Co-culture of human melanocytes and keratinocytes in a skin equivalent model: effect of ultraviolet radiation

Melanocytes grown in pure monolayer culture lack the three-dimensional organization and many of the cellular interactions that exist in vivo. This can be partially overcome by growing melanocytes together with other epidermal cells in skin equivalent models. In this study skin equivalents were prepared by seeding mixtures of cultured human keratinocytes and melanocytes in various ratios onto de-epidermized dermis. They were cultured in DMEM/Ham's F12 (3:1) for 3 days and then lifted to the air-liquid interface and maintained for 11 days. Histological examination revealed a structure that closely resembled human interfollicular epidermis. Melanocytes, identified by their dendritic appearance, positive dopa reaction and positive staining with a melanocyte-specific antibody (MEL5), were located in the basal layer. Melanin was seen both in melanocytes and in neighbouring keratinocytes. Whilst the skin equivalent became more pigmented following UV irradiation (total UVB 4760 J/m2 over 3 days), the quantity and distribution of melanin at the light microscopic level appeared to be unchanged. However, the number and dendricity of melanocytes increased, as did their staining with dopa and MEL5. These results indicate that melanocytes are functional and capable of responding to UV irradiation.