Reconstructed human pigmented skin/epidermis models achieve epidermal pigmentation through melanocore transfer.

Abstract

The skin acts as a barrier to environmental insults and provides many vital functions. One of these is to shield DNA from harmful UV radiation, which is achieved by skin pigmentation arising as melanin is produced and dispersed within the epidermal layer. This is a crucial defence against DNA damage, photo-ageing and skin cancer. The mechanisms and regulation of melanogenesis and melanin transfer involve extensive crosstalk between melanocytes and keratinocytes in the epidermis, as well as fibroblasts in the dermal layer. Although the predominant mechanism of melanin transfer continues to be debated and several plausible models have been proposed, we and others previously provided evidence for a coupled exo/phagocytosis of melanocore model. Herein, we performed histology and immunohistochemistry analyses and demonstrated that a newly developed full-thickness 3D reconstructed human pigmented skin model and an epidermis-only model exhibit dispersed pigment throughout keratinocytes in the epidermis. Transmission electron microscopy revealed melanocores between melanocytes and keratinocytes, suggesting that melanin is transferred through coupled exocytosis/phagocytosis of the melanosome core similar to our previous observations in human skin biopsies. We therefore present evidence that our in vitro models of pigmented human skin show epidermal pigmentation comparable to human skin. These findings have a high value for studies of skin pigmentation mechanisms and pigmentary disorders, whilst reducing the reliance on animal models and human skin biopsies. SignificanceCurrently, many approaches to study skin pigmentation and pigmentary disorders use 2D cultures or co-culture of melanocytes and/or keratinocytes, as well as skin biopsies obtained from consenting donors. However, challenges are faced when it is necessary to mimic the tissue microenvironment as there is a limited availability of skin biopsies, especially for the case of rare pigmentary disorders. Through this study, we show the potential of 3D in vitro reconstructed pigmented skin/epidermis models for both general and specific pigmentary research questions, which could reduce reliance on animal models and human skin biopsies.