The decline of tissue regenerative potential of skin and hair is a hallmark of physiological ageing and may be associated with age-related changes in tissue-specific stem cells and/or their environment. Human hair follicles (hHF) contain keratinocytes having the property of stem cells such as clonogenic potential. Growth capacity of hHF keratinocytes shows that most of the colony-forming cells are classified as holoclones, meroclones or paraclones when analysed in a clonal assay (Cell, Volume 76, page 1063). Despite the well-known impact of ageing on human hair growth, little is known about changes in hHF keratinocyte clonogenic potential with age. This study aimed at assessing the clone-forming efficiency (CFE) of hHF keratinocytes from three age groups of human donors. It demonstrates that ageing affects hHF keratinocyte CFE.

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Keratinocytes of the basal layer function are to maintain tissue homoeostasis and to fulfil skin repair in response to an external aggression. In wound-healing, during re-epithelialization phase, epithelial precursor cells gradually migrate from the edges of the wound. The epidermal reconstruction model called standard model allows the vertical skin regeneration process (proliferation/differentiation) to being investigated, and keratinocyte function in preserving skin homoeostasis to being assessed. Here, we developed and characterized a 3D migration model, which introduces a step of keratinocytes migration such as the one observed in the phase of re-epithelialization in woundhealing process. We validated the added value and the discriminative potential of this model by demonstrating pro-epithelializing effects of compounds. This new model allows the role of keratinocytes in different biomechanical and environmental requests to being better understood, and brings a new tool for compound screening and the study of mechanisms involved in skin regeneration. | BACKGROUNDWound-healing is a process of tissue regeneration, which involves three phases that partially overlap: (i) a vascular or inflammatory phase, (ii) a tissue repair phase and (iii) a maturation phase .[1] During phase II, keratinocytes gradually migrate from the wound edges, a period during which basal keratinocytes may differ from precursors essentially observed in the basal layer. Live imaging of stem cells recently allowed in rodent to better understand how stem cells behaviour changes in response to wounding [2] ; however, no model has succeeded in reflecting the whole wound-healing process. The current approaches consist in sequencing the process, using models that focus on one phase. In spite of its limits, this strategy, associated with clinical observations, allows the mechanisms involved to being partially described | QUESTIONS ADDRESSEDWe developed and characterized a new epidermal regeneration 3Din vitro model in a keratinocyte migration situation called "migration model" (partially mimicking the suction blister in vivo clinical model) and compared it to the existing 3D RealSkin ® regeneration standard model [6] referred here as "standard model." The standard model allows vertical regeneration (homoeostasis maintenance) to being studied while migration model introduces the additional step of re-epithelialization as observed in the phase II of wound-healing process. We studied the effect of pro-regenerating/pro-wound-healing compounds to illustrate the added value of this model. are seeded around the ring, the centre remaining empty ( Figure 1A). | EXPERIMENTAL DESIGNRing is removed 10 days after seeding the keratinocytes. Compounds to be evaluated for their capacity to impact the epidermal regeneration and epithelialization speed process are added to the culture medium. Analysis of tissue morphology, quality and thickness of the reconstructed epidermis and keratinocyte migration kinetics were performed. Experimental protocol and metho...

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A number of clinical observations have indicated that the regenerative potential and overall function of the epidermis is modified with age. The epidermis becomes thinner, repairs itself less efficiently after wounding, and presents modified barrier function recovery. In addition, the dermal papillae flatten out with increasing age, suggesting a modification in the interaction between epidermal and dermal compartments. As the epidermal regenerative capacity is dependent upon stem and progenitor cell function, it is naturally of interest to identify and understand age-related changes in these particular keratinocyte populations. Previous studies have indicated that the number of stem cells does not decrease with age in mouse models but little solid evidence is currently available concerning human skin. The objective of this study was to evaluate the clonogenic potential of keratinocyte populations isolated from the epidermis of over 50 human donors ranging from 18 to 71 years old. The data indicate that the number of epidermal cells presenting high regenerative potential does not dramatically decline with age in human skin. The authors believe that changes in the microenvironment controlling epidermal basal cell activity are more likely to explain the differences in epidermal function observed with increasing age.