© 2004 American College of Veterinary Ophthalmologists Veterinary Ophthalmology (2004) 7, 1, 11 – 22 Blackwell Publishing Ltd. REVIEW TAPETUM LUCIDUM AMONG VERTEBRATES Comparative morphology of the tapetum lucidum (among selected species) F. J. Ollivier,* D. A. Samuelson, D. E. Brooks, P. A. Lewis, M. E. Kallberg and A. M. Komáromy Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Florida, Gainesville, Florida, USA Abstract Objectives The phenomenon of ‘eye-shine’ is seen in a variety of animal species, and is generally thought to be related to the presence of an intraocular reflecting structure, the tapetum lucidum. The tapetum lucidum is a biologic reflector system that is a common feature in the eyes of vertebrates. It normally functions to provide the light-sensitive retinal cells with a second opportunity for photon-photoreceptor stimulation, thereby enhancing visual sensitivity at low light levels. The tapetum lucidum is presented here according to a classification based on the location, as well as the composition, of this reflective layer. Finally, the physical and chemical properties, as well as the origins of the different tapeta lucida, are discussed and compared. Methods The anatomic and biochemical aspects of the tapetum lucidum in various vertebrates are examined. Morphologic observations were made from paraffin and plastic embedded specimens. Specimens were treated with traditional stains and observed by light and transmission electron microscopy. Results Some species (primates, squirrels, birds, red kangaroo and pig) do not have this structure and they usually are diurnal animals. In vertebrates, the tapetum lucidum exhibits diverse structure, organization and composition. Therefore, the retinal tapetum (teleosts, crocodilians, marsupials, fruit bat), the choroidal guanine tapetum (elasmobranchs), the choroidal tapetum cellulosum (carnivores, rodents, cetacea), and the choroidal tapetum fibrosum (cow, sheep, goat, horse) are described. Conclusions The tapetum lucidum represents a remarkable example of neural cell and tissue specialization as an adaptation to a dim light environment and, despite these differences, all tapetal variants act to increase retinal sensitivity by reflecting light back through the photoreceptor layer. These variations regarding both its location and structure, as well as the choice of reflective material, may represent selective visual adaptations associated with their feeding behavior, in response to the use of specific wavelengths and amount of reflectance required. Key Words: morphology, tapetum lucidum, vertebrates Address communications to: Franck Ollivier Tel.: (352) 392–4700 Fax: (352) 392– 6125 e-mail: ollivierf@mail.vetmed.ufl.edu *Present address: Department of Small Animal Clinical Sciences College of Veterinary Medicine University of Florida Gainesville, FL 32601–0126 USA INTRODUCTION The visual senses include perceptions of light, color, form, space and movement. The light sense, by which light is perceived as such, and gradations in its intensity appreciated, is the most fundamental of the visual senses and it is highly developed in vertebrates. 1 The attainment of a high standard of light sensitivity involves certain structural specializations in the eye. The eyes of animals to which an acute perception of light is a necessity can be differentiated from animals which need excellent visual acuity. From this point of view verte- brates can be divided into three main classes: diurnal animals (adapted to bright light), nocturnal animals (adapted to near- darkness of night), and arrhythmic animals (adapted either to bright or dim illumination). The ocular microanatomy of a nocturnal and a diurnal eye are very different, with compromises needed in the arrhythmic eye. Anatomic differences in light gathering are found in the organization