Morphology of OS discs. (A) Enface transmission electron micrographs of discs in the OSs of the bull frog Rana catesbeiana. Bar, 1.5 µm. Discs in rod photoreceptors have a scalloped morphology formed by infoldings called “incisures” that penetrate deep into their radial dimension. R, red rod; G, green rod. Image reprinted from Tsukamoto (1987) with permission from Elsevier. (B) Schematic of half of a rod OS, showing two disc membranes (DM) and the plasma membrane (PM) architecture. Average measurements used to calculate the patent area of cross section available for axial diffusion of molecules are indicated.