Structure and function of the retinal pigment epithelium.

The evolution of light-perception organs started with the co-development of a double-cell structure consisting of a light-sensitive cell and a pigmented cell. In the human eye, the retinal pigment epithelium (RPE) forms the outer blood/retina barriers and has evolved into a close partnership with photoreceptors indispensable for visual function. In this interaction the RPE performs numerous functions to support the structural integrity of the photoreceptors and light sensitivity in the visual process. These functions include the absorption of light energy focused by a lens onto the retina, the removal of water from retinal tissue, the spatial buffering of K+ in the subretinal space in response to changes in illumination, the phagocytosis of destroyed photoreceptor outer segments, the reisomerization of the photon receptor molecule 11-cis retinal inside rhodopsin, which initiates the light transduction cascade, and the secretion of various factors to control the structural stability of the choriocapillaris and photoreceptors. The latter function is of utmost importance as the barrier function and secretory activity of the RPE create an immune-privileged space in the outer retina and the eye. These functional properties of the RPE ensure visual function, but loss or alteration of RPE functionality can lead to loss of vision.