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Marsha L. Kisilak, Kaitlin Bunghardt, Melanie C. Campbell, Elizabeth L. Irving; Changes in Angular Cone Density in Chick Eyes During Normal Growth are Biphasic. Invest. Ophthalmol. Vis. Sci. 2012;53(14):3466.
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© ARVO (1962-2015); The Authors (2016-present)
In vivo measurements of cones in the chick eye, an animal model of myopia are desirable. In vivo images allow longitudinal measurements of the angular photoreceptor spacing in the growing chick eye and an investigation of models of eye growth. Furthermore, in vitro measurements in the literature suggest that differentiation of the oil droplets and possibly cones are not complete on day 0. Methods developed could be extended to eyes with induced refractive errors.
The left eye of 12 Ross Ross chicks (gallus gallus domesticus) were imaged in a confocal scanning laser ophthalmoscope (CSLO, 633nm, 1.67 mm entrance pupil) and an adaptive optics corrected SLO (760nm, 2.5 mm entrance pupil) on days 0, 2, 4, 7, 11, 14, 18 and 21. Axial length measurements were taken using A-scan ultrasound. All measurements were taken close to the optical axis and the anatomical position of the area centralis. Angular cone densities were measured directly. Linear cone spacings on the retina were calculated from published schematic eye models modified for measured eye lengths and cone packing properties were assessed.
Cones were successfully imaged on all days. The angular density of cones remained constant for the first 7 days of growth, after which it significantly linearly increased. The calculated linear distance between cones increased from 6.7 microns on day 0 to 7.7 microns on day 7, after which it remained relatively constant. On average the cones showed a 36% hexagonally packed array across all days. Comparisons of angular densities between the systems indicate that higher order aberrations do not limit the number of cones imaged but at older ages, in the one setup, the pupil size did. The best system resolution for the larger pupil is close to the in vitro minimum cone separations (2 microns) suggesting that some cones may not be resolved.
Average linear cone spacings are within 10% of some literature values. Some double-green cone nearest neighbours may not have been resolved. Cones imaged near the area centralis between day 0 and day 7 show no significant change in angular density, followed by a significant increase between day 7 and day 21. This indicates a biphasic model of growth within the chick eye. Initial uniform expansion followed by either cone migration or optic pole elongation without expansion is one possible explanation. Future longitudinal measurements of the cones and their packing properties will provide insight into photoreceptor density changes with experimentally induced refractive error.
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