FINE-STRUCTURE OF THE CHOROIDAL COAT OF THE AVIAN EYE - LYMPHATIC VESSELS

Purpose. To clarify the fine structure of the avian choroid and thus h elp explain the mechanisms for normal and abnormal eye function and gr owth. Methods. Eyes from normal chickens and from experimental chicken s subjected to unilateral paracentesis were fixed either by perfusion or in situ, with or without post-fixation by microwave irradiation, an d then processed for light and electron microscopic analysis. Results. The avian choroid contains thin-walled lacunae, whose fine structure is identical to that of lymphatic vessels. The lacunae are much smalle r toward the anterior chamber and the Schlemm's canal than posteriorly in the eye bulb. Large lacunae are situated primarily in the supracho roidea, and their blind-ended capillary branches enter the choriocapil laris and the walls of large veins. The walls of the large veins conta in villous structures that protrude into their lumina and are penetrat ed by thin lacunar branches and by side lines of the venous lumen. In normal chickens, the lacunae usually are devoid of blood cells. After paracentesis of the anterior eye chamber, the lacunae become filled wi th erythrocytes on the side that was operated on, but not on the contr alateral side. Conclusions. The authors propose that the lacunae of th e avian choroid represent a system of posterior short lymphatic vessel s, which drain intraocular fluids directly into the eye's venous syste m, and that the villous structures are sites of communication between lacunae and veins. The demonstration of a choroidal lymphatic system o pens new insights into the processes of fluid removal, control of intr aocular pressure, and regulation of choroidal thickness in the avian e ye under normal and experimental conditions.