The RPE forms part of which barrier?

• A. Outer blood-retinal barrier
• B. Blood-brain barrier
• C. Inner blood-retinal barrier
• D. No barrier

Answer: (B)

The RPE is part of the outer blood-retinal barrier. Tight junctions between RPE cells create a selective seal between the choroidal blood supply and the neural retina, regulating what can pass from the blood into retinal tissue. This is crucial because the choroidal vessels are fenestrated and highly permeable, so the RPE barrier protects the retina from unwanted flux. The inner blood-retinal barrier, by contrast, is formed by the retinal capillary endothelium and Müller cells, regulating diffusion from the retinal circulation to the retina itself. The brain’s barrier, the blood-brain barrier, is a separate system formed mainly by brain capillary endothelium with astrocyte support. So the RPE forms the outer BRB, not the BBB.