Metastatic melanoma remains incurable, emphasising the acute need for improved research models to investigate the underlying biological mechanisms mediating tumour invasion and metastasis, and to develop more effective targeted therapies to improve clinical outcome. Available animal models of melanoma do not accurately reflect human disease and current in vitro human skin equivalent models incorporating melanoma cells are not fully representative of the human skin microenvironment. We have developed a robust and reproducible, fully-humanised 3D skin equivalent comprising a stratified, terminally differentiated epidermis and a dermal compartment consisting of fibroblast-generated extracellular matrix. Melanoma cells incorporated into the epidermis were able to invade through the basement membrane and into the dermis, mirroring early tumour invasion in vivo. Comparison of our novel 3D melanoma skin equivalent with melanoma in situ and metastatic melanoma indicates this model accurately recreates features of disease pathology, making it a physiologically representative model of early radial and vertical growth phase melanoma invasion.

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Background Retinoid signaling is an important regulator of the epidermis and skin appendages. Therefore, synthetic retinoids have been developed for therapeutic use for skin disorders such as psoriasis and acne. Aims In previous studies, we showed how the photostable retinoid EC23 induces neuronal differentiation in stem cell‐like cell populations, and here, we aim to investigate its ability to influence epidermal and hair follicle growth. Methods EC23 influence on skin biology was investigated initially in cultures of monolayer keratinocytes and three‐dimentional in vitro models of skin, and finally in in vivo studies of mice back skin. Results EC23 induces keratinocyte hyperproliferation in vitro and in vivo, and when applied to mouse skin increases the number of involucrin‐positive suprabasal cell layers. These phenotypic changes are similar in skin treated with the natural retinoid all‐trans retinoic acid (ATRA); however, EC23 is more potent; a tenfold lower dose of EC23 is sufficient to induce epidermal thickening, and resulting hyperproliferation is sustained for a longer time period after first dose. EC23 treatment resulted in a disorganized stratum corneum, reduced cell surface lipids and compromised barrier, similar to ATRA treatment. However, EC23 induces a rapid telogen to anagen transition and hair re‐growth in 6‐week‐old mice with synchronously resting back skin follicles. The impact of EC23 on the hair cycle was surprising as similar results have not been seen with ATRA. Conclusions These data suggest that synthetic retinoid EC23 is a useful tool in exploring the turnover and differentiation of cells and has a potent effect on skin physiology.

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