Rom J Morphol Embryol 2014, 55(2):257–261 R J M E EVIEW Romanian Journal of R Morphology & Embryology http://www.rjme.ro/

Ocular cells and light: harmony or conflict?

SANDA JURJA1), MIHAELA HÎNCU2), MIHAELA AMELIA DOBRESCU3), ANDREEA ELENA GOLU4), ANDREI THEODOR BĂLĂŞOIU4), MĂLINA COMAN5)

1)Department of Ophthalmology, Faculty of Medicine, “Ovidius” University, Constanta, Romania 2)Department of Histology, Faculty of Medicine, “Ovidius” University, Constanta, Romania 3)Department of Biology, University of Medicine and Pharmacy of Craiova, Romania 4)Research Center for Microscopic Morphology and Immunology, University of Medicine and Pharmacy of Craiova, Romania 5)Department of Histology, Faculty of Medicine, “Lower Danube” University, Galati, Romania

Abstract Vision is based on the sensitivity of the eye to visible rays of the solar spectrum, which allows the recording and transfer of visual information by photoelectric reaction. Any electromagnetic radiation, if sufficiently intense, may cause damages in living tissues. In a changing environment, the aim of this paper is to point out the impact of light radiation on ocular cells, with its phototoxicity potential on eye tissues. In fact, faced with light and oxygen, the eye behaves like an ephemeral aggregate of unstable molecules, like a temporary crystallization threatened with entropia. Keywords: ocular tissues, light, phototoxicity, electromagnetic radiations, ultraviolet radiation.

 Introduction Ocular tissues defend themselves naturally using various mechanisms: filtration, molecular protection (using The mechanism of sight is because the eye tissues enzymes and anti-oxidant agents) or biological renewal are sensitive to the visible rays of the solar spectrum, (either partially or totally by means of reformation of which extends from 380 to 700 nm and allows, by photo- the injured cell population). electric reaction, the recording and transfer of information When capacities for renewal weaken or become conveyed by the spectrum [1]. The eye has several overwhelmed, photo-induced pathologies occur [15]. protective mechanisms against light damage, including The article aims to point out the damaging potential miotic constriction of the pupils, light absorption by on ocular tissues of ambiental UV radiation and visible melanin in the retinal pigment epithelium (RPE), and light, in a changing environment. macular antioxidants, such as lutein and zeaxanthin [2]. Lutein and zeaxanthin are members of the carotenoid  Effects on the cornea family (like beta-carotene) and are the only two carotenoids in the macula [3]. Their concentration is The cornea plays an essential role in the transmission higher in the macula than anywhere else in the body; the and filtration of electromagnetic radiation, which provides retinal tissue levels of these compounds depend on vision. Its transparency is necessary for the transfer of intake and are thus variable. They may protect against all physical frequency elements reflected by the object light damage because they are efficient absorbers of or the eye’s image and focused on the by the blue light [4–6]. visible spectrum. The cornea acts as a filter, particularly All electromagnetic radiation is liable to damage the for certain ultraviolet radiation, some A and B ultraviolet various structures of the eye if its intensity is strong rays. Radiation has a traumatic effect on the cornea, enough [7–10]. However, it is mainly the visible spectrum, producing painful, disabling keratitis, the most spectacular which is responsible for retinal injury [11, 12]. The of which is snow blindness. In some regions, chronic most energetic form of radiation is ultraviolet radiation cases of keratitis are observed, that may lead to blindness. (UV). Prolonged sun exposure causes changes to the posterior Electromagnetic radiations can cause mechanical, tissue of the cornea, liable to lead, in old age, to edema- thermal or chemical reactions in the ocular tissues, type problems. Chronic solar toxicity, caused by prolonged as results of the absorption of radiation of specific exposure to ultraviolet radiation, may lead to pterygium, wavelength; photochemical damage is produced in the climatic droplet keratopathy, squamous metaplasia or ocular tissues and is defined by an unstable absorbent squamous carcinoma [16]. molecule known as chromophore. Absorption of energy We consider that, from all the eye structures, the results in the creation of an unstable molecular state with most damaged one is the cornea. It acts as a protection the formation of free radicals [13, 14]. Accumulated screen for the deep structures of the ocular globe, energy is restored in various ways resulting in numerous especially for the crystalline and retina. Prolonged or modifications to tissues due to interaction with the repeated exposure to UV radiation determines important surrounding molecules. alterations in all the microscopic structures of the cornea.

ISSN (print) 1220–0522 ISSN (on-line) 2066–8279 258 Sanda Jurja et al. The anterior epithelium may present lesions such as the UVB rays that have been transmitted by the cornea, pseudo-keratinization up to complete necrosis and whereas visible rays pass through the lens to act on the ulceration. The corneal stroma becomes the location of retina. Short wavelengths in blue and near ultraviolet are an inflammatory associated infiltrate edema, rich in the most dangerous for the crystalline lens. Cumulative lymphocytes and macrophages, and of a network of chronic exposure to ultraviolet radiation causes an angiogenesis vessels [17, 18] All these microscopic increase of chromophores in the crystalline, with coloring alterations determine the disarrangement of the conjunctive of the nucleus. At this stage, the crystalline completely network of the corneal stroma, with major effects upon absorbs short wavelengths, thus protecting the retina. the image display on the retina. The action mechanisms of However, continuous absorption of these shortwave the UV radiation are complex and incompletely clarified. It lengths alters structural proteins of this body with a is considered that the UV may have a direct phototoxic reduction in enzymatic activity. Radiant energy as action upon some cellular and tissular components, and external agent generates free radicals. These highly an indirect action through the reactive oxygen species reactive free radicals can lead to the damage of lens and of oxidative stress inside the tissues [19–21]. fibers. Peroxidation of lens fiber plasma or lens fiber Epidermoid carcinoma of the bulbar conjunctiva is plasma membrane lipids has been suggested as a factor the only cancer associated with the ultraviolet radiation. contributing to lens opacification. In the process of lipid Its frequency is increased in tropical and subtropical peroxidation, the oxidizing agent removes a hydrogen regions. atom from the polyunsaturated fatty acid, forming a In mountain regions, exposure to radiation is about fatty acid radical, which, in turn, attacks molecular four times higher and affects the fragile or poorly oxygen, forming a lipid peroxy radical. This reaction protected cornea, causing a spectacular effect, which may propagate the chain, leading to the formation of lipid occurs after six to 24 hours latency. The cornea is peroxide (LOOH), which eventually can react further to scattered with small ulcers on its epithelium. This pheno- yield malondialdehyde (MDA), a potent cross-linking menon is particularly painful and the condition can agent. It has been hypothesized that MDA cross-reacts become chronic in patients without UV protection whom with membrane lipids and proteins, rendering them are exposed for a long period, generating glare [22]. incapable of performing their normal functions. Funnell et al. (2006) report a case of a patient who Because oxygen tension in and around the lens is developed a corneal perforation secondary to UV radiation normally low, free radical reactions may not involve from a tanning lamp. The patient developed a full-thickness molecular oxygen; instead, the free radicals may react corneal perforation after 30 minutes of tanning lamp directly with molecules. DNA is easily damaged by free exposure without eye protection. The authors believe radicals. Some of the damage to the lens is reparable, that this is the first case of corneal perforation caused by but some may be permanent. Free radicals can also UV radiation [23]. attack the proteins or membrane lipids in the cortex. No Ultraviolet rays irritate the superficial corneal repair mechanisms are known to ameliorate such damage, epithelium, leading to mitosis inhibition, production of which increases with time. In lens fibers, where the protein nuclear fragmentation and loosening of the epithelial synthesis no longer takes place, free radical damage layer. Phototoxic effects occur at all levels of the may lead to polymerization and cross-linking of lipids cornea, including the stroma and endothelium. and proteins, resulting in an increase in the water- At the cornea, the UV transmission decreases away insoluble protein content [27]. from the center, fact explained by the scattering and the The lens is equipped with several enzymes that protect absorbance phenomena. The endothelial cells (cells which against free radical or oxygen damage. These enzymes line the back of the cornea) are less numerous in the include glutathione peroxidase, catalase, and superoxide center of the cornea, therefore receive a higher dose of dismutase. Superoxide dismutase catalyzes the destruction UV than those in the peripheral cornea. This is consistent of the superoxide anion, O - and produces hydrogen with the observation that endothelial cells in the corneal 2 peroxide (H O ): 2O - + 2H+ → H O + O . Catalase periphery present less oxidative DNA damage than 2 2 2 2 2 2 may break down the peroxide by the reaction: 2H O → those in the central cornea [24]. 2 2 2H2O + O2. Glutathione peroxidase catalyzes the reaction of  Effects on the crystalline lens glutathione (GSH) and lipid peroxide (LOOH), resulting Experimental research suggests that the lens is glutathione disulfide (GSSG) and lipid hydroxide (LOH): susceptible to UV radiation exposure. Epidemiologic 2GSH + LOOH → GSSG + LOH + H2O. evidence indicates that long-term exposure to is The glutathione disulfide (GSSG) is then reconverted associated with increased risk of cortical cataract [25, to glutathione (GSH) by glutathione reductase, using the 26]. Although sunlight exposure accounts for only about pyridine nucleotide NADPH (nicotinamide adenine 10% of the total risk of cortical cataract in the general dinucleotide phosphate) provided by the acid form of population, this risk is avoidable. Since exposure to UV monophosphate (HMP) shunt as the reducing agent: radiation can produce other morbidity, avoiding excessive GSSG + NADPH + H+ → 2GSH + NADP+. Thus, sunlight exposure should be encouraged. UV-absorbing glutathione acts indirectly as a major free radical corrective lenses and nonprescription sunglasses decrease scavenger in the lens. In addition, both vitamin E and UV transmission by more than 80%, and wearing a hat ascorbic acid are present in the lens. Each of these with a brim decreases ocular sun exposure by 30–50%. substances can act as a free radical scavenger and thus UVA rays are absorbed entirely by the lens, as are protect against oxidative damage. The lens stores the cells

Ocular cells and light: harmony or conflict? 259 throughout its lifetime, so that this translates by gradual dot, often surrounded by a pigment halo. Mild cases, opacification. The result is that brown nuclear cataract however, often have no fundus changes. After approxi- forms [28]. mately two weeks, a small, reddish, well-circumscribed, 100–200 μm lamellar hole or depression may evolve. This  Effects on the retina lesion may lie at or adjacent to the fovea and is usually permanent. Fluorescein angiography reveals leakage in This is the ocular tissue that is most exposed to the early stages and window defects in late stages. possibly damaging effects of luminous radiation. The It is theorized that solar retinopathy is caused by a luminous radiation that acts to form images is received photochemical injury, perhaps thermally enhanced. The and then transmitted to the retina, after the photons have extent of the damage depends on the duration of the crossed through all of the eye’s transparent media. The exposure. Histopathological studies have shown RPE effects on the retina are photochemical in type and occur damage. No beneficial treatment exists, and prevention in the photoreceptors and the pigmented epithelium. Photo- by education is critically important. chemical injury occurs due to biochemical reactions that Due to light exposure and to abundant presence of cause retinal tissue destruction without elevation of oxygen (linked to particularly important blood flows in temperature. Photochemical injury is the result of light the choroid), the retina is particularly exposed to the energy being transferred to a molecule and this excess release of “free radicals”. The latter are eliminated energy leads to reactions that produce tissue damage. under normal circumstances but their accumulation may For photochemical reactions to take place, the energy of lead to toxic reactions [32]. The lipidic membranes of the photon must exceed a certain threshold. Reactions that photoreceptor cells (cones and rods) are the main target take place can include oxidation, photoisomerization, of the newly created free radicals. However, numerous photochemical cleavage, and electrocyclic reactions defense mechanisms exist normally in the retina. First, [29–31]. Each of these may cause damage directly or there is extremely rapid renewal of photoreceptive cells, indirectly through formation of reactive molecules such particularly of their external segment and of the as lipofuscin, which are photoreactive. Such changes are molecules of the discs of which they are comprised. seen primarily at the level of the outer segments of the Combined with this, is enzymatic restoration of the photoreceptors, which are more sensitive than the inner injured molecules. Finally, the retina has its own defense segments. Solar retinopathy is an example of photo- mechanisms, based on the presence of the melanin. chemical injury. Solar retinopathy, also known as foveo- Melanin is a photon trap, capable of eliminating free macular retinitis, eclipse retinopathy or solar retinitis, is radicals. However, it is gradually reduced with age, by photochemical retinal injury caused by direct or indirect 50% between the ages of 24 and 72 years. The secretion viewing of the sun; it usually occurs after viewing a solar of hypophyseal prolactin is liable to protect the pigmented eclipse or gazing directly at the sun. The damage is felt layer to a certain extent. The cumulative effects of this to be secondary to visible blue light and shorter wave- lengths of UVA or near-UV radiation. Younger patients radiation over a lifetime are an additional factor of with cleaner lenses and patients taking drugs that photo- aggravation. If the defense mechanisms against free sensitize the eye, including tetracycline and psoralens, radicals are diminished, as in albinism or in certain are at a higher risk of solar retinopathy, compared to hereditary-degenerative disorders, such as retinitis patients with high refractive errors and dark fundus pigmentosa, the effects of short wavelength radiation pigmentation, whom are at a slightly lower risk. Patients are a real threat. Weale put forward hypothesis that complain of decreased vision, central scotomata, dys- natural degeneration of the retina may be the consequence chromatopsia, metamorphopsia, micropsia, and frontal of an excess of UV light, accelerated perhaps by genetic or temporal headache within hours of exposure. Visual factors [32]. Some retinal diseases are proof of light acuity is typically reduced to 20/25–20/100, but may be aggression. Such is the case with snow erypthropsia (red worse depending on exposure. Most patients recover vision) after extended exposure when objects appear to within 3–6 months, with vision returning to the level of be red colored. 20/20–20/40, but residual metamorphopsia and para- Although there is much speculation that ambient central scotomata may remain. exposure to UV radiation or visible light is a potential The anterior structures of the eye (cornea, lens) absorb cause of retinal toxicity or degeneration, further study much of the UV radiation of the optical spectrum; and documentation are required. despite this fact, the 315–400 nm UVA band penetrates Non-ionizing radiation generated by the sun is a into the retina. Natural sources (such as the sun) emit major factor in our environment. The eye is a privileged UV photons in long durations which do not result in energy receptor of light. This specific sensitivity explains the confinement in the retina, and thus do not produce thermal fragility of the ocular tissue with regard to the thermo- or mechanical damage, but are capable of inducing luminous aggression that is responsible for the occurrence photochemical damage. Some drugs, such as antibiotics, of various ophthalmological injuries, specifically chorio- non-steroidal anti-inflammatory drugs or psychotherapeutic retinal injuries. The damaging effects to ocular structures agents may act as photosensitizers that promote retinal UV are linked not only to the absorption capacity of the damage, if they have sufficient retinal penetration [26]. various wavelengths, but also to the energy threshold The fundus findings are variable and usually bilateral. and the duration of the emission of the radiation. The The characteristic finding in the first few days after cumulative pathogenic role of light in the appearance of exposure is a yellow-white spot in the fovea, which certain cellular degeneration is suspected. The charact- subsequently is replaced after several days by a reddish eristics of transmission and absorption by the ocular tissue

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Corresponding author Sanda Jurja, MD, Department of Ophthalmology, Faculty of Medicine, “Ovidius” University, Aleea Universităţii Campus 2, 900470 Constanţa, Romania; Phone/Fax +40241–615 048, e-mail: [email protected]

Received: February 24, 2014

Accepted: June 10, 2014