Tapetum lucidum

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The dark blue, teal, and gold tapetum lucidum from the eye of a cow.
Eye of mongrel dog with strong tapetal reflex.

The tapetum lucidum /təˈptəm/ (Latin: "bright tapestry; coverlet", plural tapeta lucida)[1] is a layer of tissue in the eye of many vertebrates. Lying immediately behind the retina, it reflects visible light back through the retina, increasing the light available to the photoreceptors, though blurring the initial image of the light on focus. The tapetum lucidum contributes to the superior night vision of some animals. Many of these animals are nocturnal, especially carnivores, while others are deep sea animals.

Similar adaptations occur in some species of spiders.[2] Most primates, including humans, lack a tapetum lucidum, and compensate for this by perceptive recognition methods.

Function and mechanism[edit]

Choroid dissected from a calf's eye, tapetum lucidum appearing iridescent blue

The tapetum lucidum enables animals to see in dimmer light than would otherwise be available to them. 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 leucophores, not iridophores.

The tapetum lucidum functions as a retroreflector which 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 constructive interference,[3] thus increasing the quantity of light passing through the retina. In the cat, the tapetum lucidum increases the sensitivity of vision by 44%, allowing the cat to see light that is imperceptible to human eyes.[4]

It has been speculated that some flashlight fish may use eyeshine both to detect and to communicate with other flashlight fish.[5]

Classification[edit]

A classification of anatomical variants of tapeta lucida[6] defines 4 types:

The functional differences between these four different types of tapeta lucida are not known.[6]

This classification does not include tapeta lucida in birds. Kiwis, stone-curlews, the boat-billed heron, the flightless kakapo and many nightjars, owls, and other night birds such as the swallow-tailed gull also possess a tapetum lucidum.[8] This classification also does not include the extraordinary focusing mirror in the eye of the brownsnout spookfish.[9]

Like humans, some animals lack a tapetum lucidum and they usually are diurnal.[6] These include most primates, squirrels, some birds, red kangaroo, and pig.[10] Among primates only the strepsirrhines, with the exception of several diurnal Eulemur species, have a tapetum lucidum.[11]

When a tapetum lucidum is present, its location on the eyeball varies with the placement of the eyeball in the head,[12] 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 lucidum itself has a color. It is often described as iridescent. In tigers it is greenish.[13] In ruminants it may be golden green with a blue periphery,[10] or whitish or pale blue with a lavender periphery. In dogs it may be whitish with a blue periphery.[10]

Eyeshine[edit]

Reflection of camera flash from the tapetum lucidum

Eyeshine is a visible effect of the tapetum lucidum. When light shines 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 photographs. In low light, a hand-held flashlight is sufficient to produce eyeshine that is highly visible to humans (despite our inferior night vision). Eyeshine occurs in a wide variety of colors including white, blue, green, yellow, pink and red. However, since eyeshine is a type of iridescence, the color varies with the angle at which it is seen and the minerals which make up the reflective tapetum-lucidum crystals.

White eyeshine occurs in many fish, especially walleye; blue eyeshine occurs in many mammals such as horses; green eyeshine occurs in mammals such as cats, dogs, and raccoons; and red eyeshine occurs in coyote, rodents, opossums and birds.[citation needed]

A three-month-old black Labrador puppy with apparent eye shine

In humans and animals, two effects can occur that may resemble eyeshine: leukocoria (white shine, indicative of abnormalities including cataracts, cancers, and other problems) and red-eye effect.

In blue-eyed cats and dogs[edit]

Odd-eyed cat with eyeshine, plus red-eye effect in one eye

Cats and dogs with a blue eye color may display both eyeshine and red-eye effect. Both species have a tapetum lucidum, so their pupils may display eyeshine. In 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/white eyeshine in the other eye. These include odd-eyed cats and bi-eyed dogs. 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 lucidum is far less extensive than the retina.

In spiders[edit]

Most species of spider also have a tapetum, which is located only in their smaller, lateral eyes; the larger central eyes have no such structure. This consists of reflective crystalline deposits, and is thought to have a similar function to the structure of the same name in vertebrates. Four general patterns can be distinguished in spiders:[14]

Uses by humans[edit]

In darkness, eyeshine reveals this raccoon

Humans use scanning for reflected eyeshine to detect and identify the species of animals in the dark, and deploying trained search dogs and search horses at night, as these animals benefit from improved night vision through this effect.

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).

Artificial tapetum lucidum[edit]

Manufactured retroreflectors modeled after a tapetum lucidum are described in numerous patents and today have many uses. The earliest patent, first used in "Catseye" brand raised pavement markers, was inspired by the tapetum lucidum of a cat's eye. 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[citation needed].

Pathology[edit]

In dogs, certain drugs are known to disturb the precise organization of the crystals of the tapetum lucidum, thus compromising the dog's ability to see in low light. These drugs include ethambutol, macrolide antibiotics, dithizone, antimalarial medications, some receptor H2-antagonists, and cardiovascular agents. The disturbance "is attributed to the chelating action which removes zinc from the tapetal cells."[15]

Gallery[edit]

Traditionally it has been difficult to take retinal images of animals with a tapetum lucidum because ophthalmoscopy devices designed for humans rely on a high level of on-axis illumination.[16] This kind of illumination causes a great deal of reflex, or back-scatter, when it interacts with the tapetum. New devices with variable illumination can make this possible however.

See also[edit]

References[edit]

  1. ^ "Latin Word Lookup". Archives.nd.edu. Retrieved 2014-03-20. 
  2. ^ pages 578-581 ofRuppert, E.E., Fox, R.S., and Barnes, R.D. (2004). "Chelicerata: Araneae". Invertebrate Zoology (7 ed.). Brooks / Cole. pp. 571–584. ISBN 0-03-025982-7. 
  3. ^ a b Locket NA (July 1974). "The choroidal tapetum lucidum of Latimeria chalumnae". Proceedings of the Royal Society B. 186 (84): 281–90. PMID 4153107. doi:10.1098/rspb.1974.0049. Retrieved 2008-09-12. 
  4. ^ GUNTER R, HARDING HG, STILES WS (August 1951). "Spectral reflexion factor of the cat's tapetum". Nature. 168 (4268): 293–4. PMID 14875072. doi:10.1038/168293a0. 
  5. ^ Howland HC, Murphy CJ, McCosker JE (April 1992). "Detection of eyeshine by flashlight fishes of the family Anomalopidae". Vision Res. 32 (4): 765–9. PMID 1413559. doi:10.1016/0042-6989(92)90191-K. Retrieved 2008-09-12. 
  6. ^ a b c Ollivier FJ, Samuelson DA, Brooks DE, Lewis PA, Kallberg ME, Komáromy AM (2004). "Comparative morphology of the tapetum lucidum (among selected species)". Vet Ophthalmol. 7 (1): 11–22. PMID 14738502. doi:10.1111/j.1463-5224.2004.00318.x. Retrieved 2008-09-12. 
  7. ^ Denton, EJ; Nichol, JAC (1964). "The chorioidal tapeta of some cartilaginous fishes (Chondrichthyes)" (PDF). J. Mar. Biol. Ass. U.K.. 44: 219–258. doi:10.1017/S0025315400024760. Retrieved 2011-09-12. 
  8. ^ Gill, Frank, B (2007) "Ornithology", Freeman, New York
  9. ^ Wagner HJ, Douglas RH, Frank TM, Roberts NW, Partridge JC (January 2009). "A novel vertebrate eye using both refractive and reflective optics". Curr. Biol. 19 (2): 108–14. PMID 19110427. doi:10.1016/j.cub.2008.11.061. 
  10. ^ a b c Orlando Charnock Bradley, 1896, Outlines of Veterinary Anatomy. Part I. The Anterior and Posterior Limbs, Baillière, Tindall & Cox, page 224. Free full text on Google Books
  11. ^ Ankel-Simons, Friderun (2007). Primate Anatomy (3rd ed.). Academic Press. p. 375. ISBN 0-12-372576-3. 
  12. ^ Lee, Henry (1886). "On the Tapetum Lucidum". Med Chir Trans. 69: 239–245. PMC 2121549Freely accessible. PMID 20896672. 
  13. ^ 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. Full free text on Google Books
  14. ^ Rainer F. Foelix (1996). Biology of Spiders, 2nd ed. Oxford University Press. pp. 84–85. ISBN 0-19-509594-4. 
  15. ^ Cohen, Gerald D. (1986). Target organ toxicity. Boca Raton: CRC Press. pp. 121–122. ISBN 0-8493-5776-4. 
  16. ^ Slatter's Fundamentals of Veterinary Ophthalmology By David Maggs, Paul Miller, Ron Ofri pp94