Aging and effects of ultraviolet A exposure may be quantified by fluorescence excitation spectroscopy in vivo

The fluorescence properties of skin chromophores such as tryptophan and col lagen cross-links might be useful markers of aging and photoaging. As the f luorescence of pepsin-digestible collagen cross-links was found to increase with aging and decrease with photoaging we investigated the characteristic s of this dependence. In vivo fluorescence excitation spectra (emission at 380 nm) of SKH hairless mouse model skin are characterized by two bands cen tered near 295 nm and 335 nm due, respectively, to epidermal tryptophan moi eties and pepsin-digestible collagen cross-links, Several groups of hairles s mice were followed over a period of 18 mo to document changes in skin flu orescence with aging. Other groups of animals were exposed to either broad band or narrowband ultraviolet A radiation to determine the effects of ultr aviolet A exposure on the fluorescence of the dermal collagen cross-links a nd to determine an action spectrum for the induced changes. We also found t hat the intensity of pepsin-digestible collagen cross-links in vivo increas es linearly with age and that the fluorescence of epidermal tryptophan decr eases linearly with age. We found that the fluorescence of pepsin-digestibl e collagen cross-links decreases immediately following exposure to ultravio let A whereas epidermal tryptophan fluorescence increases. Both changes wer e dose dependent but the increase in tryptophan fluorescence occurred exclu sively in young animals (2-6 mo old). We found that the ultraviolet-induced fluorescence decrease of pepsin-digestible collagen cross-links is wavelen gth specific. The action spectrum for the ultraviolet A effect on the in vi vo fluorescence of pepsin-digestible collagen cross-links shows a distinct maximum at 335 nm that corresponds to the maximum in the fluorescence excit ation spectrum due to pepsin-digestible collagen cross-links. Our results s eem to indicate that in vivo fluorescence of epidermal tryptophan moieties and collagen cross-links in the dermal matrix may serve as markers for skin aging, for photoaging, and for immediate assessment of exposure to ultravi olet A radiation.