Editing Retina (section)

=== Inverted versus non-inverted retina ===
The vertebrate retina is ''inverted'' in the sense that the light-sensing cells are in the back of the retina, so that light has to pass through layers of neurons and capillaries before it reaches the photosensitive sections of the rods and cones.<ref name="Kolb"/> The ganglion cells, whose axons form the optic nerve, are at the front of the retina; therefore, the optic nerve must cross through the retina en route to the brain. No photoreceptors are in this region, giving rise to the [[blind spot (vision)|blind spot]].<ref>{{cite web|last1=Kolb|first1=Helga|title=Photoreceptors|url=https://webvision.med.utah.edu/book/part-ii-anatomy-and-physiology-of-the-retina/photoreceptors/|website=Webvision|access-date=11 January 2018}}</ref> In contrast, in the [[cephalopod]] retina, the photoreceptors are in front, with processing neurons and capillaries behind them. Because of this, cephalopods do not have a blind spot.

Although the overlying neural tissue is partly transparent, and the accompanying [[glial cell]]s have been shown to act as [[Optical fiber|fibre-optic]] channels to transport photons directly to the photoreceptors,<ref name="pmid17485670">{{cite journal |vauthors = Franze K, Grosche J, Skatchkov SN, Schinkinger S, Foja C, Schild D, Uckermann O, Travis K, Reichenbach A, Guck J |title = Muller cells are living optical fibers in the vertebrate retina |journal = Proc. Natl. Acad. Sci. U.S.A. |volume = 104 |issue = 20 |pages = 8287–8292 |year = 2007 |pmid = 17485670 |pmc = 1895942 |doi = 10.1073/pnas.0611180104 |bibcode = 2007PNAS..104.8287F |doi-access = free }}</ref><ref>{{cite magazine|last1=Baker|first1=Oliver|title=Focus: Eye Cells as Light Pipes|magazine=Physical Review Focus|date=23 April 2010|volume=25|issue=15|doi=10.1103/physrevfocus.25.15}}</ref> [[light scattering]] does occur.<ref name="Bringmann2018">{{cite journal | vauthors = Bringmann A, Syrbe S, Görner K, Kacza J, Francke M, Wiedemann P, Reichenbach A | title = The primate fovea: Structure, function and development | journal = Prog Retin Eye Res | volume = 66 | pages = 49–84 | date = 2018 | pmid = 29609042 | doi = 10.1016/j.preteyeres.2018.03.006 | s2cid = 5045660 }}</ref> Some vertebrates, including humans, have an area of the central retina adapted for high-acuity vision. This area, termed the [[fovea centralis]], is avascular (does not have blood vessels), and has minimal neural tissue in front of the photoreceptors, thereby minimizing light scattering.<ref name="Bringmann2018"/>

The cephalopods have a non-inverted retina, which is comparable in [[Angular resolution|resolving power]] to the eyes of many vertebrates. Squid eyes do not have an analog of the vertebrate [[retinal pigment epithelium]] (RPE). Although their photoreceptors contain a protein, retinochrome, that recycles retinal and replicates one of the functions of the vertebrate RPE, cephalopod photoreceptors are likely not maintained as well as in vertebrates, and that as a result, the useful lifetime of photoreceptors in invertebrates is much shorter than in vertebrates.<ref>{{Cite journal |last1 = Sperling |first1 = L. |last2 = Hubbard |first2 = R. |date = 1 February 1975 |title = Squid retinochrome. |journal = The Journal of General Physiology |volume = 65 |issue = 2 |pages = 235–251 |doi = 10.1085/jgp.65.2.235 |issn = 0022-1295 |pmid = 235007 |pmc=2214869}}</ref> Having easily replaced stalk eyes (some lobsters) or retinae (some spiders, such as'' Deinopis''<ref>{{cite web |url = https://australian.museum/learn/animals/spiders/how-spiders-see-the-world/ |title = How spiders see the world | publisher = Australian Museum | access-date = 5 December 2017 | archive-url = https://web.archive.org/web/20170912224747/https://australianmuseum.net.au/how-spiders-see-the-world | archive-date = 12 September 2017 | url-status = live }}</ref>) rarely occurs.

The cephalopod retina does not originate as an outgrowth of the brain, as the vertebrate one does. This difference suggests that vertebrate and cephalopod eyes are not [[homology (biology)|homologous]], but have evolved separately. From an evolutionary perspective, a more complex structure such as the inverted retina can generally come about as a consequence of two alternate processes - an advantageous "good" compromise between competing functional limitations, or as a historical maladaptive relic of the convoluted path of organ evolution and transformation. Vision is an important adaptation in higher vertebrates.

A third view of the "inverted" vertebrate eye is that it combines two benefits - the maintenance of the photoreceptors mentioned above, and the reduction in light intensity necessary to avoid blinding the photoreceptors, which are based on the extremely sensitive eyes of the ancestors of modern hagfish (fish that live in very deep, dark water).<ref>{{cite journal |title=Metabolism and enzyme activities of hagfish from shallow and deep water of the Pacific Ocean |date=June 2011 |volume=159 |issue=2 |pages=182–187 |journal=[[Comparative Biochemistry and Physiology|Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology]] |doi=10.1016/j.cbpa.2011.02.018 |pmid=21356325 |last1=Drazen |first1=J. C. |last2=Yeh |first2=J. |last3=Friedman |first3=J. |last4=Condon |first4=N. }}</ref>

A recent study on the evolutionary purpose for the inverted retina structure from the APS (American Physical Society)<ref>{{Cite journal |last1=Labin |first1=A. M. |last2=Ribak |first2=E. N. |date=2010-04-16 |title=Retinal Glial Cells Enhance Human Vision Acuity |url=https://link.aps.org/doi/10.1103/PhysRevLett.104.158102 |journal=Physical Review Letters |volume=104 |issue=15 |pages=158102 |doi=10.1103/PhysRevLett.104.158102|pmid=20482021 |bibcode=2010PhRvL.104o8102L }}</ref> says that "The directional of glial cells helps increase the clarity of human vision. But we also noticed something rather curious: the colours that best passed through the glial cells were green to red, which the eye needs most for daytime vision. The eye usually receives too much blue—and thus has fewer blue-sensitive cones.

Further computer simulations showed that green and red are concentrated five to ten times more by the glial cells, and into their respective cones, than blue light. Instead, excess blue light gets scattered to the surrounding rods. This optimization is such that color vision during the day is enhanced, while night-time vision suffers very little".