Chronic exposure of Sk‐1 hairless mice to narrow‐band ultraviolet A (320–355 nm)

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Abstract
Several recent investigations collectively suggest that the role of ultraviolet A (UVA) in chronic actinic skin damage may be greater than originally thought. In the present work, the output of a xenon‐arc solar‐simulator passed through a Bausch & Lomb monochromator in conjunction with a 2‐mm Schott WG‐320 filter produced narrow‐band UVA centered at 338 nm, half‐band width 24 nm, Io=3.4±0.3 mW/cm2. We chronically irradiated 10 Sk‐1 albino hairless mice 5 times per week for 18 weeks, starting with 1.25 J/cm2, for 33 irradiation days, sequentially followed by 1.50 J/cm2 (34 days), 1.8 J/cm2 (10 days), 2.0 J/cm2 (22 days) to afford a total UVA dose of 154.3 J/cm2 over 99 irradiation days. Erythema was noted clinically by day 6, which persisted throughout the irradiation. During the irradiation period, some scaling, consistent with mild epidermal hyperplasia was noted during irradiation days 37–56. This response later regressed despite continued chronic irradiation. Hematoxylin and eosin examination immediately after the final irradiation revealed a mild inflammatory response, with some dermal restructuring. At the end of the experiment, no significant signs of epidermal hyperplasia or (pre)malignant lesions were seen, although some stratum corneum thickening was noted. Marked dermal collagen damage and moderate elastosis was also evident. We believe that the observed differences in results reported in previous studies are in large part due to differences in light sources and irradiation protocols.