Choroid is a thin layer of the eye between the retina and sclera containing many blood vessels. The tapetum is an additional cell layer in the choroid that envelopes the retina from behind and, like the choroid, is densly vascularized. It is assumed to function as an intensifier of light impulses, which are reflected back by the tapetum, passing again through the retina and thus enhancing the dim light. There are two types of tapetum: tapetum lucidum cellulosum and tapetum lucidum fibrosum. The color of the light reflected by different tapetums varies among groups of animals. Carnivores and ungulates reflect green to blue light, while marsupials and some primates reflect yellow to red.4 Lack of tapetum is not an abnormality; it is absent in all owls. Normal number of tapetal cells are present at birth; however, progressive degeneration of the tapetal cell layer occurs later in life. Tapetal degeneration occurs especially in dogs with white or dilute hair color such as the Dalmatian, white cats, some merle collies, and some small breed dogs. The Chihuahua has very small tapetal areas. An autosomal, recessively hereditary tapetal degeneration has been described in the Beagle.
Zinc deficiency has been shown to result in a variety of eye abnormalities. This trace element influences cell metabolism through a variety of mechanisms and appears to play an integral role in maintaining normal eye function. The element is present in high concentrations in eye tissue, particularly in the retina and choroid, and is believed to interact with taurine and vitamin A. It also regulates the light-rhodopsin reaction and serves as an antioxidant. Both taurine and zinc are localized on the periphery of the tapetal rods and contribute to the stability of the membrane. Cats fed on taurine-depleted dog food often develop tapetal degeneration.
Zinc pyrithione (ZPT) is an antibacterial and antifungal agent that is often utilized in antidandruff shampoos. If ingested, it may cause adverse effects. In some experimental studies the administration of ZPT to cats resulted in ocular lesions characterized by tapetal degeneration and atrophy. ZPT also caused ocular lesions in Beagle dogs. The initial lesions consisted of degeneration of tapetal cells, soon accompanied by intense tapetal inflammation which progressed to inflammation of the retina, bleeding, retinal detachment, and blindness.3
Note: The bright glowing reflection of the tapetum lucidum should not be confused with the red or orange eye-shine that is captured in photographs of primates, including humans, and in the photographs of many owls. In this case, the color comes from the reflection of light off blood vessels in the rear wall of the eyeball and contributes nothing to the visual sensitivity of the eye.5
- Comparative Ophthalmology Notes: Chapter 12 - Retina, Optic Nerve, Vitreous Body (www.vetmed.ucdavis.edu)
- Bruce H. Grahn, DVM, Phyllis G. Paterson, PhD, Katherine T. Gottschall-Pass, PhD and Zhen Zhang, MSc. Zinc and the Eye. Journal of the American College of Nutrition, Vol. 20, No. 2, 106-118 (2001), American College of Nutrition
- Ying Shih, Jyh-Myng Zen, Annamalai Senthil Kumarb and Pei-Yen Chen. Flow injection analysis of zinc pyrithione in hair care products on a cobalt phthalocyanine modified 'screen-printed carbon electrode. Talanta Volume 62, Issue 5, 19 April 2004, Pages 912-917
- Friderun Ankel-Simons. Primate Anatomy: An Introduction.
- Wayne Lynch. Owls of the United States and Canada