English: Organization of rod photoreceptors in the mammalian retina. (A) Vertical section of a vertebrate retina stained with toluidine blue. The retina contains five classes of neurons (photoreceptors, bipolar cells, horizontal cells, amacrine cells, ganglion cells), and is divided into three nuclear layers (outer nuclear layer (ONL), inner nuclear layer (INL), ganglion cell layer (GCL)) and two synaptic layers (outer plexiform layer (OPL), inner plexiform layer (IPL)); (B) Schematic of a vertebrate rod photoreceptor consisting of the long, light-sensitive outer segment (OS) enclosed by the retinal pigment epithelium (RPE) which is connected via a small intracellular bridge, the connecting cilium (CC), to the metabolically active inner segment (IS) including the basal body complex (BBC; shaded in blue) and the apical region. The OS represents a modified primary cilium. The CC corresponds to the transition zone of a prototypic cilium with a (9 × 2 + 0) microtubule array; (C–H) Transmission electron micrograph of a photoreceptor-connecting cilium; (C) The CC connects the OS and IS. The BBC consists of the basal body (BB) and its centriole (Ce); (D) Cross-section of the CC showing the characteristic (9 × 2 + 0) microtubule array; (E) RPE with pigment granules and the Bruch’s membrane (BM); (F) OS composed of stacks of tightly packed membrane discs; (G) Higher magnification view of the ciliary apparatus; (H) Synaptic ribbon (SR) with invaginations of postsynaptic horizontal cells (HC) and bipolar cells (BC). Scale bars: 100 µm (A), 200 nm (D,F,H), 500 nm (E,G).