What is a pigmented epidermal skin model?

A reconstructed human pigmented epidermal model e.g. epiCS-M consists of normal epidermal keratinocytes and melanocytes cultured at the air liquid interface (AL) and is characterised by a fully differentiated epidermis with underlying melanocytes. This highly differentiated model of the human epidermis shows epithelial stratification and cornification in the upper layers with an intact barrier function while the melanocytes and proliferating keratinocytes are located in the basal compartment. The location of melanin and melanocytes in the basal layers of the model can be visualized by Fontana-Masson or melanocyte-specific antibody (e.g., HMB45) staining (Fig. 1). Melanogenesis can be observed throughout the culture period. The degree of pigmentation in this model can be varied by using different melanocyte populations from e.g., Caucasian, Asian-Caucasian or Afro-American donors (Fig. 2).

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 cosmetics application. Some substances can also be applied systemically by adding to the cell culture medium. Both ways of application allow pigmentation studies. This model is an ideal tool for skin tanning or lightening studies as well as for melanogenesis research. To study the tanning potential of the model the melanin production can be stimulated with UV light, IBMX (3-Isobutyl-1-Methyl-Xanthine), or α-MSH (Melanin Stimulating Hormone). In contrast, lightening of matured or tanned tissue models is achieved e.g., with Kojic Acid (5-hydroxy-2-(hydroxymethyl)-4-pyrone), a known inhibitor of melanogenesis generally used for whitening in cosmetic products. The melanin content can be quantified by solvent extraction and determination of the optical density at 492 nm wavelength. 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 the pigmented epidermal skin model be characterised?

The reconstructed human pigmented epidermal model is characterised by a fully differentiated stratified epidermis where melanocytes are located in the basal cell layer. Melanin is synthesized by melanocytes and transferred to the neighbouring keratinocytes by their dendrites. This in vitro skin model can be used to investigate the tissue morphology for up to four weeks starting on day seven AL. HMB45 staining shows the location of melanocytes in the basal layers of the model during the airlift culture period. Melanin can be visualised with Fontana-Masson staining of the tissues.

Fig. 1 This figure demonstrates H&E staining of epiCS-M and markers specific for cornification (filaggrin) and melanocytes (HMB45) at days 7 or 14 of AL.

Fig. 2 Development of pigmentation of epiCS-M over time. Caucasian melanocyte donor display different stages of pigmentation during three weeks AL culture.

How can pigmentation be quantified?

Photometric quantification or digital image analysis is described in more detail in the sections Tanning and Lightening.

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