Figure 6. Electron microscopy of HESC-derived RPE cells after exposure to human retinal explant in vitro. A: Control HESC-derived RPE cells, which were not exposed to human retina, have apical microvilli (AMv) and contain melanin granules (white arrows). B: HESC-derived RPE exposed to the photoreceptor surface of human retina for 48 h appear more mature. Note the close association of the RPE apical microvilli with the photoreceptor outer segment (OS), the abundance of pigmented melanin granules within the apical region of the cell, the nucleus (N), and the numerous lipid deposits (L) located toward the basal portion of the cell. C: Apical microvilli surround a human photoreceptor outer segment. D: An outer segment is engulfed by the HESC-derived RPE cell. E: A high magnification image of lipid deposits (L) observed in the basal portion of a HESC-derived RPE after exposure to human retina. The formation of lipid deposits is indicative of the end stages of phagocytosis. Several features associated with RPE cell function are present in the cells including (F) tight junctions (black arrow) and coated pits (red box, indicated with arrows at higher magnification in G). Cells also contain a high number of coated vesicles within the apical portion of the cell (H) and develop basal end feet and infolding of the basal membrane (I). Scale bars equal 2 μm in A-C, 1 μm in D-F,I, and 500 nm in G,H.