In contrast to neurons, epithelial polarity is not inherent in the cell but is induced by the environment. The environment of the RPE changes dramatically as the neural retina and the choriocapillaris differentiate. Just as the polarity of RPE changes during development, the intercellular junctions that bind neighboring RPE cells also mature gradually. These junctions retard diffusion across the RPE through the paracellular spaces. In many epithelia these junctions are relatively leaky, but in the RPE they must be tight to form a blood-retinal barrier. Our studies indicate that this plasticity of the membranes and junctions depends upon interactions with the neural retina. For reviews see:

 

Wilt SD, Rizzolo LJ: Unique aspects of the blood-brain barrier. In Tight Junctions, Ed. M. Cereijido and J.M. Anderson, CRC Press, in press, 2001.

Rizzolo LJ: Polarity and the Development of the Outer Blood-Retinal Barrier. Histol. Histopath. 12:1057-1067, 1997.