Investigation into the Mechanism of UV Transmission to Follicular Stem Cells and Implications for Melanoma Development
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Children are particularly vulnerable to sun exposure; excessive sun exposure during their childhood can result in increased melanoma incidence in later life. It is hypothesized that the reasons for the vulnerability of children to sun exposure is related to their hair follicles. The melanocyte stem cells (McSCs) in hair follicles have been identified as a possible origin of melanoma upon exposure to ultraviolet radiation (UVR). Those cells in the vellus hairs (predominant type of hair before puberty) are much shallower than in the terminal hairs (predominant type of hair after puberty). Using the Monte Carlo (MC) method for photon transport in skin alone, we have shown that the McSCs in vellus hair follicles (VHF) would receive and absorb significantly higher UV than those in the terminal hair follicles (THF). Furthermore, as a consequence of the thinner epidermis in children, the cells would absorb about 1.9- and 3.2-times greater UVA and UVB respectively compared to adult skin. Due to the unique morphologies of vellus hairs, it is also hypothesized that they contribute to the solar UV transmission in the skin to the McSCs. To validate this, Caucasian scalp and body hairs have been used to measure their transmission properties in the UV wavelength range, using a CRAIC™ microspectrophotometer. The measured properties are then implemented into realistic skin-hair models. The simulated results show that a higher level of UV is delivered to the McSCs in the skin model with vellus hairs, as compared to hairless skin. The relative increase in energy absorbed in the stem cells when vellus hair is present to hairless skin varies from 4.6% to 52.0% over the UVA – UVB wavelength ranges. For skin with terminal hair, this relative increase in energy absorbed varies from 16.7% to 55.6% over the same wavelength range. Skin with shaved vellus hair will further enhance the UV transmission into the skin. In conclusion, this research provides possible explanations as to why children are particularly vulnerable to sun exposure: 1) the shallower depth of McSCs in the VHF than in the THF result in significantly higher UV absorption; 2) relatively thinner epidermis of child’s skin increases the UV absorption even more; 3) the presence of vellus hair provides an additional optical pathway, contributing to the overall solar UV transmission into the skin. These findings also explain the positive correlation between the incidence of melanoma in adults’ bodies and the number of vellus hair in these areas. This research may lead to the improvement of melanoma prevention, e.g. improvement of the efficacy of sunscreens.