Tapetum lucidum
203610
225452584
2008-07-13T19:43:34Z
Una Smith
3349439
/* Blue-eyed cats and dogs */ detail
{{otheruses|Tapetum}}
[[Image:Eyeshine-BW-cat.jpg|thumb|right|Cat with eyeshine]]
The '''tapetum lucidum''' ([[Latin]]: "bright tapestry", plural '''tapeta lucida''')<ref>[http://www.archives.nd.edu/cgi-bin/lookup.pl?stem=tapet&ending=um Latin Word Lookup<!-- Bot generated title -->]</ref> is a layer of tissue in the [[eye]] of many [[vertebrate]] animals, that lies immediately behind or sometimes within the [[retina]]. It [[Reflection (physics)|reflects]] visible [[light]] back through the retina, increasing the light available to the [[Photoreceptor cell|photoreceptors]]. This improves vision in low-light conditions, but can cause the perceived image to be blurry from the interference of the reflected light.{{Fact|date=June 2008}}<!-- the TL is a *retroreflector*, so there should be no blur --> The tapetum lucidum contributes to the superior [[night vision]] of some animals. Many of these animals are [[nocturnality|nocturnal]], especially [[carnivore]]s that hunt at night, and their [[prey]]. Others are [[deep sea]] animals. Although some [[primate]]s have a tapetum lucidum, humans do not.
==Eyeshine==
[[Image:raccoon red eye.JPG|thumb|right|In darkness, eyeshine reveals this [[raccoon]]]]
'''Eyeshine''' is a visible effect of the tapetum lucidum. When a light is shone into the eye of an animal having a tapetum lucidum, the [[pupil]] appears to glow. Eyeshine can be seen in many animals, in nature and in [[Flash (photography)|flash]] [[photograph]]s. In low light, a hand-held flashlight is sufficient to produce eyeshine that is highly visible to humans (despite our inferior [[night vision]]); this technique, [[spotlighting]], is used by naturalists and hunters to search for animals at night. Eyeshine occurs in a wide variety of [[color]]s including [[white]], [[blue]], [[green]], [[yellow]], [[pink]] and [[red]]. However, because eyeshine is a form of [[iridescence]], the color varies slightly with the angle at which it is seen and the color of the source light.
White eyeshine occurs in many fish.
Blue eyeshine occurs in many mammals, including [[horse]]s.
Yellow eyeshine occurs in many mammals, including [[cat]]s.
Red eyeshine occurs in [[oppossum]]s.
[[Image:Man and dog.jpg|thumb|right|[[Dog]] has eyeshine, human does not]]
The human eye has no tapetum lucidum, hence no eyeshine. However, in humans and animals two effects can occur that may resemble eyeshine: [[leukocoria]] (white shine) and [[red-eye effect]] (red shine).
===Blue-eyed cats and dogs===
Cats and dogs with blue eyes (see [[Eye color]]) may display both eyeshine and red-eye effect. Both species have a tapetum lucidum, so their pupils may display eyeshine. In [[Flash (photography)|flash]] color photographs, however, individuals with blue eyes may also display a distinctive red eyeshine. Individuals with [[heterochromia]] may display red eyeshine in the blue eye and "normal" yellow / green / blue eyeshine in the other eye. The red-eye effect is independent of the eyeshine: in some photographs of individuals with a tapetum lucidum and heterochromia, the eyeshine is dim yet the pupil of the blue eye still appears red. This is most apparent when the individual is not looking into the camera, because the tapetum ludicum is far less extensive than the retina.
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==Classification==
A [[classification]] of anatomical variants of tapetum lucidum<ref name="Ollivier+2004"/><!-- classification is not Ollivier's; dates from 1967 or before --> defines 4 types:
*''Retinal tapetum'', as seen in [[teleost]]s, [[crocodile]]s, [[marsupial]]s and [[fruit bat]]s. The tapetum lucidum is within the [[retina]]; in the other 3 types the tapetum is within the choroid behind the retina.
*''[[Choroid]]al guanine tapetum'', as seen in [[elasmobranchii]] ([[skate]]s, [[batoidea|rays]], and [[shark]]s). The tapetum is a palisade of cells containing stacks of flat hexagonal crystals of [[guanine]].<ref name="Locket1974">{{cite journal
| author = Locket NA
| title = The choroidal tapetum lucidum of Latimeria chalumnae
| journal = Proc. R. Soc. Lond., B, Biol. Sci.
| volume = 186
| issue = 084
| pages = 281–90
| year = 1974
| month = July
| pmid = 4153107
| doi =
| url =
| issn =
}}</ref>
*''Choroidal tapetum cellulosum'', as seen in [[carnivore]]s, [[rodent]]s and [[cetacea]]. The tapetum consists of layers of cells containing organized, highly refractive crystals. These crystals are diverse in shape and makeup.
*''Choroidal tapetum fibrosum'', as seen in [[cow]]s, [[sheep]], [[goat]]s and [[horse]]s. The tapetum is an array of extracellular fibers.
The functional differences between these 4 different types of tapetum lucidum are not known.<ref name="Ollivier+2004"/>
Like humans, some animals (some primates, squirrels, birds, red kangaroo and pig<ref name="Charnock1896">Orlando Charnock
Bradley, 1896, ''Outlines of Veterinary Anatomy. Part I. The Anterior and Posterior Limbs,'' Baillière, Tindall & Cox, page 224. [http://books.google.com/books?id=ZQOq9yYDK1AC Free full text on Google Books]</ref>) do not have a tapetum lucidum and they usually are [[diurnal]].<ref name="Ollivier+2004">{{cite journal
| author = Ollivier FJ, Samuelson DA, Brooks DE, Lewis PA, Kallberg ME, Komáromy AM
| title = Comparative morphology of the tapetum lucidum (among selected species)
| journal = Vet Ophthalmol
| volume = 7
| issue = 1
| pages = 11–22
| year = 2004
| pmid = 14738502
| doi = 10.1111/j.1463-5224.2004.00318.x
| url = http://www.blackwell-synergy.com/openurl?genre=article&sid=nlm:pubmed&issn=1463-5216&date=2004&volume=7&issue=1&spage=11
}}</ref> When a tapetum lucidum is present, its location on the eyeball varies with the placement of the eyeball in the head:<ref name="Lee1886">Lee, Henry (1886) ''On the Tapetum Lucidum'', Med Chir Trans. 69: 239–245. [http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=2121549 Free full text in Pubmed Central]</ref>, such that in all cases the tapetum lucidum enhances [[night vision]] in the center of the animal's [[field of view]].
Apart from its eyeshine, the tapetum luminum itself has a color. It is often described as iridescent. In [[tiger]]s it is greenish.<ref>Fayrer, Sir Joseph (1889) ''The deadly wild beasts of India'', pages 218–240 in James Knowls (ed)
''The Nineteenth Century: A Monthly Review'', Henry S. King & Co., v.26; page 219. [http://books.google.com/books?id=kTMAAAAAYAAJ Full free text on Google Books]</ref> In [[ruminant]]s it may be golden green with a blue periphery,<ref name="Charnock1896"/> or whitish or pale blue with a lavender periphery. In [[dog]]s it may be whitish with a blue periphery.<ref name="Charnock1896"/>
==Mechanism==
[[Image:Calf-Eye-Posterior-With-Retina-Detached-2005-Oct-13.jpg|thumb|right|Choroid dissected from a calf's eye, tapetum lucidum appearing iridescent blue]]
The tapetum lucidum, which is [[iridescent]], reflects light roughly on the [[interference]] principles of [[thin-film optics]], as seen in other iridescent tissues. However, the tapetum lucidum cells are [[Chromatophore#Iridophores and leucophores|leucophore]]s, not [[Chromatophore#Iridophores and leucophores|iridophore]]s.
The tapetum lucidum is a [[retroreflector]] of the [[transparent]] [[sphere]] type. Because it is a retroreflector, it reflects light directly back along the light path. This serves to match the original and reflected light, thus maintaining the sharpness and contrast of the image on the retina. The tapetum lucidum reflects with [[interference|constructive interference]]<ref name="Locket1974"/>, thus increasing the quantity of light passing through the retina. In the cat, the tapetum lucidum lowers the minimum threshold of vision 6-fold, allowing the cat to see light that is invisible to human eyes.<ref>PMID 14875072</ref>
==Uses==
The tapetum lucidum enables an animal equipped with it to see better in low light than humans do. This is of use to the animal, but it is of use also to humans. Human uses include scanning for reflected eyeshine to detect and identify the species of animals in the dark, and deploying trained [[search dog]]s and [[search and rescue horse|search horse]]s at night. Historically, its function was regarded as simply to increase the light intensity of an image on the retina.<ref>Dunglison, Robley (1854) ''Medical lexicon'', Blanchard and Lea, 903 pages, page 843. [http://books.google.com/books?id=NOCL44dvZrgC Free full text in Google Books]</ref>
Using eyeshine to identify animals in the dark employs not only its color but also several other features. The color corresponds approximately to the type of tapetum lucidum, with some variation between species. Other features include the distance between pupils relative to their size; the height above ground; the manner of blinking (if any); and the movement of the eyeshine (bobbing, weaving, hopping, leaping, climbing, flying).
[[Image:Photostomias2.jpg|thumb|left|Flashlight fish (''Photostomia'', Stomiidae)]]
Manufactured retroreflectors modeled after a tapetum lucidum are described in numerous patents and today have many uses. The earliest patent, first used in [[Cat's eye (road)|"Catseye"]] brand [[raised pavement marker]]s, was inspired by the tapetum lucidum of a cat's eye (see [[Cat's eye (road)]]). A more recent use of retroreflectors, helping to provide secure communications between two stations in [[line of sight]], is modeled after the combination of tapetum lucidum and [[bioluminescent]] "flashlight" in [[flashlight fish]] of the families [[Anomalopidae]] and [[Stomiidae]] (see [[Retroreflector]]). Some of these fish have been shown<ref name="pmid1413559">{{cite journal
| author = Howland HC, Murphy CJ, McCosker JE
| title = Detection of eyeshine by flashlight fishes of the family Anomalopidae
| journal = Vision Res.
| volume = 32
| issue = 4
| pages = 765–9
| year = 1992
| month = April
| pmid = 1413559
| doi = 10.1016/0042-6989(92)90191-K
| url = http://linkinghub.elsevier.com/retrieve/pii/0042-6989(92)90191-K
}}</ref> to use eyeshine both to communicate and to detect other flashlight fish.
==Pathology==
In dogs, certain drugs are known to disturb the precise organization of the crystals of the tapetum lucidum, thus compromise the dog's ability to see in low light. These drugs include [[ethambutol]], [[macrolide]] antibiotics, [[dithizone]], [[antimalarial compound]]s, some receptor H(sub)2-antagonists, and cardiovascular agents. The disturbance "is attributed to the chelating action which removes zinc from the tapetal cells."<ref>
Target Organ Toxicity
By Gerald M. Cohen, 1986
CRC Press,240 pages
ISBN:0849357764
[http://books.google.com/books?id=CQmPlVTR5_g]
page 121-122
</ref>
==See also==
*[[Defense mechanism (biology)]]
*[[Walleye]]
==External links==
{{Commonscat|Eyeshine}}
==References==
{{reflist}}
{{Eye}}
{{Photography}}
[[Category:Eye]]
[[Category:Animal anatomy]]
[[Category:Animal communication]]
[[Category:Science of photography]]
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