Introduction The retina is the sensory portion of the neuroepithelial tunic. It has ten layers, including the photoreceptor layer, which is an essential component for visualization. The pho- toreceptor layer contains cone and rod cells. Irreversible photoreceptor cell damage from visible light is a complex process that differs among species. Even in a well-studied model such as the rat, the mechanism of light damage is complicated because age, diet, and genetic factors can all influence the susceptibility of the retina to intense light. 1,2 Retinal light damage in rats has been shown to be rhodopsin-mediated and a function of retinal irradiance, wave length, and duration of exposure. 3–5 Noell 6 classified these forms of light damage into two types: type I, in which both the retina and the retinal pig- mented epithelium (RPE) are damaged, and type II, in which the visual cells are primarily affected but there is little or no damage to the RPE. 6 The schedule by which light is administered can also affect the outcome of retinal cell damage. Noell et al. 4 found that high-intensity light caused damage to the rat retina due to hyperthermia. Various his- tological studies have been undertaken on photoreceptor Jpn J Ophthalmol 2009;53:635–639 DOI 10.1007/s10384-009-0721-4 © Japanese Ophthalmological Society 2009 LABORATORY INVESTIGATION Effect of Intermittent Light on Photoreceptor Cells in the Rabbit Retina Arash Esfandiari 1 , Alireza Yousofi 2 , Asghar Dehghan 3 , and Ali Safavi 4 1 Department of Anatomical Sciences, School of Veterinary Medicine, Islamic Azad University, Kazerun Branch, Kazerun, Iran; 2 Department of Pathological Sciences, School of Veterinary Medicine, Islamic Azad University, Kazerun Branch, Kazerun, Iran; 3 Department of Clinical Sciences, School of Veterinary Medicine, Islamic Azad University, Kazerun Branch, Kazerun, Iran; 4 Department of Electron Microscopy, School of Veterinary Medicine, Shiraz University, Shiraz, Iran Abstract Purpose: The objective of the present study was to determine what histological changes occur in the photoreceptor layer of male rabbits exposed to intermittent light of different intensities. Methods: Twelve adult male rabbits were randomly divided into three groups: control group (CON), experimental group I (EXP-1), and experimental group II (EXP-2). The EXP-1 and EXP-2 groups were exposed to a 3-h/3-h light–dark cycle, with light provided by a 60-W or 200-W fluorescent bulb, respec- tively. The eyes of the rabbits were removed, routinely fixed, and studied by transmission electron microscopy. Results: The EXP-1 rabbits showed the least photoreceptor damage, such as only slight vacuolization in the inner segment, but normal appearance in the other layers. The EXP-2 rabbits showed disorganization and distention in the outer segments and sublethal injury in the inner segment, such as vacuolization, mitochondrial distention and disorganization, increased extracellular space, and distention of cisternae in the rough endoplasmic reticulum. In the EXP-2 group, the outer limiting membrane was absent at some sites, and swelling and pyknosis were present. Conclusions: These observations suggest that intermittent high-intensity light causes more damage in the photoreceptor layer of the rabbit retina than does intermittent low-intensity light. Jpn J Ophthalmol 2009;53:635–639 © Japanese Ophthalmological Society 2009 Keywords: light intensity, photoreceptor, rabbit, ultrastructure Received: October 21, 2008 / Accepted: April 13, 2009 Correspondence and reprint requests to: Arash Esfandiari, Depart- ment of Anatomical Sciences, School of Veterinary Medicine, Islamic Azad University, Kazerun Branch, P.O. Box 73135-168, Kazerun 73198-66451, Iran e-mail: esfandiari.arash@gmail.com