Tapetum Lucidum - Classification

Classification

A classification of anatomical variants of tapeta lucida defines 4 types:

• Retinal tapetum, as seen in teleosts, crocodiles, marsupials and fruit bats. The tapetum lucidum is within the retina; in the other 3 types the tapetum is within the choroid behind the retina.

• Choroidal guanine tapetum, as seen in elasmobranchii (skates, rays, and sharks) and chimaeras. The tapetum is a palisade of cells containing stacks of flat hexagonal crystals of guanine.
• Choroidal tapetum cellulosum, as seen in carnivores, rodents 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 cows, sheep, goats and horses. 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. 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 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.

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 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 tigers it is greenish. In ruminants it may be golden green with a blue periphery, or whitish or pale blue with a lavender periphery. In dogs it may be whitish with a blue periphery.