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N.V. Bilton, S.A. McFadden; A Schematic Eye for the Normal Developing Chick . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3319.
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Schematic eyes have been developed for the 30–day–old chick1 and for chicks between 0 and 14 days of age2. However, in both cases, the data was from fellow eyes of chicks unilaterally treated with occluders or lenses, and the induction of myopia or hyperopia in one eye causes the interorbital septum to move medially to adjust for an increase or decrease in axial length respectively. We sought to develop a schematic eye for the normal developing chick using both longitudinal and cross sectional methods and extended the measurements previously taken to also include the changes in the posterior layers of the eye.
White leghorn hatchlings (Ross 308 strain, Bartter–Steggles, NSW) were raised until 15 days of age. Room illumination was 58 cd/m2 on a 12:12 h light:dark cycle. In the longitudinal study, axial ocular parameters (A Scan ultrasound) were measured daily (n=5) and in another group, refractive error (streak retinoscopy), corneal curvature (infrared video keratometry) and ocular parameters were measured every three days (n=15). Chicks were anaesthetised with halothane for all measures.
The Ross 308 chick emmetropises rapidly. Whether measured repeatedly or cross–sectionally, the greatest shift in refractive error occurred between age 0 to 3 days (–1.93 D, p < 0.01). After day 3, the mean refractive error was +1.27D. The axial length, anterior segment, and lens thickness increased steadily throughout the first 15 days of life. However, corneal curvature and vitreous chamber depth did not start changing until after the chicks were 3 days old. Additionally, the posterior tunics increased in thickness over the first 3 days, but remained constant thereafter (retina, 289; choroid, 210; sclera, 146; µm). Based on our model eyes, the predicted focal length of the whole eye increased 0.0793 mm per day and the small eye artifact decreased from 2.56 to 1.54 D as age increased.
At birth, our chicks were 3 diopters less hyperopic and had axial lengths that were 920 µm longer than that previously reported for White cross broilers2. Despite the different start points and rates of emmetropisation, the growth rates of the various ocular parameters were similar. In the Ross 308 chick, the majority of the emmetropization occurs within 3 days, during which time the posterior layers are still thickening. However, corneal power increases and vitreous elongation starts after this major refractive shift has occurred, with lens changes accounting for most of this initial emmetropization. 1. Schaeffel F, Howland HC. Clin Vision Sci. 1988;3:83–98. 2. Irving EL, Sivak JG, Curry TA, Callender MG. J Comp Physiol [A] 1996;179:185–94.
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