| Summary: | This thesis examines the photoadaptive response of human skin. Photoadaptation can be divided into two major components: pigmentation and epidermal thickening. These components safeguard the skin against future UV damage. This thesis is based on four main studies. The first study examines the photoadaptive response as a whole; this was found to be present over many weeks, decreasing to normal levels at approximately 10 weeks. The second study then examines the pigmentary component of the photoadaptive response. Following a single dose of UVB, pigmentation, measured by L*, was maximal at Week 1; this pigmentary response subsequently decreased, however, was still present at Week 12. A pigmentary dose-response to UVB was clearly demonstrated. Subjects with red hair bearing homozygous or compound heterozygous MC1R mutations were also studied; these redheads pigmented to a lesser extent compared to non-redhead individuals; however, their rate of pigmentation loss was the same as non-redheaded individuals. The third study then examines the other component of the photoadaptive response: epidermal thickening, using UV Transmission Spectrophotometry. Following UVB exposure, there was an increase in skin darkness (measured by L*), melanosome density (eumelanin and pheomelanin) and epidermal thickness. The increase in thickness coincided with a greater level of photoprotection. The increase in epidermal thickness was greater for subjects of lighter constitutive pigmentation. The method of UV Transmission Spectrophotometry for measuring melanin density was found to correlate positively with a known chemical assay that measures eumelanin and pheomelanin. The fourth study examines the photoadaptive response of vitiligo skin. The melanin content of vitiligo skin was determined and found to be significantly reduced or absent. Given this, epidermal thickness is thought to be the main mechanism by which vitiligo skin photoadapts. This study found the epidermis to be thicker in areas of vitiligo. Flowever, the greater epidermal thickness did not provide an equivalent level of photoprotection to normal pigmented skin. Flaving examined photoadaptation as a whole, and its two components separately, there is good evidence to support the fact that both, the pigmentary response and epidermal thickening, are important and contribute to towards the photoadaptive response as a whole.
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