What is lightening?
Lightening describes a process which reduces the production and content of melanin in human skin. In vitro, lightening can be induced in epidermal skin models comprising functional melanocytes. Lightening of matured or tanned tissue models is achieved by topical or systemic application of selected chemicals which suppress melanin formation. For example, Kojic Acid (5-hydroxy-2-(hydroxymethyl)-4-pyrone), a known inhibitor of melanogenesis generally used as whitener in cosmetic products, is topically applied at a concentration of 1% (Fig. 1). Other test substances might be applied systemically (Fig. 1). Because epiCS-M can be cultured for a few weeks at airlift culture sufficient time is provided to study skin differentiation, pigmentation or de-pigmentation.
How can melanin content be quantified?
The most common method to quantify the melanin content of the models is solvent extraction (e.g., 60 min, 100°C). Commercially available melanin can be used to generate a standard curve. The optical density can be determined at 492 nm wavelength to calculate the melanin content of single epiCS-M tissues. Alternatively, the melanin content can be quantified by digital image analysis of tissue cross sections stained with Fontana-Masson (Fig. 2 and Fig. 3).
Evaluation by photometric methods
Fig. 1 Examples for inhibition of melanin production with 1% Kojic Acid, which was topically applied and another test substance, which was systemically applied. Due to the skin barrier function of the epidermis, liquid, creamy and solid substances can be applied topically -onto the stratum corneum- to closely mimic the in vivo situation of cosmetic applications.