## 4 Discussion  
Chronological ageing is well described today and the main cause is oxidative stress [19]. This stress is triggered by external and internal factors, such as UV, pollution, chemicals, cigarette smoke and psycho-emotional stress [9; 20]. Recently, research on hair ageing has also pointed to oxidative stress as a major cause of \"hair greying\" [10; 11; 21]. In an attempt to understand the phenomenon of canities and offer a smart solution to prevent it, we designed ex vivo models and exposed them to oxidative stress in order to mimic this phenomenon. First, we repeatedly exposed scalp skin explants to free radicals to induce premature canities. After this exposure, we analysed the gene expressions and demonstrated that oxidative stress had clearly reduced the antioxidant defences of the cells by down-regulating the expression of numerous genes. The expressions of genes involved in autophagy and stemness, which are also known to play a role in perturbation of skin pigmentation, were also disturbed [22; 23; 24]. The expression of major actors (DKK1, TYR, EDN1) of melanogenesis was significantly down-regulated, especially the expression of DCT and MITF, which are important during the differentiation of melanogenically active melanocytes [2; 25]. Interestingly, it has also been evidenced that MITF has a link with the maintenance of stemness in melanocyte precursors [3; 26]. This result confirmed the observations made for COX2, FST and KRT19 in our own work but also the work of [PERSON] and colleagues, showing a down-regulation of KRT19 in unpigmented hair [13]. We also confirmed the results obtained by [PERSON] and colleagues, by showing that the genes encoding for melanosome formation and transfer (CTNS, HPS5, MLPH, SLC2A5) were disturbed with oxidative stress, especially MY05A [1; 14]. Unfortunately, we did not find any significant modulation of KIT or Bcl-2 or Pmel17/gp-100, which are widely described as essential in human hair pigmentation [1; 2; 11; 19; 25]. The fine regulation of redox balance is crucial in melanocytes and the slightest deviation could disturb melanogenesis and trigger apoptosis [10; 14]. Exposition of hair follicles to oxidative stress allowed us to work on a robust model of premature canities. The exposure to oxidative stress triggered a higher oxidation level, evidenced by the induction of ROS production. This stress had a major impact on hair follicles and the consequence was the depletion of melanocyte and melanoblast reservoirs, as observed with the NKI/beteb staining, which is already evidenced in the literature [4]. On the basis of these results, we designed a smart combination of molecules to act on the key players of hair pigmentation during canities. The glucosylation of taxifolin (dihydroquercetin, DHQ) and epigallocatechin gallate (EGCG) provided better solubility to aglycones. In this work, we evaluated the efficacy of this active ingredient on the main causes of canities development. Indeed, these compounds are well known for their ROS-scavenging property. We hypothesised that their antioxidant property could play a major role in canities development since oxidative stress plays a central role in its induction. Interestingly, it is described in the literature that antioxidants can preserve stemness by controlling redox homeostasis and maintaining a low level of ROS. Moreover, we have already demonstrated that antioxidant compounds can preserve the pool of stem cells by reducing cell apoptosis and stress induced by the oxygen level [27]. We proposed the hypothesis that our active ingredient could also protect melanocyte stem cells against oxidative stress and thereby control hair pigmentation [27; 28; 29; 30]. In addition, a precursor for melanin synthesis was added to boost and potentialise the reactivation of melanogenesis. In this work, we demonstrated that this smart active ingredient can reactivate the hair pigmentation mechanism and fight against canities by acting at various levels.