, but potentially not all cone classes.
Previously we have shown in vivo imaging in alert chicks (different birds on day 2 and day 18). We have now imaged in vivo cones longitudinally in normally developing alert chicks (day 0 and day 14). The ability to image cones in the young chick eye indicates that the optical quality is better than the retinal sampling by the observed cones. A non significant change in the angular density of the cones imaged is consistent with a linear expansion of the retina at the same rate as the axial length change. This is consistent with uniform expansion of the chick eye during this period of growth. The change in cone density with growth differs from the previously reported small reduction in central ganglion cell density (Straznicky and Chehade, 1987) which is not consistent with retinal expansion.
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