The blood-ocular barriers: past, present, and future.

The blood-ocular barriers system is formed by two main barriers: the blood-aqueous barrier and the blood-retinal barrier. They combine to maintain the eye as a privileged site and are essential for normal visual function. After reviewing where the blood-aqueous barrier and blood-retinal barrier are located and the main transport mechanisms involved in the regulation of the microenvironment of ocular tissues, special attention is given to the clinical significance of breakdown of the blood-retinal barrier. New perspectives on the clinical significance of breakdown of the blood-retinal barrier are offered by the demonstration of a specific alteration of the glucose transport in diabetes, by the development of a new diagnostic instrumentation, and by the utilization of the blood-retinal barrier for new strategies for drug delivery to the retina. New diagnostic instrumentation includes the topographic imaging vitreous fluorometer, which simultaneously measures the localized blood-retinal barrier and images the retinal region, and the retinal thickness analyzer for mapping retinal edema. Drug delivery to the retina may be improved by modification of blood-retinal barrier permeability, chemical modification of the drug for better blood-retinal barrier penetration, and liposome encapsulation or coupling of the drug to specific vectors.