Two essential features of the RPE are its polarity and barrier properties. The RPE is polarized, because it separates the neural retina from the fenestrated capillaries in the choroid. The apical membrane of RPE interacts with the photoreceptors; the basal membrane interacts with the choroid. Like other epithelia, the apical and basolateral membranes have different protein compositions that enable each to interact with different environments. The barrier properties are regulated by two components. Transepithelial transport through the cells is regulated by plasma membrane pumps and transporters. Passive diffusion through the paracellular spaces is regulated by the strands of tight junctions that encircle each cell. Tight junctions are semi-selective, which means some solutes cross them more readily than others. By regulating both the transcellular and paracellular pathways, the RPE regulates the ionic composition of the subretinal space. For review 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.

Rizzolo, L.J. (2007) Development and role of tight junctions in the retinal pigment epithelium, Int. Rev. Cytol. 258:195-234.