Original articleSpectral transmission of the pig lens: Effect of ultraviolet A + B radiationTransmission spectrale du cristallin du porc : l’effet du rayonnement ultraviolet A + B
Introduction
Numerous studies state that ultraviolet radiation (UV) is one of the factors that most influences the loss of transparency of the crystalline lens (cataracts) in both humans and animals [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. Several studies with rats [5], [6], [7], [8], [9], [10], [11] and rabbits [13] show the influence UV radiation bears on the development of cataracts.
For different reasons, swine ophthalmology has historically received little attention [14]. Consequently, there are few studies on the porcine eye in the literature and fewer focusing on its optical properties. However, currently a greater interest has emerged in the study of the eye of the pig because it is used as a model in investigation [15], [16], [17] since it does not have the ethical and economic restrictions of other species and it shares many similarities with the human eye. The porcine eye is phylogenetically close to the human eye, since, for example, it does not have a tapetum lucidum, it has holangiotic retinal vascularisation, it has cones on the external retina, and the thickness, shape and size of the sclera is similar to that of the human lens [18]. Thus, some inferences may be made to the human lens.
In the human eye, it is complicated to determine the influence of the UV radiation precisely on the development of opacities as it is difficult to isolate its effect from other factors that can also bear an influence. Nonetheless, some epidemiologic [1], [2] studies have been performed in humans with a view to analysing the effect that exposure to sunlight, especially ultraviolet B (UVB) radiation, has on crystalline lens opacification. Sunlight is the principal source of ultraviolet radiation for most of the world's population. This fact now has considerable interest in the era of ozone depletion and potential for even greater exposure to UVB. The Chesapeake Bay Waterman Study [1] was the first investigation to develop a detailed model of personal ocular exposure to UVB, and correlate it with a detailed standardised system for cataract assessment. An increased risk of cortical opacity was found with increasing average annual ocular UVB exposure. Likewise, the Beaver Dam Eye Study [2] reveals a significant correlation between exposure to UVB radiation and cortical opacities.
Deduction from animal data to the human situation is always questionable. However, it is the only option for development of empirically based safety recommendations for avoidance of cataract after exposure to UV radiation.
The use of the porcine eye as a research model involves studying its specific characteristics, therefore diverse studies on the pig eye have been performed [18], [19], [20], [21]. Regarding the action of UV radiation on the pig lens, Oriowo et al. [22] analysed in vitro the action spectrum for acute UV cataractogenesis using whole cultured lenses. They used wavebands ranging from 270 to 370 nm and studied the intensity thresholds that produce permanent and reversible damage. Along the same lines, Okuno et al. [23] performed a similar study to determine ultraviolet action spectra for cell killing of primary porcine lens epithelial cells, using a waveband range from 235 to 311 nm. These research works are particularly important for studying cataractogenesis and they show that the waveband between 270 and 315 nm is the most dangerous in general.
However, practically no solar radiation reaches the Earth's surface [24], [25], [26] below 300 nm nor do artificial sources emit below 400 nm [27]. Therefore, in natural conditions, the formation of lens opacities due to UV radiation cannot be as a result of wavelengths under 300 nm more or less.
The purpose of this study is to determine the mean spectral transmission curve of the common pig lens, with a view to defining said curve for the species, and how ultraviolet A + B radiation affects porcine lens transmission. Then we analysed the effect controlled exposure to ultraviolet radiation ranging from 300 to 400 nm had on in vitro porcine lens. This UV radiation band corresponds approximately to that which reaches the Earth's surface from the sun. The effect such radiation produces on the porcine lens will be analysed in two ways: (1) how its spectral transmission varies, and if so what type and quantity of radiation the lens lets through, and (2) if opacities occur and if so, what type.
Finally, we will compare the results with the existing data in the literature on the human lens.
Section snippets
Pig lens
We used the lenses of freshly slaughtered common pigs from an industrial abattoir for our experiments. The pigs were slaughtered at six months of age and the eyes were removed at the same time, stowed in suitable recipients in saline solution, and sent to our laboratory five or six hours afterwards where the lenses were then removed. Therefore, all the eyes we used belonged to genetically similar pigs that were fed the same diet and that were slaughtered at the same age: thus, our study was
Results
We measured the transmission of twelve crystalline lenses, randomly selected, to establish the baseline. The Kolmogorov-Smirnov test confirmed that the variable follows a normal distribution (P > 0.50 for any wavelength). As the standard deviations were small for all values of wavelength (λ), we investigated if three measurements would be enough to establish a transmission curve. Fig. 1 shows the mean and the confidence interval from three spectral transmission curves of three porcine lenses. In
Spectral transmission of the common pig lens
The spectral transmission curve of the common pig lens (Fig. 1) shows that this lens lets the whole visible spectrum through with practically no attenuation (about 95 % transmission). However, from the 400 nm wavelength onwards and towards the short wavelengths, an abrupt decline in transmission commences which stops at about 370 nm; from there on the transmission remains constant at about 15 % up to 320 nm where it drops abruptly. This wavelength can be considered as the cut-off point since
Disclosure of interest
The authors declare that they have no conflicts of interest concerning this article.
Funding: partially supported by Cátedra Alcon-Universitat de València.
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