Ocular responses of genetically blind chicks to the light environment and to lid suture

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Abstract
Blind rc chicks were examined as to growth characteristics of the developing eye. We also explored the relationship between hereditary blindness and two environmental interventions affecting eye development, light-induced avian glaucoma (LIAG) and the axial lengthening associated with lid-suture. Genetically blind rc chicks could be identified by their behavior even during the first hours after hatching. However, they retained the pupillary reflex, at least to 5 weeks of age. Blind chicks reared under a diurnal lighting schedule had eyes 18% larger (by eye weight) than sighted hatchmates, with corresponding increase in globe diameters. Of the anterior segment parameters, neither corneal diameter, anterior chamber depth nor corneal curvature was demonstrably different from sighted controls. When rc chicks were reared under continuous light, the environmental condition which brings on LIAG, their already enlarged eyes showed no superimposed increases in either eye weight or globe dimensions. The corneas showed some characteristic LIAG effects: small anterior chamber depth and increased corneal radius of curvature (flat cornea), as compared with diurnal rc birds. Some birds were subjected to unilateral eyelid-suture, a protocol which usually induces axial lengthening and corneal bulging. Blind rc birds did not respond to this surgical intervention; in comparison by paired t-test between sutured and contralateral eye, there was little or no difference in eye weight, globe diameters or anterior segment parameters. Thus vision, or at least an intact retina/RPE, seems to be necessary for development of the characteristic axial lengthening in response to eyelid suture. The globe enlargement of LIAG also appears to be vision-dependent, although the anterior segment manifestations of LIAG are systmeic effects.