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 ultraviolet 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 model. Herein, we performed histology and immunohistochemistry analyses and demonstrated that a newly developed full‐thickness three‐dimensional 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, or melanocore, 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.

Investigation into the effect of skin tone modulators and exogenous stress on skin pigmentation utilizing a novel bioengineered skin equivalent

“…The formation of melanin SPNCs within keratinocytes is a well‐known phenomenon that protects nuclear DNA from UV‐induced damage 62 . A modest number of pigmented HSEs have been reported in the literature, however they lack extensive characterization, with limited evidence provided relating to the formation of SPNCs 32,34,41,63–65 . To our knowledge, presence of SPNCs in vitro has only been reported by Gibbs et al 66 in a reconstructed epidermal system requiring patient‐derived tissue for construction.…”

“…62 A modest number of pigmented HSEs have been reported in the literature, however they lack extensive characterization, with limited evidence provided relating to the formation of SPNCs. 32,34,41,[63][64][65] To our knowledge, presence of SPNCs in vitro has only been reported by Gibbs et al 66 in a reconstructed epidermal system requiring patient-derived tissue for construction. Although the use of patient-derived tissue can lead to personalized medicine applications, the availability of tissue can be a limiting factor in the creation of such models, restricting their use for industrial pursuits and large scale applications.…”

Adaptability of melanocytes post ultraviolet stimulation in patients with melasma

“…[19][20][21] On the other hand, Tarafder et al suggested coupled exo/endocytosis of melanocores as a major mechanism in constitutive skin pigmentation. 16,17,24 In fact, keratinocytes internalize both membrane-bound melanosomes and melanocores, 21,25 and both forms are found in the extracellular space, 16 suggesting different transfer mechanisms could co-exist in physiological and pathological circumstances. The exact mechanism how melanocytes transfer melanosomes to keratinocytes in melasma skin warrants incorporating in vivo and in vitro studies.…”

“…It has been proposed that melanosomes are transferred using the melanosome‐rich package (pigment globules) in darkly pigmented skin 19–21 . On the other hand, Tarafder et al suggested coupled exo/endocytosis of melanocores as a major mechanism in constitutive skin pigmentation 16,17,24 . In fact, keratinocytes internalize both membrane‐bound melanosomes and melanocores, 21,25 and both forms are found in the extracellular space, 16 suggesting different transfer mechanisms could co‐exist in physiological and pathological circumstances.…”

Photoprotective melanin is maintained within keratinocytes in a storage lysosome

“…Within the 2h pulse, organelles with the lamellar morphology typically associated with lysosomes that also contained aggregated gold could be observed fusing with melanin-containing vesicles (Figure 2A). We then moved to Reconstructed Human Pigmented Epidermis (RHPE) as an in vitro human model system, which we previously showed to mimic the essential features of human pigmentation (Hall and Lopes-Ventura et al, 2022). We observed by TEM similar examples of melanin within compartments with lysosomal morphology (Figure 2B).…”