## 3 Results
### Evaluation of Elastin Synthesis  
Together with collagen, elastin is the second principal component of ECM representing 2% of the dermis weight. Its role is closely linked to collagen fibers and it is mainly responsible for the elasticity of the skin. After evaluating the production of new collagen in normal human dermal fibroblasts, the synthesis of elastin was also investigated after 24 h treatment with 0.625 and 1.25 mg/mL. As shown in Figure 4, treatment with the product determines a significant increase in elastin levels. In particular, the amount of elastin measured results to be statistically significant with an increase equal to 77% after treatment with the highest concentration (1.25 mg/mL) of the tested product (**_p_ values \(\leq\) 0.01). Figure 3: Collagen levels expressed as \(\mathrm{\mu g/mL}\) (mean \(\pm\) SD) after treatment with the RETINOIDS SERUM compared to control (CTRL, untreated cells). Values of **_p_ \(<\) 0.01 and ***_p_ \(<\) 0.001 were considered statistically significant compared to CTRL (\(n\) = 3, replicates = 2). Figure 2: Representative images of immunofluorescence BrdU assay (**A**) DNA is marked in blue with Hoechst 33258 while BrdU is visible in green. (**B**) Quantitative assessment of BrdU positive cells expressed as a percentage (mean \(\pm\) SD) compared to control after 24 h of treatment with the RETINOIDS SERUM (0.625 and 1.25 mg/mL). Values of *_p_ \(<\) 0.05 were considered statistically significant compared to untreated cells (CTRL). loss of tone and elasticity of the skin, irregular pigmentation together with a reduction in the thickness of the epidermis and dermis, represent the typical signs of skin-ageing. All of these changes are induced by chronological ageing or can be accelerated by the externals factor [13]. Several studies prove that a daily and constant use of cosmetic products enriched with functional compounds, such as vitamins, can help to counteract the signs of ageing and photoaging. In particular, in the last two decades, retinoids such as vitamin A and its derivates, have been frequently added to cosmetic formulations. All of them have important effects on skin cells: they can stimulate or maintain the skin keratinocyte's differentiation from the basal layer to the most superficial one, they are fundamental in epidermal renewal both caused by normal skin turnover or in case of injury, and they also influence the ECM composition. Several in vitro and in vivo studies demonstrate the ability of retinol and its derivatives to penetrate across the epithelium exerting a biological effect in keratinocyte cells differentiation, corneocyte exfoliation, and regulating fibroblast proliferation in the dermis by angiogenesis, collagen and elastin synthesis stimulation [14; 15]. The retinoids receptor signaling mediated by the interaction with specific nuclear receptors, such as retinoid X receptors (RXRs) and retinoic acid receptors (RARs), increases the production of procollagen, thus leading to the synthesis of new collagen and, at the same time, inhibits Matrix Metalloproteinases ( MMPs) [16; 17; 18; 19; 20; 21; 22]. In addition, many articles show the potential effect of a new dermocosmic containing retinaldehyde (RAL), delta-tocopherol glucoside, and glycylglycine oleamide with in vitro and clinical studies as well as the restoration and the improvement of the ECM complex in UV-irradiated fibroblast models [23; 24; 25; 26; 27]. In our study, we investigated the in vitro effects of a new formulation produced by Matex Lab Spa, containing vitamin A derivatives in a mix that included 0.1% of retinol, 0.1% retinaldehyde, and 1% of bioretionl, using human dermal fibroblasts (NHDF) and epidermal keratinocytes (HaCaT). First of all, the viability on the selected cell models was evaluated by the MTT assay to identify the first two non-cytotoxic concentrations to perform efficacy tests. The obtained results showed 0.625-1.25 mg/mL and 0.313-0.625 mg/mL for NHDF and HaCaT cells, respectively.