The blood-ocular barriers

The introduction of the concept of blood-ocular barriers in the ophthalmic literature is briefly reviewed. Two main blood-ocular barriers are proposed: the blood-aqueous barrier and the blood-retinal barrier. The blood-aqueous barrier is formed by an epithelial barrier located in the nonpigmented layer of the ciliary epithelium and in the posterior iridial epithelium, and by the endothelium of the iridial vessels. Both these layers have tight junctions of the "leaky" type. The pereability of the blood-aqueous barrier shows a significant degree of pressure-dependent diffusion associated with transport activity, resembling the standing gradient osmotic flow model. The blood-retinal barrier is located at two levels, forming an outer barrier in the retinal pigment epithelium and an inner barrier in the endothelial membrane of the retinal vessels. Both these membranes have tight junctions of the "nonleaky" type. The permeability of the blood-retinal barrier resembles cellular permeability in general, diffusion being directly related to the predominant roles of lipid solubility and transport mechanisms. Finally, the clinical significance of the blood-ocular barrier is analyzed. The metabolism of cornea and lens and the regulation of intraocular fluids are directly influenced by the blood-aqueous barrier. Similarly, an alteration of the blood-retinal barrier appears to play an important role in the development of vascular retinopathies, pigment epitheliopathies, and retinal edema.