# In Vivo Assessment of Water Content, Trans-Epidermial Water Loss and Thickness in Human Facial Skin
## 5 Discussion
reported that skin thickness in the cheeks was higher than in the forehead, which is again consistent with our results, although the difference did not reach statistical significance in our experiments. Interestingly, the epidermis showed the same three clusters as the dermis, but, notably, with a different ordering in terms of thickness, which in this case was: (1) lips and neck, (2) chin and nose and (3) cheeks and forehead (from thickest to thinner). Unfortunately, in this case there are fewer results available to compare with. In [32] [PERSON] et al. determined that the lips had the highest epidermis thickness, which is line with our results; but differently from the results found here, they found that the neck had lower thickness than most of the other areas. Finally, our analysis of the stratum corneum returned two clusters of statistically-indistinguishable areas, which, from thickest to thinner were: (1) lips and neck and (2) cheeks, chin, forehead and nose. Regrettably, no comparison with published results is possible in this case, as we are aware of no earlier studies investigating the distribution of the thickness of the stratum corneum in the different facial areas. We believe the results of this investigation can provide valuable insights for the development and/or evaluation of cosmetic products and medical and surgical treatments. The correct assessment of functional and anatomical differences between facial sites plays in fact a fundamental role for the preparation of cosmetic, dermatological and aesthetic treatments and surgical procedures tailored to the patient's needs. Specifically, trans-epidermal water loss is a strong indicator of the integrity of the stratum corneum and is widely adopted to evaluate the skin barrier function, with high values of TEWL being related to increases in skin irritability [25]. It is also believed that TEWL plays a major role in inflammatory skin conditions such as eczema and dermatitis [45]. Here we determined that TEWL was significantly higher in the lips and chin compared to the cheeks, thereby confirming previous results [25]. Water content is also a crucial element for a healthy skin, and de-hydrated skin is one of the major concerns of today's cosmetics consumer base worldwide. In this case we identified three different clusters of facial areas, the least hydrated being composed of forehead, lips and nose (Figure 6), a finding consistent with previous works [23; 29]. This result may have direct practical applications, as a recent study by [PERSON] et al. [29] demonstrated that moisturising treatments were most effective on the facial areas that had the lowest hydration baseline. Finally, understanding regional thickness differences is essential for optimising reconstructive procedures and minimally invasive treatments. The results presented here can help surgeons in a number of tasks, such as skin reconstruction after cancer resection and determination of optimal needle location for injectable neuro-modulators and/or tissue fillers [32]. Interesting questions arise about what each cluster means physiologically, and more generally, regarding possible links between the results found here and other biophysical and/or anatomical properties of the facial areas investigated. As for water content, a number of factors have been advocated as possible determinants--among them the presence in the skin of natural (endogenous) hygroscopic agents (e.g., glycerol and hyaluronic acid) and that of inter-cellular lipids [6]. However,apart from lipids (sebum), the effects of such factors are difficult to evaluate due to the absence of literature data, and should be therefore explored in future studies. As for sebum, previous works [23; 46] have reported a significantly higher concentration in the forehead than in the cheeks; however, neither of the two studies found higher water content in the more sebaceous areas. With respect to TEWL, it is generally accepted that the size of superficial corneocytes correlates negatively with trans-epidermal water loss [47; 48].