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V. Choh, C.F. Wildsoet; The Eyes of Albino Chicks Show Abnormal Responses to Optic Nerve Section . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3336.
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© ARVO (1962-2015); The Authors (2016-present)
Albinism in humans is associated with high refractive errors, suggesting abnormal emmetropisation. Probably either or both the neural retina and RPE in albino animals are functionally abnormal. Optic nerve section isolates local ocular growth mechanisms and in normally pigmented chicks, results in choroidal thickening and increased hyperopia. We assessed ocular growth responses in albino chicks using optic nerve section.
11 day–old White Leghorn (Gallus gallus domesticus) normally pigmented (n=5) and albino (n=5) chicks were used. Baseline high frequency A–scan ultrasonography (US) measurements of both eyes were taken 4 hours prior to optic nerve section (ONS) surgery of one eye, with subsequent US readings taken after 1 (20 hours post–ONS), 3, 5 and 7 days. Contralateral, unoperated eyes served as controls. ONS–induced refractive error and axial changes (presented as mean interocular differences ± s.d.) to the anterior chamber, crystalline lens, vitreous chamber, retina, choroid and sclera were analysed.
Prior to optic nerve section (ONS), albino eyes had shorter anterior chamber depths (p<0.0001), thicker lenses (p=0.0057), thinner choroids (p=0.0095) and thinner scleras (p=0.0043) than the pigmented birds and they also showed reduced hyperopia (p<0.0001). After ONS, the pigmented birds showed the expected choroidal thickening response (difference at 3 days post–ONS: 0.111 ± 0.115 mm; p=0.0085), while the albino birds showed thinning instead (difference at 7 days post–ONS: –0.050 ± 0.16 mm; p=0.0024). Vitreous chamber changes in albino birds were also opposite to those in normal birds, increasing instead of decreasing (difference at day 5: 0.285 ± 0.171 mm, p<0.0001 vs –0.138 ± 0.100 mm, p=0.0043 respectively). The ONS eyes of the albino birds were also relatively myopic compared to the increased hyperopia in the ONS eyes of the pigmented birds (difference at day 7: –11.68 ± 5.93 D vs. +1.30 ± 3.01 D, respectively).
The altered ocular dimensions of the albino chicks compared to the normally pigmented chicks imply altered emmetropisation. The subsequent finding that ONS induces the opposite changes in the eyes of albino chicks compared to those in normal chicks implies abnormalities at either the level of the inner retina (altered amacrine cell numbers have been noted in albino rats and cats; ganglion cells are lost after ONS), or RPE, which is the communication link between the retina and choroid. Further investigation is required to distinguish between these possibilities.
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