The important factors for UV sensitivity in humans are considered to be the skin pigmentation and the epidermal thickness. In this study on 73 Caucasians (age 20-85 years), we investigated in UV unexposed buttock skin the relationship between the UV sensitivity and constitutive skin pigmentation and thickness of the stratum corneum and the cellular part of the epidermis, in 34 normal people and in 39 skin cancer patients (20) patients with cutaneous malignant melanoma and 19 patients with basal cell carcinoma of the skin). Skin pigmentation was measured by skin reflectance spectroscopy, and UV sensitivity by phototest with a solar simulator. Thicknesses of the stratum corneum and the cellular part of the epidermis were determined by light microscopic evaluation of skin biopsies from the phototest areas. We found that epidermal thickness was independent of skin type and was not correlated to constitutive skin pigmentation. Thickness of the stratum corneum was statistically not different in normal persons and in skin cancer patients (P = 0.41) and was independent of gender (P = 0.61) and age (P = 0.56), while thickness of the cellular epidermis decreased with age (P < 0.01). Stratum corneum thickness was found to be of minor importance for the constitutive UV sensitivity (accounting for on average 11% of the total photoprotection), which was mainly determined by the constitutive skin pigmentation (goodness-of-fit for correlation r = 0.83). A theoretical model for the relationship of UV dose to induction of clinical erythema grade and skin pigmentation and thickness of the stratum corneum was developed. Objective measurements of skin pigmentation in UV unexposed skin by skin reflectance spectroscopy in Caucasians, normal people and people with cutaneous malignant melanoma and basal cell carcinoma of the skin predicts the constitutive UV sensitivity with a high degree of precision.

It is found that thickness measurements of the stratum corneum and the cellular epidermis are reliably performed on cryostatic cut sections stained with haematoxylin-eosin.

Urocanic acid (UCA), present in the stratum corneum as trans-UCA, absorbs ultraviolet (UV) radiation and isomerizes to cis-UCA. Cis-UCA has been demonstrated to initiate suppression of selected immune responses in several experimental systems. Topical application of UCA-containing products reduces UV-induced erythema, but a role for endogenous UCA in photoprotection has not been reported. In this study the relationship between UCA isomers, pigmentation, minimal erythema dose (MED), and stratum corneum thickness was investigated. Pigmentation, concentration of total UCA, and the percentage present as the cis-isomer was measured in 36 healthy subjects, skin type I-IV, at six UV-exposed and nonexposed body sites: forehead, chest, back, outer upper arm, inner upper arm, and buttock. The MED was determined by phototesting on buttock skin, and a punch biopsy for measurement of stratum corneum thickness was taken adjacent to the site of the phototest. The percentage of cis-UCA was significantly higher in UV-exposed than on nonexposed areas. A small intraindividual variation in total UCA was found, being high on the buttock and the arm, lowest on the forehead. The subject to subject variation of total UCA was considerable at all body sites. No correlation was found between total UCA and MED, skin type, pigmentation, or stratum corneum thickness.

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