ACTA lecture: The retinal pigment epithelium, friend or foe?

Purpose: To review the physiology and pathophysiology of the retinal pigment epithelium, RPE, with focus on some of the work of myself and my wonderful collaborators and students during the last 20 years. Methods: In vitro studies of RPE preparations from frogs and of cultured human and porcine RPE cells. In vivo studies of experimental surgery in pigs using histopathology, immunohistochemistry and functional testing by multifocal electoretinograhpy. Results: There are active fluid transport mechanisms in the RPE, which are capable of transporting water aganist an osmotic gradient. This has important bearings on the fundus appearance in states of macular oedema. RPE cell physiology is dependent on the surface on which they grow, and the maturity of the cells. Older RPE cells are more stress resistant than younger RPE cells. This has important bearings on studies where cultured RPE cells are used to assess toxicity of substances used in the clinic. In vivo RPE cells are non‐dividing under normal circumstances. Posterior injury to the epithelium induces proliferation of the peripheral, but not the posterior RPE. This indicates that a g zone of putative RPE stem cells might exist in the peripheral retina. The functional consequences of experimental posterior RPE injury can be assessed by multifocal electroretinography Conclusions: The RPE plays an important role in the normal physiology og the outer retina, as well as in the pathogenesis of retinal diseases. The development of animal models of RPE injury increase our understanding of the role of RPE in health and disease.