Membrane organization of the basal OS. Electron micrographs of the basal OS in unlabeled, non-transfected (A) and DAB-labeled, transfected (B, C) rods. Arrowheads in (A) point to the nascent discs/cisternae at the basal-most OS, which are surrounded by cytosol and enveloped by the OS PM. Arrows in B point to small tubulovesicles containing DAB precipitates inside their lumens; some of them look as if they are undergoing fusion with the adjacent discs. White arrows in (B) and (C) point to the intense DAB precipitate inside the lumens of the disc membranes. The bottom 1st to 15th discs (in C) are clearly surrounded by the OS cytosol (Cyto). Despite the occasional PM discontinuity (arrowheads in C), the differential electron density between the OS cytosol and extracellular (Ec) space clearly shows that the base of the OS is “closed” without evaginated PM. IS inner segment.