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L. Ai, C. F. Wildsoet, H. Guan, J. M. Miller; Emmetropization and Eye Growth in Young Aphakic Chickens. Invest. Ophthalmol. Vis. Sci. 2007;48(13):1529.
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There is on-going debate over the role of the crystalline lens as a source of critical growth factors for early eye development, with implications for the timing of surgery for congenital cataracts. We established a chick model to study the effect of early lens removal on eye growth and the additional influence of optical correction of surgically induced refractive errors.
36 White Leghorn 18 day-old chicks were randomly assigned to 4 groups: (1) lens extraction (LE), (2) LE plus +30D lens, (3) LE plus +20D lens, and (4) sham surgery plus -30D lens. Lensectomy left the posterior capsule intact; sham surgery involved a near identical protocol except for the lens removal steps. Throughout the experimental period, retinoscopy and high frequency A-scan ultrasonography were used to monitor refractive errors and the axial dimensions of ocular compartments respectively. Both these measurements and the surgery were performed under general anesthesia. On post-operative day 28 (final day), external ocular dimensions of eyes enucleated from sacrificed chicks were measured with digital calipers.
Compared to their fellow eyes, experimental (Exp) eyes of the sham/-30D, LE (imposed -30D), and LE/+20D (undercorrected by - 6 to -7D) groups showed myopic refractive shifts whereas the LE/+30D (overcorrected by +2 to +3 D) group exhibited a small hyperopic shift. None of the eyes developed posterior capsule opacities.Exp eyes of Sham/-30D, LE, and LE/+20D groups exhibiting myopic refractive shifts showed increased axial growth, although the LE group showed significantly less elongation than the sham/-30D group, even though both groups experienced similar initial defocus. The LE/+30D group showed slowed axial elongation and choroidal thickening, contrasting with changes in the other 3 groups but consistent with their refractive changes. All aphakic eyes recorded enlarged equatorial dimensions relative to their fellows, irrespective of the optical manipulation. This change was not seen in sham/-30D eyes.
The refractive changes observed in young, aphakic eyes are consistent with compensation to the defocus experienced, and thus emmetropization. However, the relatively slow eye growth in LE eyes, which experienced similar defocus to eyes undergoing sham surgery, leaves open the possibility that the lens supplies essential growth factors. Finally, the oblate shapes of aphakic eyes are a likely response of peripheral retinal areas to the very large and uncorrectable hyperopic refractive errors induced by the surgery.
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