The Indicator Extraction Technique. A Method for Studying the Blood-Retinal Barriers

Abstract

In the last decade, the properties of the blood-brain barrier have been the object of many investigations. The eye possesses a similar barrier which is involved in regulating the environment of the retina, but as yet, little is known about its properties. Previous investigations have demonstrated that the blood-retinal barrier excludes large molecules, such as horseradish peroxidase (1,2) and microperoxidase (3) and even the low molecular weight compound, fluorescein (4) from entering the retinal tissue. If the entry of water-soluble substances into the retina by free diffusion is hindered, one can anticipate that there are carrier mechanisms for the transport of essential substrates and metabolites across the blood-retinal barrier, similar to those in the brain. In man, as well as in cats and pigs, the retina has a dual vascular supply; the inner parts are nourished by the retinal vessels, whereas the outer parts are supplied by the choroidal circulation. Thus, in these species, the blood- retinal barrier has two interfaces. The endothelium of the retinal capillaries is continuous and lacks fenestrations, similar to that of the cerebral capillaries. Adjacent cells are attached to each other by tight junctions, which constitute the main barrier to diffusion through the capillary wall. In contrast, the choroidal capillary wall is thin and fenestrated, each fenestration being closed by a thin membrane. These capillaries form a dense network, the choriocapillaris, that is restricted to a single plane and separated from the photoreceptors by the retinal pigment epithelium. This epithelium constitutes the other part of the blood-retinal barrier.