Conjunctival and episcleral blood vessels are permeable to blood-borne horseradish peroxidase.

The vessels of the conjunctival and episcleral plexuses of Macaca mulatta eye are of the continuous type. Most of the vessels in the conjunctival plexus have the diameter of capillaries, while the vast majority of the vessels in the episcleral plexus are venules. Both types of vessels have a simple wall, which consists of an endothelium and a discontinuous layer of pericytes. The aim of this study was to establish their permeability properties to blood-borne horseradish peroxidase (HRP). After intravenous injection of HRP, in 200 microns chopper sections of the anterior segment of the eye examined with the light microscope, the subconjunctival and episcleral tissues appear intensely and diffusely stained by the reaction product. The electron microscope shows that HRP escapes from the vessels lumen by crossing the interendothelial clefts and, in addition, a great number of pinocytotic vesicles loaded with HRP are present on the luminal, tissue front and in the cytoplasm of the endothelial cells. HRP, which rapidly penetrates the loose connective tissue of the region, reaches the spaces between the cells of the conjunctival epithelium where it is finally blocked by the zonulae occludentes that connect the most superficial epithelial cells. A slow diffusion into the compact tissue of the cornea and of the sclera was also observed. Thus, under normal conditions, blood-borne macromolecules can freely diffuse into the subconjunctival and episcleral loose connective tissues. On the other hand, one can equally expect that the aqueous humor that reaches the episcleral and conjunctival blood plexuses through the canal of Schlemm and collector channels can freely diffuse into the subconjunctival spaces across the walls of these permeable vessels.