Skin-derived precursors (SKPs) have been described as multipotent dermal precursors. Here, we provide a review of the breadth and depth of scientific literature and studies regarding SKPs, accounting for a large number of scientific publications. Interestingly, these progenitors can be isolated from embryonic and adult skin, as well as from a population of dermal cells cultured in vitro in monolayer. Gathering information from different authors, this review explores different aspects of the SKP theme, such as the potential distinct origins of SKPs in rodents and in humans, and also their ability to differentiate in vitro and in vivo into multiple lineages of different progeny. This remarkable capacity makes SKPs an interesting endogenous source of precursors to explore in the framework of experimental and therapeutic applications in different domains. SKPs are not only involved in the skin's dermal maintenance and support as well as wound healing, but also in hair follicle morphogenesis. This review points out the interests of future researches on SKPs for innovative perspectives that may be helpful in many different types of scientific and medical domains.

In human skin, melanogenesis is a tightly regulated process. Indeed, several extracellular signals are transduced via dedicated signalling pathways and mostly converge to MITF, a transcription factor integrating upstream signalling and regulating downstream genes involved in the various inherent mechanisms modulating melanogenesis. The synthesis of melanin pigments occurs in melanocytes inside melanosomes where melanogenic enzymes (tyrosinase and related proteins) are addressed with the help of specific protein complexes. The melanosomes loaded with melanin are then transferred to keratinocytes. A more elaborate level of melanogenesis regulation comes into play via the action of non-coding RNAs (microRNAs, lncRNAs). Besides this canonical regulation, melanogenesis can also be modulated by other non-specific intrinsic pathways (hormonal environment, inflammation) and by extrinsic factors (solar irradiation such as ultraviolet irradiation, environmental pollution). We developed a bioinformatic interaction network gathering the multiple aspects of melanogenesis and skin pigmentation as a resource to better understand and study skin pigmentation biology.

The microRNAs (miRNAs) function as global negative regulators of gene expression and have been associated with a multitude of skin biological processes such as aging. To understand genome-wide changes in miRNA expression in human skin from Asian individuals during aging, we analysed by miRNA array full-thickness skin samples categorized into young, intermediate and elderly groups. Based on filtering criteria of a fold change ≥5 and an adjusted p-value ≤0.05, the expression of 21 miRNAs was found to be modulated during skin aging. More specifically, the expression of 8 miRNAs were altered between young and elderly groups, while 17 were modulated between intermediate and elderly groups. Strikingly, no miRNA appeared to be altered between young and intermediate groups. Using advanced bioinformatics, we identified predicted target genes and cellular pathways related to identified miRNAs. The miRNA-targeted genes were found to be associated with several biological processes related to methylation, cell cycle, apoptotic process, mitochondrion and regulation of stress fibre assembly. We also found a subset of the potential dysregulated-miRNA target genes belonged to the insulin signalling pathway. We confirmed with qPCR that mRNA expression of IGF-1 and its receptor were downregulated in the elderly group. Determining a miRNA signature in the skin of Chinese individuals for aging is a first step towards a better understanding of the interplay between ncRNAs and protein-coding genes during intrinsic aging. Moreover, miRNAs differentially expressed in the skin of Chinese individuals may be the foundation to understand specific ethnic-related physiological characteristics.

Skin aging is associated with phenotypic changes in cutaneous structures, and with various features such low resistance to oxidative stress and DNA damage, decreased extracellular matrix protein synthesis, and the appearance of markers of aging. Cellular senescence is an irreversible state of cell cycle arrest that is induced during cellular aging. It is considered as one of the nine hallmarks of aging. Thee accumulation of senescent cells with age and the expression of factors impacting the surrounding environment (inflammatory cytokines, proteases) contribute in turn to tissue aging. Many genes have been reported to be involved in the regulation of senescence. In this study, several 3D engineered skin models were developed in order to study cellular senescence and skin aging, based on the silencing of specific genes such as FOXO3A. Comparative characterization of the reconstructed skin models was undertaken in order to delineate the consequences of gene silencing, and to point out specific features according to the gene silenced. Consequently, these skin reconstructed models represent a promising tool for monitoring in vitro markers of aging following the application of chemical substances such as biofunctional ingredients.

Acne vulgaris is the most common skin disease worldwide, with a prevalence of 80 to 85% among adolescents. It is a complex multifactorial skin disease of the pilosebaceous follicles characterized by comedones, papules, pustules, and nodules that leave physiological as well as physical scars. The main pathogenic factors associated with acne development are follicular hyperkeratinization, increased sebum excretion (and alterations of the sebum composition), Propionibacterium acnes colonization, and inflammation. However, the sequence of events involved in acne lesion initiation, augmentation, and resolution have eluded acne researches. Bioinformatics is a powerful approach to understand relationships between genes involved in a biological process. As acne is correlated with modulated expression of specific genes, we developed in this study a bioinformatic model of the interactions between the genes involved in acne pathogenesis, with the goal of better understanding the relationships between hyperproliferation, exces sebum and inflammation. Besides this in silico approach, several in vitro test models were developed using skin tissue engineering and covering the hyperproliferation, sebum and inflammatory aspects. This complementary approach will facilitate the study the modulating potential of biofunctional ingredients or chemicals on aspects of acne pathogenesis in vitro.

In the skin, increased oxidative stress due to ultraviolet irradiation has been associated with the aging process. DNA damage checkpoints are cellular surveillance mechanisms ensuring DNA integrity after oxidative stress. UV irradiation induces cell cycle arrest via two main DNA checkpoint pathways known as ATR/Chk1 and ATM/Chk2, concomitantly with the activation of DNA damage response. Cell cycle arrest at the G1/S checkpoint allows checking of DNA integrity before its replication, whereas the G2/M checkpoint allows the cell cycle to pause before mitosis starts. In this study, we analyzed cell cycle by flow cytometry after UV irradiation of synchronized human skin fibroblasts in culture and studied the effect of various ingredients on the cell cycle profile, as well as the phosphorylation at Ser124 of Cdc25A, which represents the event that is common to the activation of both ATR and ATM pathways.

Acne vulgaris is the most common skin disease worldwide, with a prevalence of 80 to 85% among adolescents. It is a complex multifactorial skin disease of the pilosebaceous follicles and is characterized by comedones, papules, pustules, and nodules that leave psychological as well as physical scars. The main pathogenic factors associated with acne development are follicular hyperkeratinization, increased sebum excretion (and alterations of the sebum composition), Propionibacterium acnes colonization, and inflammation. However, the sequence of events involved in acne lesion initiation, augmentation, and resolution have eluded acne researches. Bioinformatics is a powerful approach to understand the relationships between genes involved in a biological process. As acne is correlated with modulated expression of specific genes, we developed in this study a bioinformatic model of the interactions between the genes involved in acne pathogenesis, with the goal of better understanding the relationships between hyperproliferation, excess sebum and inflammation. In addition to this in silico approach, several in vitro test models were developed using skin tissue engineering and covering the hyperproliferation, sebum and inflammatory aspects. This complementary approach will enable further study of the modulating potential of biofunctional ingredients or other chemicals on aspects of acne pathogenesis in vitro.

Epidermis undergoes a continuous renewal through the proliferation and differentiation of keratinocytes anchored to the dermo-epidermal junction. This multistep process requires a complex and coordinated program of gene expression and inhibition. Recent findings suggest that microRNAs and long non-coding RNAs (lncRNAs) play an essential role in the renewal of epidermis, in the maintenance of the skin barrier, and the progression of skin senescence. With the aim of establishing several different 3D engineered skin models to study epidermal renewal, differentiation and skin senescence, we studied the effect of the modulation in vitro of some major microRNAs (miR-203), lncRNAs (TINCR or "tissue differentiation-inducing non-protein coding RNA"), or protein-coding genes (filaggrin). Indeed, miR-203 is a master epidermal regulator controlling the shift between basal proliferation of keratinocytes and suprabasal differentiation, via the involvement of p63. In parallel, TINCR regulates epidermal differentiation genes – including filaggrin, a protein of the epidermal differentiation complex – and is a key player in skin barrier function. We developed four different 3D engineered skin models by down-regulating miR-203, TINCR, or filaggrin; or on the contrary, by increasing the content in miR-203 in the reconstructed epidermal layer. The characterization of each model and the comparison with normal Reconstructed Human Epidermis (RHE) was carried out to understand the induced tissular and molecular specificities for each model. These new models with deregulated epidermal differentiation and barrier will facilitate the study of the effects in vitro of the application of biofunctional ingredients or chemicals on skin equivalents.

Cellular senescence is an irreversible state of cell cycle arrest that is induced during cellular aging. In the skin, senescence is associated with phenotypic changes in cutaneous structure and cells, lowered resistance to oxidative stress and DNA damage, decreased epidermal cell renewal, decreased synthesis of extracellular matrix protein and the appearance of markers of aging (senescence-associated beta-galactosidase, etc.). Many genes have been reported to be involved in the regulation of senescence: tumor suppressor genes (e.g., p53), senescence genes (e.g., p16, p21) and senescence suppressor genes (e.g., telomerase), oncogenes (e.g., MYC), stemness genes (e.g., SOX2), genes related with epigenetics (e.g., SIRT1), inflammation-related genes, genes implicated in DNA damage repair, cytoskeletal remodeling (e.g., TGF, WNT), and also various transcription factors. All these genes participate in a network of interactions regulating senescence, ending with the highly regulated p53–p21 and p16–pRB pathways. In this study we developed a bioinformatic network model of the relationships between the main genes involved in skin senescence. This network facilitated the categorization of subsets of genes and the prediction of potential modulating microRNAs. In parallel, we developed several in vitro models to study senescence, based on the silencing of specific genes, allowing us to obtain skin cells in culture with a senescent phenotype. Characterization of the cellular phenotype by immunodetection of proteic markers and qPCR was undertaken in order to identify specific features according to the gene silenced. This complementary in silico and in vitro testing approach will foster study of the modulating potential of biofunctional ingredients or other chemicals on cellular senescence in vitro.

Recent discoveries regarding the skin microbiome help to develop a stronger common understanding of skin physiology. On the skin surface, when homeostasis and symbiotic relationships are well maintained and facilitated, the skin barrier is reinforced, immunity is effective with a low inflammation state, and pathobionts are suppressed. Skin health has also been related to probiotic status inside the body and on the skin, and gut flora. When this balance is absent, one of the most common human chronic skin disorders that can arise is Acne vulgaris, which is highly prevalent among adolescents. Acne is a disease of the pilosebaceous follicle, with a complex multifactorial origin. The main pathogenic factors associated with acne development are follicular hyperkeratinization, increased sebum excretion, alteration of the sebum composition, Propionibacterium acnes colonization, inflammation burst, stress response and involvement of neuropeptidergic pathways, and dysregulation of the hormonal microenvironment. Acne involves both epidermal keratinocytes and sebocytes of the sebaceous gland and is characterized by comedones, papules, pustules, and nodules that can leave not only physical scars, but also potentially psychological ones. Our approach to this problem was toy study acne by using multi-angle approaches. This was accomplished firstly in silico with bioinformatics, a powerful tool, to generate complex networks of the interactions between the genes involved in acne. Then, in vitro testing was conducted with human tissue models based on engineered 3D epithelial models (RHE and RHPE), facilitating several different types of experiments (heat-inactivated P. acnes, cytokine cocktail application, induced pigmentation, squalene peroxide application). In combination, this represents a powerful complementary approach that will foster further study in vitro of the effects of the application of anti-acne oriented ingredients, biofunctionals or other chemicals on skin equivalents.

Cellular senescence is an irreversible state of cell cycle arrest that is induced during cellular aging. In the skin, senescence is associated with phenotypic changes in cutaneous structure and cells, lowered resistance to oxidative stress and DNA damage, decreased epidermal cell renewal, decreased synthesis of extracellular matrix protein and the appearance of markers of aging (senescence-associated beta-galactosidase, etc.). Many genes have been reported to be involved in the regulation of senescence: tumor suppressor genes (e.g., p53), senescence genes (e.g., p16, p21) and senescence suppressor genes (e.g., telomerase), oncogenes (e.g., MYC), stemness genes (e.g., SOX2), genes related with epigenetics (e.g., SIRT1), inflammation-related genes, genes implicated in DNA damage repair, cytoskeletal remodeling (e.g., TGF, WNT), and also various transcription factors. All these genes participate in a network of interactions regulating senescence, ending up with the highly regulated p53–p21 and p16–pRB pathways. In this study we developed a bioinformatic network model of the relationships between the main genes involved in skin senescence. This network allowed the categorization of subsets of genes and to predict potential modulating microRNAs. In parallel, we developed several in vitro models to study senescence, based on the silencing of specific genes allowing to obtain skin cells in culture with a senescent phenotype. Characterization of the cellular phenotype by flow cytometry, expression of proteic markers and qPCR was undertaken in order to point out specific features according to the gene silenced. This complementary in silico and in vitro testing approach will facilitate the study of the modulating potential of biofunctional ingredients or chemicals on the cellular senescence in vitro.

The main pathogenic factors associated with acne development are follicular hyperkeratinization, increased sebum excretion with alteration of the sebum composition, Propionibacterium acnes colonization, and inflammation. In this study, we focused on sebum composition and particularly on the study and role of squalene derivatives after oxidation. In the skin, sebocytes synthetize and secrete sebum, which is mostly composed of triglycerides, fatty acids, waxes, cholesterol and approximately 12% [JY1] of squalene. Evidence suggests that inflammation may be related both to the level of squalene oxidation and to the nature of oxidized by-products. As the first step to develop an in vitro acne testing model, we experimentally oxidized squalene under controlled conditions: ultraviolet irradiation, oxygen and controlled incubation. Our goal was to compare the nature of the generated compounds (squalene derivatives) to the ones described in the sebum acne patients. Oxidized squalene derivatives were analyzed and quantified by gaz chromatography coupled to mass spectroscopy (GC/MS) and nuclear magnetic resonance (NMR). In parallel, we developed an in vitro model to study acne, based on the application of oxidized squalene derivatives on reconstructed human epidermises (RHEs) that were maintained in culture. Following treatment, the morphology of the RHEs was studied, and specific markerssuch as inflammatory cytokines were measured. The intent of this study was to observe the effect of oxidized squalene application on RHEs in culture, and to correlate these effects with the nature of the compounds obtained after squalene oxidation. This model will be useful to evaluate in vitro the potential of ingredients and compounds to inhibit acne development.

Human skin is the first natural shield to protect against solar radiation that can be responsible for premature extrinsic photo-aging. Skin uses pigmentation as a natural barrier. Skin pigmentation is classified among six main phototypes, which are extended to 70 possible tones observed across the ethnicities of the world. The pigmentation process involves the two main cell types of the epidermis: melanocytes and keratinocytes. In keratinocytes, melanosomes (organized as supranuclear caps called microparasols) are essential to protect the keratinocyte nucleus from UV-induced damage. Pigmentation is the coordination center of a multi-parametric process. Epigenetics and the role of miRNAs in skin pigmentation related to ethnicity represent a very promising area to explore. Using bioinformatics, pathways and mechanisms of melanogenesis were studied in silico to better understand the overall skin pigmentation process. Commonalities emerged from all skin types but the skin tones are the result of fine up- or down-regulation of the pigmentation processes. We studied ethnicity, in vitro, using in-house 3D tissue engineering and reproduced skin tone differences from donors of different origins. The multiplicity of donors incorporates epigenetic variations. All these approaches represent the basis of new solid testing models to develop a biofunctional with skin brightening or tanning potential, in vitro. In clinic, a brightening effect on skin (age spots) was observed after a few weeks of treatment with a biofunctional, compared to placebo. Identifying at the skin level extrinsic signs of aging, the mechanisms activated, their role in the process of melanogenesis, and the key contributing factors enable better understanding of skin pigmentation, its involvement in relation to ethnicity, and how to specifically counteract photo-aging. All these in vitro and clinical observations help us to study and develop better cosmetic ingredients to fight the visible signs of aging as well as to propose new solutions related to specific ethnic needs.