An electron microscopical study of the blood-aqueous barrier in the ciliary body and iris of the rabbit

Abstract

Horseradish peroxidase was administered to rabbits by intravenous injection, and its distribution in the iris and in the different regions of the ciliary body was studied using an electron microscope. In the iris, exogenous peroxidase was localized in the lumina of the blood vessels and in some micropinocytotic vesicles within the endothelial cells. None was found beyond the vascular endothelium. The micropinocytotic vesicles were few in number and did not appear to transport peroxidase in great amounts, while the tight junctions between the endothelial cells were probably responsible for preventing its intercellular passage. These findings indicate the presence of a “barrier” at the endothelium of the iris vessels. In the different regions of the ciliary body exogenous peroxidase was localized in the lumen of the vessels, in the vesicles throughout the cytoplasm, and in the intercellular clefts between the endothelial cells. Transport may occur through fenestrations rather than through the cytoplasm of the endothelial cells. Peroxidase easily penetrated the stroma and also the basement membrane of the vessels and pigmented cells and the intercellular spaces between the pigmented cells. The ultrastructural observations indicated that the non-pigmented epithelial cells and their intercellular junctions constitute the site of the barrier to proteins under normal conditions. Only in the ciliary processes and the pars plana was the reaction product seen to penetrate into the intercellular spaces between two adjoining non-pigmented cells. The significance of the different permeabilities in the junctions of the non-pigmented epithelial cells in different regions of the ciliary body is discussed, particularly with reference to histochemical and cytochemical differences.