Tapetum lucidum

Tapetum lucidum

Overview


The tapetum lucidum is a layer of tissue in the eye
Eye
Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement...

 of many vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

 animals.

It lies immediately behind or sometimes within the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...

. It reflects
Reflection (physics)
Reflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...

 visible light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

 back through the retina, increasing the light available to the photoreceptors. This improves vision in low-light conditions, but can cause the perceived image to be blurry from the interference of the reflected light.
Discussion
Ask a question about 'Tapetum lucidum'
Start a new discussion about 'Tapetum lucidum'
Answer questions from other users
Full Discussion Forum
 
Unanswered Questions
Recent Discussions
Encyclopedia


The tapetum lucidum is a layer of tissue in the eye
Eye
Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement...

 of many vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

 animals.

It lies immediately behind or sometimes within the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...

. It reflects
Reflection (physics)
Reflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...

 visible light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

 back through the retina, increasing the light available to the photoreceptors. This improves vision in low-light conditions, but can cause the perceived image to be blurry from the interference of the reflected light. The tapetum lucidum contributes to the superior night vision
Night vision
Night vision is the ability to see in low light conditions. Whether by biological or technological means, night vision is made possible by a combination of two approaches: sufficient spectral range, and sufficient intensity range...

 of some animals. Many of these animals are nocturnal, especially carnivore
Carnivore
A carnivore meaning 'meat eater' is an organism that derives its energy and nutrient requirements from a diet consisting mainly or exclusively of animal tissue, whether through predation or scavenging...

s that hunt their prey at night, while others are deep sea
Deep sea
The deep sea, or deep layer, is the lowest layer in the ocean, existing below the thermocline and above the seabed, at a depth of 1000 fathoms or more. Little or no light penetrates this part of the ocean and most of the organisms that live there rely for subsistence on falling organic matter...

 animals.

Eyeshine



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
Pupil
The pupil is a hole located in the center of the iris of the eye that allows light to enter the retina. It appears black because most of the light entering the pupil is absorbed by the tissues inside the eye. In humans the pupil is round, but other species, such as some cats, have slit pupils. In...

 appears to glow. Eyeshine can be seen in many animals, in nature and in flash
Flash (photography)
A flash is a device used in photography producing a flash of artificial light at a color temperature of about 5500 K to help illuminate a scene. A major purpose of a flash is to illuminate a dark scene. Other uses are capturing quickly moving objects or changing the quality of light...

 photograph
Photograph
A photograph is an image created by light falling on a light-sensitive surface, usually photographic film or an electronic imager such as a CCD or a CMOS chip. Most photographs are created using a camera, which uses a lens to focus the scene's visible wavelengths of light into a reproduction of...

s. In low light, a hand-held flashlight is sufficient to produce eyeshine that is highly visible to humans (despite our inferior night vision
Night vision
Night vision is the ability to see in low light conditions. Whether by biological or technological means, night vision is made possible by a combination of two approaches: sufficient spectral range, and sufficient intensity range...

); this technique, spotlighting
Spotlighting
Spotlighting or Lamping is a method of hunting nocturnal animals using off-road vehicles and high-powered lights, spotlights, lamps or flashlights, that makes special use of the eyeshine revealed by many animal species. A further important aspect is that many animals Spotlighting or Lamping (also...

, is used by naturalists and hunters to search for animals at night. Eyeshine occurs in a wide variety of color
Color
Color or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...

s including white
White
White is a color, the perception of which is evoked by light that stimulates all three types of color sensitive cone cells in the human eye in nearly equal amounts and with high brightness compared to the surroundings. A white visual stimulation will be void of hue and grayness.White light can be...

, blue
Blue
Blue is a colour, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 440–490 nm. It is considered one of the additive primary colours. On the HSV Colour Wheel, the complement of blue is yellow; that is, a colour corresponding to an equal...

, green
Green
Green is a color, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 520–570 nanometres. In the subtractive color system, it is not a primary color, but is created out of a mixture of yellow and blue, or yellow and cyan; it is considered...

, yellow
Yellow
Yellow is the color evoked by light that stimulates both the L and M cone cells of the retina about equally, with no significant stimulation of the S cone cells. Light with a wavelength of 570–590 nm is yellow, as is light with a suitable mixture of red and green...

, pink
Pink
Pink is a mixture of red and white. Commonly used for Valentine's Day and Easter, pink is sometimes referred to as "the color of love." The use of the word for the color known today as pink was first recorded in the late 17th century....

 and red
Red
Red is any of a number of similar colors evoked by light consisting predominantly of the longest wavelengths of light discernible by the human eye, in the wavelength range of roughly 630–740 nm. Longer wavelengths than this are called infrared , and cannot be seen by the naked eye...

. However, because eyeshine is a form of iridescence
Iridescence
Iridescence is generally known as the property of certain surfaces which appear to change color as the angle of view or the angle of illumination changes...

, 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, especially walleye
Walleye
Walleye is a freshwater perciform fish native to most of Canada and to the northern United States. It is a North American close relative of the European pikeperch...

; blue eyeshine occurs in many mammals such as horse
Horse
The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...

s; yellow eyeshine occurs in mammals such as cats, dogs, and raccoons; and red eyeshine occurs in rodents, opossums and bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

s.

The human eye has no tapetum lucidum, hence no eyeshine. However, in humans and animals two effects can occur that may resemble eyeshine: leukocoria
Leukocoria
Leukocoria is an abnormal white reflection from the retina of the eye. Leukocoria resembles eyeshine, but leukocoria can occur in humans and other animals that lack eyeshine because their retina lacks a tapetum lucidum....

(white shine, indicative of abnormalities including cataract
Cataract
A cataract is a clouding that develops in the crystalline lens of the eye or in its envelope, varying in degree from slight to complete opacity and obstructing the passage of light...

s, cancer
Cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...

s, and other problems) and red-eye effect
Red-eye effect
The red-eye effect in photography is the common appearance of red pupils in color photographs of eyes. It occurs when using a photographic flash very close to the camera lens , in ambient low light. The effect appears in the eyes of humans and animals that have no tapetum lucidum, hence no...

.

Blue-eyed cats and dogs




Cats and dogs with blue eyes (see Eye color
Eye color
Eye color is a polygenic phenotypic character and is determined by two distinct factors: the pigmentation of the eye's iris and the frequency-dependence of the scattering of light by the turbid medium in the stroma of the iris....

) may display both eyeshine and red-eye effect. Both species have a tapetum lucidum, so their pupils may display eyeshine. In flash
Flash (photography)
A flash is a device used in photography producing a flash of artificial light at a color temperature of about 5500 K to help illuminate a scene. A major purpose of a flash is to illuminate a dark scene. Other uses are capturing quickly moving objects or changing the quality of light...

 color photographs, however, individuals with blue eyes may also display a distinctive red eyeshine. Individuals with heterochromia
Heterochromia
In anatomy, heterochromia refers to a difference in coloration, usually of the iris but also of hair or skin. Heterochromia is a result of the relative excess or lack of melanin...

 may display red eyeshine in the blue eye and "normal" yellow / green / blue / white eyeshine in the other eye. These include odd-eyed cat
Odd-eyed cat
An odd-eyed cat is a cat with one blue eye and one green, yellow or brown eye. It is a feline form of complete heterochromia, a condition which occurs in some other animals...

s 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.

Classification


A classification
Biological classification
Biological classification, or scientific classification in biology, is a method to group and categorize organisms by biological type, such as genus or species. Biological classification is part of scientific taxonomy....

 of anatomical variants of tapeta lucida defines 4 types:
  • Retinal tapetum, as seen in teleosts, crocodile
    Crocodile
    A crocodile is any species belonging to the family Crocodylidae . The term can also be used more loosely to include all extant members of the order Crocodilia: i.e...

    s, marsupial
    Marsupial
    Marsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central...

    s and fruit bat
    Fruit Bat
    Fruit Bat can refer to:* Megabats, a species of bat which eats fruit* Les "Fruitbat" Carter, guitarist of Carter the Unstoppable Sex Machine* Fruit Bats , an American band...

    s. The tapetum lucidum is within the retina
    Retina
    The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...

    ; in the other 3 types the tapetum is within the choroid behind the retina.

  • Choroid
    Choroid
    The choroid, also known as the choroidea or choroid coat, is the vascular layer of the eye, containing connective tissue, and lying between the retina and the sclera. The human choroid is thickest at the far extreme rear of the eye , while in the outlying areas it narrows to 0.1 mm...

    al guanine tapetum
    , as seen in elasmobranchii
    Elasmobranchii
    Elasmobranchii is a subclass of Chondrichthyes or cartilaginous fish, that includes the sharks and the rays and skates .-Evolution:...

     (skate
    Skate
    Skates are cartilaginous fish belonging to the family Rajidae in the superorder Batoidea of rays. There are more than 200 described species in 27 genera. There are two subfamilies, Rajinae and Arhynchobatinae ....

    s, rays
    Batoidea
    Batoidea is a superorder of cartilaginous fish commonly known as rays and skates, containing more than 500 described species in thirteen families...

    , and shark
    Shark
    Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....

    s) and chimaeras. The tapetum is a palisade of cells containing stacks of flat hexagonal crystals of guanine
    Guanine
    Guanine is one of the four main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine . In DNA, guanine is paired with cytosine. With the formula C5H5N5O, guanine is a derivative of purine, consisting of a fused pyrimidine-imidazole ring system with...

    .
  • Choroidal tapetum cellulosum, as seen in carnivore
    Carnivore
    A carnivore meaning 'meat eater' is an organism that derives its energy and nutrient requirements from a diet consisting mainly or exclusively of animal tissue, whether through predation or scavenging...

    s, rodent
    Rodent
    Rodentia is an order of mammals also known as rodents, characterised by two continuously growing incisors in the upper and lower jaws which must be kept short by gnawing....

    s and cetacea
    Cetacea
    The order Cetacea includes the marine mammals commonly known as whales, dolphins, and porpoises. Cetus is Latin and is used in biological names to mean "whale"; its original meaning, "large sea animal", was more general. It comes from Ancient Greek , meaning "whale" or "any huge fish or sea...

    . 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 cows, sheep, goat
    Goat
    The domestic goat is a subspecies of goat domesticated from the wild goat of southwest Asia and Eastern Europe. The goat is a member of the Bovidae family and is closely related to the sheep as both are in the goat-antelope subfamily Caprinae. There are over three hundred distinct breeds of...

    s and horse
    Horse
    The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...

    s. The tapetum is an array of extracellular fibers.


The functional differences between these 4 different types of tapeta lucida are not known.

This classification does not include tapeta lucida in birds. Kiwi
Kiwi
Kiwi are flightless birds endemic to New Zealand, in the genus Apteryx and family Apterygidae.At around the size of a domestic chicken, kiwi are by far the smallest living ratites and lay the largest egg in relation to their body size of any species of bird in the world...

s, Stone-curlew
Stone-curlew
The Stone-curlews, also known as Dikkops or Thick-knees are a group of largely tropical birds in the family Burhinidae. Despite the group being classified as waders, most species have a preference for arid or semi-arid habitats...

s, the Boat-billed Heron
Boat-billed Heron
The Boat-billed Heron - colloquially known as the Boatbill - is an atypical member of the heron family, and was formerly thought to be in a monotypic family, Cochlearidae....

, the flightless Kakapo
Kakapo
The Kakapo , Strigops habroptila , also called owl parrot, is a species of large, flightless nocturnal parrot endemic to New Zealand...

 and many Nightjar
Nightjar
Nightjars are medium-sized nocturnal or crepuscular birds with long wings, short legs and very short bills. They are sometimes referred to as goatsuckers from the mistaken belief that they suck milk from goats . Some New World species are named as nighthawks...

s, Owl
Owl
Owls are a group of birds that belong to the order Strigiformes, constituting 200 bird of prey species. Most are solitary and nocturnal, with some exceptions . Owls hunt mostly small mammals, insects, and other birds, although a few species specialize in hunting fish...

s, and other night birds such as the Swallow-tailed Gull
Swallow-tailed Gull
The Swallow-tailed Gull is an equatorial seabird in the gull family Laridae. It is the only species in the genus Creagrus, which derives from the Latin Creagra and the Greek kreourgos which means butcher, also from kreas, meat; according to Jobling it would mean "hook for meat" referring to the...

 also possess a tapetum lucidum This classification also does not include the extraordinary focusing mirror in the eye of the brownsnout spookfish.



Like humans, some animals lack a tapetum lucidum and they usually are diurnal. These include most primates, squirrels, some birds, red kangaroo, and pig. Primates that have a tapetum lucidum include the aye aye and sportive lemur
Sportive lemur
The sportive lemurs are the medium sized primates that make up the Lepilemuridae family. The family consists of only one extant genus, Lepilemur, as well as the extinct genus Megaladapis. They are closely related to the other lemurs and exclusively live on the island of Madagascar...

.

When a tapetum lucidum is present, its location on the eyeball varies with the placement of the eyeball in the head, such that in all cases the tapetum lucidum enhances night vision
Night vision
Night vision is the ability to see in low light conditions. Whether by biological or technological means, night vision is made possible by a combination of two approaches: sufficient spectral range, and sufficient intensity range...

 in the center of the animal's field of view
Field of view
The field of view is the extent of the observable world that is seen at any given moment....

.

Apart from its eyeshine, the tapetum luminum itself has a color. It is often described as iridescent. In tiger
Tiger
The tiger is the largest cat species, reaching a total body length of up to and weighing up to . Their most recognizable feature is a pattern of dark vertical stripes on reddish-orange fur with lighter underparts...

s it is greenish. In ruminant
Ruminant
A ruminant is a mammal of the order Artiodactyla that digests plant-based food by initially softening it within the animal's first compartment of the stomach, principally through bacterial actions, then regurgitating the semi-digested mass, now known as cud, and chewing it again...

s it may be golden green with a blue periphery, or whitish or pale blue with a lavender periphery. In dog
Dog
The domestic dog is a domesticated form of the gray wolf, a member of the Canidae family of the order Carnivora. The term is used for both feral and pet varieties. The dog may have been the first animal to be domesticated, and has been the most widely kept working, hunting, and companion animal in...

s it may be whitish with a blue periphery.

Mechanism



The tapetum lucidum, which is iridescent, reflects light roughly on the interference principles of thin-film optics
Thin-film optics
Thin-film optics is the branch of optics that deals with very thin structured layers of different materials. In order to exhibit thin-film optics, the thickness of the layers of material must be on the order of the wavelengths of visible light...

, as seen in other iridescent tissues. However, the tapetum lucidum cells are leucophores, not iridophores.

The tapetum lucidum is a retroreflector
Retroreflector
A retroreflector is a device or surface that reflects light back to its source with a minimum scattering of light. An electromagnetic wave front is reflected back along a vector that is parallel to but opposite in direction from the wave's source. The device or surface's angle of incidence is...

 of the transparent
Transparency and translucency
In the field of optics, transparency is the physical property of allowing light to pass through a material; translucency only allows light to pass through diffusely. The opposite property is opacity...

 sphere
Sphere
A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...

 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 constructive interference, 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.

Uses


The tapetum lucidum enables an animal with it to see in dimmer light than the animal would otherwise be able to. 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 dogs and search horses at night. Historically, its function was regarded as simply to increase the light intensity of an image on the retina.

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).

Manufactured retroreflectors modeled after a tapetum lucidum are described in numerous patents and today have many uses. The earliest patent, first used in "Catseye"
Cat's eye (road)
The cat's eye is a retroreflective safety device used in road marking and was the first of a range of raised pavement markers. It originated in the UK in 1933 and is today used all over the world....

 brand raised pavement marker
Raised pavement marker
A raised pavement marker is a safety device used on roads. These devices are usually made with plastic, ceramic, or occasionally metal, and come in a variety of shapes and colors. Many varieties include a lens or sheeting that enhance their visibility by reflecting automotive headlights...

s, 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
Line-of-sight propagation
Line-of-sight propagation refers to electro-magnetic radiation or acoustic wave propagation. Electromagnetic transmission includes light emissions traveling in a straight line...

, is modeled after the combination of tapetum lucidum and bioluminescent "flashlight" in flashlight fish
Flashlight fish
The flashlight fish are a family, the Anomalopidae, of beryciform fish. There are some unrelated fish with similar features, some of which are also called flashlight fish. Notable among these are the deep sea lanternfish, of the family Myctophidae, of which there are over 200 species.Flashlight...

 of the families Anomalopidae and Stomiidae
Stomiidae
Stomiidae is a family of deep-sea ray-finned fish, including the barbeled dragonfishes, stareaters and loosejaws.Stomiids are generally elongated fish with black or near-black bodies, but they are highly variable in form, and are sometimes grouped into multiple different families as a result. The...

 (see Retroreflector
Retroreflector
A retroreflector is a device or surface that reflects light back to its source with a minimum scattering of light. An electromagnetic wave front is reflected back along a vector that is parallel to but opposite in direction from the wave's source. The device or surface's angle of incidence is...

). Some of these fish have been shown to use eyeshine both to detect and to communicate with other flashlight fish.

Pathology


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
Ethambutol
Ethambutol is a bacteriostatic antimycobacterial drug prescribed to treat tuberculosis. It is usually given in combination with other tuberculosis drugs, such as isoniazid, rifampicin and pyrazinamide....

, macrolide
Macrolide
The macrolides are a group of drugs whose activity stems from the presence of a macrolide ring, a large macrocyclic lactone ring to which one or more deoxy sugars, usually cladinose and desosamine, may be attached. The lactone rings are usually 14-, 15-, or 16-membered...

 antibiotics, dithizone
Dithizone
Dithizone is a sulfur-containing organic compound. It is a good ligand, and forms complexes with many metals such as lead and mercury.Dithizone may be prepared by reacting phenylhydrazine with carbon disulfide, followed by reaction with potassium hydroxide....

, antimalarial compounds, some receptor H(sub)2-antagonists, and cardiovascular agents. The disturbance "is attributed to the chelating action which removes zinc from the tapetal cells."

See also


  • Defense mechanism (biology)
  • Emission theory (vision)
    Emission theory (vision)
    Emission theory or extramission theory is the proposal that visual perception is accomplished by rays of light emitted by the eyes. This theory has been replaced by intromission theory, which states that visual perception comes from something representative of the object entering the eyes...

  • Walleye
    Walleye
    Walleye is a freshwater perciform fish native to most of Canada and to the northern United States. It is a North American close relative of the European pikeperch...