Structural Characterization of the Retinopathy, Globe Enlarged (rge) Chicken Cornea

Abstract

Purpose: : Rge is an inherited retinal disease of chickens resulting in blindness by ~30 days post-hatch. Secondary to retinal dysfunction, rge chicks display globe enlargement accompanied by corneal thickening and flattening as the eye develops post-natally. Previously we reported alterations in collagen organisation in the corneal periphery of 9 month old rge chickens. Here we further characterize corneal structure in rge chickens and compare it with normal birds during post-natal development.

Methods: : Wide-angle x-ray scattering (WAXS), performed at the UK Synchrotron Source (Daresbury), was used to map collagen fibril orientation in the corneas of 1 month, 3 month and 9 month old rge chickens and age-matched normal birds. Small-angle x-ray scattering (SAXS) was employed to measure fibril spacing and diameter in 1 month and 3 month old corneas. Further ultrastructural information on 3 month old corneas was provided by electron microscopy, employing cuprolinic blue to visualise stromal proteoglycans.

Results: : WAXS results indicate that normal collagen fibril orientation is not significantly altered in 1 month old rge corneas. However, data from 3 and 9 month old chickens indicate that tangential fibrils in the corneal periphery/limbus, as displayed by controls, become progressively re-oriented in favour of a predominantly orthogonal arrangement in rge birds >1 month. SAXS analysis yielded no significant differences in fibril spacing or diameter between normal and rge chickens up to 3 months old, consistent with electron microscopy findings. In addition electron microscopy revealed no obvious differences in the appearance of proteoglycans between normal and rge corneas. For both genotypes, proteoglycans appeared longer and more electron-dense in the posterior compared to anterior- and mid-stromal regions.

Conclusions: : Normal collagen fibril orientation is progressively altered in the peripheral cornea/limbus of rge chicks >1 month. Corneal flattening in rge chicks may be associated with biomechanical changes brought about by altered peripheral corneal/limbal collagen structure. From the current data corneal thickening in rge appears not to be due to any significant increase in the separation or diameter of stromal collagen fibrils. The distribution and morphology of collagen-associated proteoglycans in the rge corneal stroma appear normal.

Keywords: cornea: basic science • cornea: stroma and keratocytes • degenerations/dystrophies