Which statement best describes the inner and outer blood-retinal barriers?

• A. Tight junctions between retinal endothelial cells
• B. Tight junctions between photoreceptor outer segments
• C. Desmosomes between Muller cells
• D. Gap junctions between photoreceptors

Answer: (A)

The inner and outer blood-retinal barriers rely on tight junctions to keep the retinal environment stable by blocking unwanted paracellular diffusion. The inner barrier is formed by tight junctions between retinal capillary endothelial cells, which prevent substances in the blood from freely entering the neural retina. The outer barrier is formed by tight junctions between retinal pigment epithelium cells, separating the neural retina from the choroidal blood supply and regulating ion and fluid movement to protect photoreceptors.

Among the options, tight junctions between retinal endothelial cells best describe this barrier mechanism, since tight junctions are the hallmark feature that creates the barrier. The other options describe structures or junctions that are not responsible for forming the blood-retinal barrier, such as features related to photoreceptors or Muller cells, or gap junctions that enable cell-to-cell communication rather than restricting diffusion.