Characterization of reconstructed skin models

The aim of the present study was to evaluate tissue architecture and lipid composition of commercially available reconstructed human skin models; EpiDerm, SkinEthic and Episkin in comparison to in-house reconstructed epidermis on a de-epidermized dermis (RE-DED) model and native tissue. For this purpose, the tissue architecture was examined using light microscopy, electron microscopy and immunohistochemistry; epidermal lipid composition was analyzed by HPTLC. Histological examination showed a completely stratified epithelium in all skin models closely resembling normal human epidermis. Low intra-batch variation in tissue architecture was observed in all skin models, but moderate to considerable inter-batch variation was noticed. In the stratum corneum extracellular space, lipid lamellae consisting of multiple alternating electron-dense and electron-lucent bands were present. Lipid analyses revealed the presence of all major epidermal lipid classes. Compared with native epidermis and RE-DED in EpiDerm, SkinEthic and Episkin models, the content of polar ceramides 5 and 6 was lower, ceramide 7 was absent, and the content of free fatty acids was very low. Evaluation of the expression and localization of a number of differentiation-specific protein markers revealed that all skin models showed an aberrant expression of keratin 6, skin-derived antileukoproteinase, small-proline-rich proteins, involucrin and transglutaminase. Although variation within batches was low, in particular keratin 6, involucrin and skin-derived antileukoproteinase expression demonstrated some inter-batch variation. In conclusion, all skin models provide a promising means for studying the effects of topically applied chemicals, although the observed deviations in tissue homeostasis and barrier properties need to be diminished. All skin models tested reproduced many of the characteristics of normal human epidermis and therefore provide a morphologically relevant in vitro means to assess skin irritation and perform other skin-related studies.