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K Pendrak, C Capehart, T Lin, AS Gill, R Sugimoto, RA Stone; A Visual Mechanism for Equatorial Eye Expension . Invest. Ophthalmol. Vis. Sci. 2002;43(13):201.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To learn if a visual mechanism stimulates ocular expansion selectively in the equatorial dimension and whether it affects biochemical markers linked to myopia. Methods: A full goggle or one of several goggles modified with offset perforations of varied size was secured over one eye of one day-old chicks. At two weeks of age, the chicks were anesthetized, and the eyes were measured with refractometry and A-scan ultrasonography. While still under general anesthesia, the chicks were decapitated; the axial and equatorial dimensions of enucleated eyes were measured with calipers. Retinas were dissected into central, peripheral nasal and peripheral temporal retinal regions and assayed for dopamine and DOPAC using HPLC with electrochemical detection. Ciliary ganglia were assayed for choline acetyltransferase. Results: As well described, wearing a full goggle caused pronounced ipsilateral myopia and diffuse ocular enlargement; ipsilateral retinal dopamine/DOPAC and ciliary ganglion enzyme activity were all reduced. With perforated goggles, increased size and inferonasal offset of the hole caused progressively less myopia and axial elongation while still inducing equatorial expansion. With the largest and most offset goggle perforation, eyes remained emmetropic with minimally increased axial length but with still pronounced equatorial expansion. In contrast to full goggles, the biochemical measures were unaffected in eyes and ciliary ganglia ipsilateral to these goggles. Conclusions: Prior studies with pharmacology, photoperiod and autonomic denervations have suggested that distinctive mechanisms regulate vitreous cavity expansion in axial and equatorial dimensions of chick. A goggle with a large offset perforation permitting unimpaired visual input to the superotemporal retina, the region with highest ganglion cell density, stimulated vitreous cavity expansion mainly in the equatorial dimension but did not alter retinal dopamine/DOPAC or ciliary ganglion choline acetyltransferase. Thus, nonidentical neurochemical pathways seemingly modulate vitreous cavity growth in the axial and equatorial dimensions.Given that ocular form can identify experimental conditions altering eye growth in chick and given the parallels between human and chick eye growth, more directed study of human eye shape may prove useful in identifying mechanisms underlying refractive development.
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