# Retinal
**Retinal** is the aldehyde form of vitamin A (retinol) and, in its 11-cis isomer, the universal light-absorbing chromophore of vertebrate vision. Bound covalently to the opsin proteins of rod and cone photoreceptors, it absorbs a photon, isomerizes to the all-trans form, and triggers the biochemical cascade that converts light into a neural signal; the spent chromophore is then regenerated by the visual cycle.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC4593475/)
| Key fact | Detail |
|---|---|
| Chemical identity | Aldehyde of vitamin A; the biologically active isomer in photoreceptors is 11-cis-retinal |
| Binding to opsin | Covalently attached through a protonated Schiff base to Lys296 in the seventh transmembrane domain, with Glu113 in transmembrane domain 3 acting as the counterion[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC4593475/) |
| Absorption | Free 11-cis-retinal absorbs at 360 nm; binding to opsin shifts rhodopsin absorption to about 500 nm, the "opsin shift"[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC4593475/) |
| Photochemistry | Absorption of a photon isomerizes 11-cis-retinal to all-trans-retinal, activating the receptor; the chromophore is later released and recycled[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC4593475/) |
| Regeneration | The RPE visual cycle, whose key enzyme is RPE65, converts all-trans-retinyl esters back to 11-cis-retinol and then 11-cis-retinal[2](https://doi.org/10.1016/j.preteyeres.2021.101013) |
| Cone supply | Cones additionally draw on a retina visual cycle run by Müller glial cells, which produce 11-cis-retinol that only cones can oxidize to the aldehyde[3](https://www.annualreviews.org/content/journals/10.1146/annurev-vision-100820-083937?TRACK=RSS) |
| Scale of use | Rods make up about 80% of the cells in the human, mouse, and bovine retina, and the average human retina holds roughly 92 million rods and 4.6 million cones, each renewing its pigment supply[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC4593475/) • [4](https://www.sciencedirect.com/science/article/pii/B978044322193400007X) |
## Role in phototransduction
In rod photoreceptors, 11-cis-retinal is bound to the apoprotein opsin to form rhodopsin, the light-sensitive pigment of the rod outer segment, where rhodopsin constitutes more than 90% of the membrane protein.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC4593475/) The chromophore sits in the hydrophobic core of the seven-transmembrane receptor, about two-thirds of the way from the cytoplasmic surface, attached to Lys296 through a protonated [Schiff base](https://www.edgepedia.org/schiff-base).[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC4593475/)
When a photon is absorbed, the 11-cis double bond flips to the all-trans configuration. The protein passes through unstable intermediates, metarhodopsin I and metarhodopsin II, within milliseconds; it is metarhodopsin II that activates the [G protein](https://www.edgepedia.org/g-protein) transducin and initiates the phototransduction cascade.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC4593475/) • [5](https://www.ncbi.nlm.nih.gov/books/NBK11522/) The all-trans-retinal eventually dissociates from the binding pocket and is returned to the retinal pigment epithelium (RPE), a monolayer of cells behind the retina, for regeneration.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC4593475/) • [5](https://www.ncbi.nlm.nih.gov/books/NBK11522/)
## Spectral tuning
The color of vision depends on the same chromophore in different protein environments. Free 11-cis-retinal absorbs ultraviolet light at 360 nm, yet rhodopsin absorbs at about 500 nm; this red shift, produced by the interaction of the protonated Schiff base with the protein, is called the opsin shift.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC4593475/) Cone opsins interact with the same chromophore to produce both shorter-wavelength (hypsochromic) and longer-wavelength (bathochromic) absorption shifts, producing the spectral tuning that underlies color vision.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC4593475/)
## Regeneration: the visual cycles
Because light destroys the active chromophore, continuous vision requires its renewal. In the canonical RPE visual cycle, all-trans-retinal released from bleached pigment is reduced to all-trans-retinol (vitamin A), transported to the RPE, esterified, and then converted back to 11-cis-retinol and oxidized to 11-cis-retinal for delivery to photoreceptors.[5](https://www.ncbi.nlm.nih.gov/books/NBK11522/) The hallmark step is catalyzed by RPE65, a 65 kDa retinoid isomerase in the RPE that couples hydrolysis of all-trans-retinyl esters with geometric isomerization to yield 11-cis-retinol, a reaction described as possibly biochemically unique.[2](https://doi.org/10.1016/j.preteyeres.2021.101013)
**A second, cone-specific cycle.** Cones also receive chromophore through a parallel pathway based in the neural retina itself. Müller glial cells, which span the retina, take up all-trans-retinol released from bleached cone pigment and convert it to 11-cis-retinol; only cones, not rods, can oxidize this retinol to 11-cis-retinal and use it for pigment regeneration.[3](https://www.annualreviews.org/content/journals/10.1146/annurev-vision-100820-083937?TRACK=RSS) • [6](https://pmc.ncbi.nlm.nih.gov/articles/PMC3265844/) This retina visual cycle is independent of the pigment epithelium and supports the rapid dark adaptation and extended dynamic range that cones need in bright light; its function has been demonstrated in species from salamander and zebrafish to mouse, primates, and humans.[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC3265844/) In the mouse, the retina visual cycle promotes cone adaptation about 8-fold faster than the RPE cycle, while complete cone recovery requires both pathways.[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC3265844/)
## History of discovery
The red color of rod cells was first noticed by Heinrich Müller in 1851, but he attributed it to hemoglobin.[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC2801150/) The initial discovery of a light-sensitive retinal pigment is generally attributed to the German physiologist Franz Boll, who around 1876 observed the purple-red color of the rod layer in the retina of a dark-adapted frog and recognized that the color bleaches in light and regenerates in darkness.[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC3858459/) • [7](https://pmc.ncbi.nlm.nih.gov/articles/PMC2801150/) Willy Kühne, working in [Heidelberg](https://www.edgepedia.org/heidelberg), named the pigment "sehpurpur" (visual purple) and later rhodopsin, extracted it from bovine retina, and showed that a photobleached retina regained its red color when placed in contact with the RPE in the dark, the observation that opened the visual cycle.[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC3858459/) • [1](https://pmc.ncbi.nlm.nih.gov/articles/PMC4593475/)
The chemical identity of the chromophore waited until the work of [George Wald](https://www.edgepedia.org/george-wald), who provided evidence that rhodopsin consists of the protein opsin plus a covalently linked 11-cis-retinal, and who first sketched the visual cycle.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC4593475/) • [2](https://doi.org/10.1016/j.preteyeres.2021.101013) The retinal attachment site was later identified as Lys296, and Glu113 was found to act as the counterion for the protonated Schiff base.[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC2801150/)
## Retinal in health and disease
Because the chromophore is derived from vitamin A, dietary absence of carotenoids and retinoids has long been known to cause progressive retinal degeneration and blindness.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC4593475/) Prolonged 11-cis-retinal shortage, as in vitamin A deficiency, leads to pronounced visual deficits.[9](https://www.oculogenetica.com/?mdocs-file=1039)
Genetic defects in the supply chain cause inherited retinal disease. Mutations in RPE65, the isomerase at the center of the RPE visual cycle, are associated with [Leber congenital amaurosis](https://www.edgepedia.org/leber-congenital-amaurosis) type 2, and defects in genes encoding visual cycle components more broadly can produce retinal dystrophies.[2](https://doi.org/10.1016/j.preteyeres.2021.101013) • [10](https://discovery.ucl.ac.uk/19985/1/19985.pdf) In Stargardt disease, toxic fluorophores accumulate in the visual cycle pathway, and a modified vitamin A (C20-D3-vitamin A) has been tested in clinical trials; it reacts normally throughout the visual cycle while slowing toxic byproduct formation.[11](https://pmc.ncbi.nlm.nih.gov/articles/PMC9339908/)
The pathway is also a drug target. [Visual cycle](https://www.edgepedia.org/visual-cycle) modulators tested in clinical trials include [emixustat](https://www.edgepedia.org/emixustat), which inhibits RPE65, and fenretinide and [tinlarebant](https://www.edgepedia.org/tinlarebant), which reduce plasma concentrations of the vitamin A carrier RBP4; a principal challenge is delivering such drugs to the retina across the outer blood-retinal barrier.[12](https://doi.org/10.1016/j.preteyeres.2025.101360) When the visual cycle is nonfunctional, as in Leber congenital amaurosis, regeneration of pigment with 9-cis-retinal (forming isorhodopsin) has been suggested as a possible clinical approach.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC4593475/)
## References
1. [Hofmann L, Palczewski K (2015). The G Protein-Coupled Receptor Rhodopsin: A Historical Perspective. Methods Mol Biol.](https://pmc.ncbi.nlm.nih.gov/articles/PMC4593475/)
2. [Kiser PD (2021). Retinal pigment epithelium 65 kDa protein (RPE65): An update. Progress in Retinal and Eye Research.](https://doi.org/10.1016/j.preteyeres.2021.101013)
3. [Sato S, Kefalov VJ (2024). The Retina-Based Visual Cycle. Annual Review of Vision Science 10:293-321.](https://www.annualreviews.org/content/journals/10.1146/annurev-vision-100820-083937?TRACK=RSS)
4. [Retinal anatomy: Comparison of primate (including human) and mouse retina. Handbook of Clinical Neurology, Volume 217 (2026).](https://www.sciencedirect.com/science/article/pii/B978044322193400007X)
5. [Photoreceptors. Webvision, NCBI Bookshelf.](https://www.ncbi.nlm.nih.gov/books/NBK11522/)
6. [Rod and Cone Visual Pigments and Phototransduction through Pharmacological, Genetic, and Physiological Approaches. PMC.](https://pmc.ncbi.nlm.nih.gov/articles/PMC3265844/)
7. [Rhodopsin and the others: a historical perspective on structural studies of G protein-coupled receptors. PMC.](https://pmc.ncbi.nlm.nih.gov/articles/PMC2801150/)
8. [Kiser PD, Golczak M, Palczewski K (2014). Chemistry of the Retinoid (Visual) Cycle. Chemical Reviews. PMC.](https://pmc.ncbi.nlm.nih.gov/articles/PMC3858459/)
9. [The Visual Cycle. Storm Eye Institute, Medical University of South Carolina.](https://www.oculogenetica.com/?mdocs-file=1039)
10. [Patterning and Development of Photoreceptors in the Human Retina (thesis summary, UCL Discovery).](https://discovery.ucl.ac.uk/19985/1/19985.pdf)
11. [Vitamin A, systemic T-cells, and the eye: Focus on degenerative retinal disease. PMC.](https://pmc.ncbi.nlm.nih.gov/articles/PMC9339908/)
12. [Advances and therapeutic opportunities in visual cycle modulation. Progress in Retinal and Eye Research (2025).](https://doi.org/10.1016/j.preteyeres.2025.101360)
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*Topic: Encyclopedia › Life and health › Human health and medicine › Human structure and function › Nervous and sensory systems › Sensory systems › Visual system and the eye › Retinal and visual physiology › Visual cycle and chromophore regeneration*
*Initially written Sep 17, 2026 · Reviewed: Oct 21, 2026 · Edited: Oct 15, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license