Thetapetum lucidum (Latin for 'bright tapestry, coverlet';/təˈpiːtəmˈluːsɪdəm/tə-PEE-təmLOO-sih-dəm;pl.:tapeta lucida)[1] is a layer of tissue in theeye of manyvertebrates and some other animals. Lying immediately behind theretina, it is aretroreflector. Itreflects visiblelight back through the retina, increasing the light available to thephotoreceptors (although slightly blurring the image).
The tapetum lucidum contributes to the superiornight vision of some animals. Many of these animals arenocturnal, especiallycarnivores, while others aredeep-sea animals. Similar adaptations occur in some species ofspiders.[2]Haplorhine primates, including humans, arediurnal and lack a tapetum lucidum.[Note 1]
The presence of a tapetum lucidum enables animals to see in dimmer light than would otherwise be possible. The tapetum lucidum, which isiridescent, reflects light roughly on theinterference principles ofthin-film optics, as seen in other iridescent tissues. However, the tapetum lucidum cells areleucophores, notiridophores.[dubious –discuss]
The tapetum functions as aretroreflector 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 withconstructive interference,[4] thus increasing the quantity of light passing through the retina. In the cat, the tapetum lucidum decreases the absolute threshold for vision, but does not materially changespectral sensitivity.[5] When a tapetum lucidum is present, its location on the eyeball varies with the placement of the eyeball in the head.[6]
Apart from its eyeshine, the tapetum lucidum itself has a color. It is often described as iridescent. Intigers, it is greenish.[7] Inruminants, it may be golden green with a blue periphery,[8] or whitish or pale blue with a lavender periphery. Indogs, it may be whitish with a blue periphery.[8] The color inreindeer changes seasonally, allowing the animals to better avoid predators in low-light winter at the price of blurrier vision.[9]
Aclassification of anatomical variants of tapeta lucida[3] defines four types:
The functional differences between these four structural classes of tapeta lucida are not known.[3]
Humans, likehaplorhine primates, lack a tapetum lucidum as they arediurnal.[3]Strepsirrhine primates are mostly nocturnal and, with the exception of several diurnalEulemur species, have a tapetum lucidum of riboflavin crystals.[12]
In canids, the tapetum lucidum is found in the dorsal half of the eye's fundus. It consists of 9-20 layers of specialized rectangular cells between the choroid and retinal pigment epithelium, thinning towards the periphery. The cells contain zinc-rich rodlets arranged in parallel. The structure appears yellow-green in adults, though blue in puppies until four months of age. Zinc concentration varies among species, with red foxes showing highest levels, followed by Arctic foxes, then domestic dogs. A hereditary zinc-deficiency condition in some beagles results in degenerated tapetal cells with disrupted rodlet arrangement.[13]
The tapetum lucidum in cats is renowned for its brilliance, even inspiring ancient Egyptians to believe it reflected the sun at night. This reflective layer is composed of 15-20 layers of cells arranged in a central pattern. This structure, denser than that of dogs, results in high reflectance, nearly 130 times that of humans. Its color is heterogeneous, varying with age and species due to factors like rodlet spacing, refractive index, and light interactions. Young cats exhibit a blue appearance, which shifts to yellow with age, with adult coloration ranging from light orange to green. While enhancing night vision, increased light scatter within the tapetum slightly compromises visual acuity.[14]
Kiwis,stone-curlews, theboat-billed heron, the flightlesskākāpō, and manynightjars,owls, and other night birds such as theswallow-tailed gull possess a tapetum lucidum.[15] Nightjars use a retinal tapetum lucidum composed of lipids.[16]
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:[17]
Animals without tapetum lucidum include haplorhine primates, squirrels, some birds,red kangaroo, and pigs.[8]
Eyeshine is a visible effect of the tapetum lucidum. When light shines into the eye of an animal having a tapetum lucidum, thepupil appears to glow. Eyeshine can be seen in many animals, in nature, and inflashphotographs. In low light, a hand-held flashlight is sufficient to produce eyeshine that is visible to humans (despite their inferiornight vision). Eyeshine occurs in a wide variety ofcolors includingwhite,blue,green,yellow,pink, andred. However, since eyeshine is a type ofiridescence, the color varies with the angle at which it is seen and the minerals which make up the reflective tapetum lucidum crystals. Individuals withheterochromia may display red eyeshine in the blue eye and other-colored eyeshine in the other eye. These includeodd-eyed cats andbi-eyed dogs.
Although human eyes lack a tapetum lucidum, they still exhibit a weak reflection from thechoroid, as can be seen in photography with thered-eye effect and withnear-infrared eyeshine.[18][19] Another effect in humans and other animals that may resemble eyeshine isleukocoria, which is a white shine indicative of abnormalities such ascataracts andcancers.
Humans can scan for eyeshine to detect and identify the species of animals in the dark and deploysearch dogs andsearch horses at night. The color corresponds approximately to the type of tapetum lucidum, with some variation between species.[This paragraph needs citation(s)]
It has been speculated that someflashlight fish may use eyeshine both to detect and to communicate with other flashlight fish.[20] American scientistNathan H. Lents has proposed that the tapetum lucidum evolved in vertebrates, but not incephalopods, which have a very similar eye, because of the backwards-facing nature of vertebrate photoreceptors. The tapetum boosts photosensitivity under conditions of low illumination, thus compensating for thesuboptimal design of the vertebrate retina.[21]
Traditionally, it has been difficult to take retinal images of animals with a tapetum lucidum becauseophthalmoscopy devices designed for humans rely on a high level of on-axis illumination.[22] This kind of illumination causesbackscatter when it interacts with the tapetum. New devices with variable illumination can make this possible, however.
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 includeethambutol,macrolide antibiotics,dithizone,antimalarial medications, some receptorH2-antagonists, andcardiovascular agents. The disturbance "is attributed to thechelating action which removes zinc from thetapetal cells."[23]
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