The health and integrity of retinal neurons depend on a well-regulated extracellular environment. A crucial function of the RPE that contributes to this regulation is control of the volume and composition of fluid in the subretinal space through transport of ions, fluid, and metabolites. The distribution of transport proteins residing in the apical and basolateral membrane domains of the cell is asymmetric, and this allows the epithelium to carry out vectorial transport. The membrane proteins remain in their proper location because of tight junction proteins. The polarity of the cell is maintained because of the intracellular molecular machinery that synthesizes new proteins and delivers them preferentially to the apical or basolateral cell membranes. Cytoskeletal proteins are fundamental in determining cell polarity and regulating transport.
Figure 13-5 RPE. A, Interdigitization of the apical processes of the RPE with photoreceptors in the subretinal space. B, RPE apical microvilli surround photoreceptor outer segments. Cytosolic melanin granules are also shown. C, Phagocytosed photoreceptor outer segments within the RPE.
(Reproduced with permission from Spalton D, Hitchings R, Hunter P.
Atlas of Clinical Ophthalmology.
3rd ed. New York: Elsevier/Mosby; 2005:403.)
Figure 13-6 Example of fundus autofluorescence imaging, which is facilitated by lipofuscin molecules present within the RPE. Changes in fundus autofluorescence patterns reflect disorders of the RPE in the presence of hyperfluorescence and atrophic RPE in the presence of hypofluorescence (see BCSC Section 12, Retina and Vitreous).
(Courtesy of Vikram S. Brar, MD.)
The aqueous environment of the subretinal space is actively maintained by the iont-ransport systems of the RPE, which regulate transport of a variety of ions (K+, Ca2+, Na+, Cl–, and ). This transport is vectorial in most cases; for example, Na+ is actively transported from the choriocapillaris toward the subretinal space, whereas K+ is transported in the opposite direction. The apical membrane of the RPE appears to be the major locus of this transport. As mentioned previously, ouabain-sensitive Na+,K+-ATPase is present at the apical, but not the basal, side. Similarly, an active bicarbonate-transport system appears to be located in the apical RPE membrane. High carbonic anhydrase activity seems to be associated with both the apical and basal sides of the cell.
Net ionic fluxes in the RPE are responsible for the transepithelial electrical potential that can be measured across the RPE apical membrane—a potential that is rapidly modified in the presence of a variety of metabolic inhibitors (eg, ouabain and dinitrophenol). Ion gradients across the RPE drive the transport of water from the subretinal space to the choriocapillaris. The RPE also transports lactic acid produced by metabolic activity in the retina away from the subretinal space. Active vectorial transport systems for other retinal metabolites (eg, taurine, methionine, and folate) have also been demonstrated. The RPE, therefore, appears to be important for maintaining the ionic environment of the subretinal space, which in turn is responsible for maintaining the integrity of the RPE–photoreceptor interface. The trans-RPE potential is the basis for the electro-oculogram (EOG), which is the most common electrophysiologic test for evaluating the RPE (see Chapter 12, Fig 12-8).
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.