Which of the following is NOT true regarding the RPE?

• A. Its apical side faces the retina
• B. Microvilli are found on the apical side of the RPE and are responsible for phagocytosis of photoreceptor outer segments
• C. The Blood retinal barrier is caused by tight junctions found on the apical RPE
• D. The Blood retinal barrier is caused by tight junctions found on the basal RPE

Answer: (D)

The key idea is how the RPE is oriented and what forms the blood-retinal barrier. The apical surface of the RPE faces the photoreceptors and carries microvilli that engage in phagocytosis of shed photoreceptor outer segments, a daily renewal process essential for visual function. Tight junctions on this apical side create the outer blood-retinal barrier, regulating exchange between the choroidal blood supply and the retina. The basal surface faces Bruch’s membrane and the choroid, but these tight junctions do not form the BRB. The inner BRB, by contrast, is established by tight junctions between retinal capillary endothelium, not by RPE. So stating that the BRB is caused by tight junctions on the basal RPE is not true.