sᵻkɹˈiːʃən: ðɪ ˌɑːɹpˌiːˈiː ɪz ɐn ˌɛpɪθˈiːliəm wˌɪtʃ klˈoʊsli ˌɪntɚɹˈækts wɪð fˌoʊɾoʊɹᵻsˈɛptɚz ˌɔn wˈʌn sˈaɪd bˌʌt mˈʌst ˈɔːlsoʊ biː ˈeɪbəl tʊ ˌɪntɚɹˈækt wɪð sˈɛlz ɔnðə blˈʌd sˈaɪd ʌvðɪ ˌɛpɪθˈiːliəm, sˈʌtʃ ɐz ˌɛndoʊθˈɛlɪəl sˈɛlz ɔːɹ sˈɛlz ʌvðɪ ɪmjˈuːn sˈɪstəm.
Secretion: The RPE is an epithelium which closely interacts with photoreceptors on one side but must also be able to interact with cells on the blood side of the epithelium, such as endothelial cells or cells of the immune system.