"Skin ageing is a multifactorial process attributed to both intrinsic and extrinsic factors. Intrinsic ageing is associated with changes that occur with age such as cumulative molecular and cellular damage, while extrinsic ageing exacerbates these changes through the UV exposure. Nowadays, ageing has become an important issue in the world: life expectancy and population have increased, while social and psychological factors have motivated a stronger desire for healthy ageing and a youthful appearance. Alongside other environmental factors, solar UV radiation is associated with extrinsic skin ageing, as it drives up to 80% of premature skin aging1 and it is one of the most potent carcinogens known. UV radiation induces structural and functional changes in both the epidermal and dermal compartments, such as erythema sunburn, tanning, DNA damage, inflammation, remodelling of the extracellular matrix, changes in epidermal barrier function, cellular proliferation and differentiation. The extent of these effects are mainly due to the degree of constitutive pigmentation of the skin.
Human neonatal skin cells were used to generate in vitro human skin equivalents as previously described in Roger et al.2. A UV irradiator system was used to simulate UV exposure on skin equivalents and they were characterised using a colorimeter, histology, immunofluorescence, melanin quantification and advanced microscopy.
UV-irradiated non-pigmented skin equivalents, which lack melanin protection, reveal structural epidermal changes, sunburn cells, DNA damage in form of cyclobutane pyrimidine dimers, apoptotic cells, and decreased expression of the differentiation marker filaggrin compared to the sham-irradiated non-pigmented skin equivalents. This effect appears to be dose-dependent, with a greater response identifiable following chronic irradiation. Conversely, UV-irradiated pigmented skin tans following chronic exposure, which is confirmed by an increased melanin content and demonstrates the protective effects of melanin by demonstrating a well-differentiated and organised epidermis and an absence of UV-induced damage. This melanin photoprotection is related to the tone of the pigmented skin equivalent, where the lightest skin tone has a greater UV impact than darker skin types.
We describe the characterisation of UV-induced skin equivalents, which recapitulate UV exposure of human skin and the role of melanin, providing a platform to test new and current formulations for cosmetic products designed to protect and treat the skin from harmful UV-exposure and ageing.
- Gilchrest, B. A. Journal of the American Academy of Dermatology, 21(3), 610–613 (1989).
- Roger, M. et al., Journal of Anatomy, 234(4), 438–455 (2019)."