A darkness induced eye abnormality in the domestic chicken

Two-hundred broiler-type chickens were reared under either normal lighting or darkness for 16 weeks. Each week, birds were subjected to applanation tonometry for measurement of intraocular pressures (IOP). The birds were then weighed, eye enucleated, and dimensions measured. All eyes were prepared for histological examination. Chickens reared in darkness for 4 to 6 weeks displayed an abnormal eye enlargement, accompanied by an increase in IOP in comparison to chickens reared under conventional management. From the 6th to the 16th week, the eyes of dark-reared chickens continued to enlarge while IOP decreased. The sclera, choroid, retina, and retinal layers of dark-reared chicks were not as thick as controls throughout the 16 weeks. Two principle retinal pathologies were induced by darkness. One appeared as darkened areas of the peripheral retina. In histological sections the darkened retinal areas had a thin choroid and thin retinal layers. The second retinal abnormality was one or more white, nonpigmented bands directed perpendicular to the pectin. Histologically, the depigmented bands were areas in which the vessels of the choroid, the pigmented epithelium, and the outer retinal layers were absent. The corneas of dark-reared chicks exhibited a reduction in their curvature and thickness compared to control corneas.