Figure 1.

(A) Cellular organization of the retina. The retina contains the retinal neuronal cell types, such as the retinal pigment epithelium (RPE), which faces choroidal blood vessels at the basal side, and cones (purple) and rods (blue) at the apical side. The photoreceptor nuclei constitute a layer called the outer nuclear layer (ONL), whereas their axons and processes meet with horizontal (violet) and bipolar (red) cells in the outer plexiform layer (OPL). More anterior, the inner nuclear layer (INL) harbors nuclei of the bipolar (red), amacrine (pink), and horizontal (violet) cells, and Müller glia, while the inner plexiform layer (IPL) contains the processes and synapses of bipolar (red) cells, amacrine (pink) cells, and retinal ganglion cells or RGCs that are reduced in number by the stage of photoreceptor maturation (yellow). (B) Structure of rod and cone photoreceptors. Photoreceptors are polarized sensory neurons. Rods (blue) and cones (red) have three cellular compartments. Outer segments (OS) are stacks of membrane disks rich in the visual pigment rhodopsin. This is where phototransduction originates. Interestingly, this cellular part does not contain any protein synthesis machinery. All OS proteins are synthesized in the inner segments (IS) and then transported to this cellular part. IS also contain other vital organelles, i.e., mitochondria, and the nucleus. Neuronal impulses created in the OS pass through the IS until they reach the synaptic terminals, where they are transmitted to other retinal neurons. (Created with BioRender.com).