Background Dermatologists and cosmetic scientists are becoming increasingly interested in stratum corneum (SC) hydration because the SC plays an important role in keeping the skin surface soft and smooth. However, conventional in vivo noninvasive methods do not provide direct information about the depth profiles of water content or SC components that hold water.
Objectives To study the depth profiles of water and SC components in vivo by Raman spectroscopy, and to analyse the changes due to age, anatomical location, season and water application.
Methods In vivo Raman spectra of the skin of the cheek and the volar forearm were obtained from 50 healthy Japanese volunteers of different ages (age range 22–76 years) with a confocal Raman spectrometer. The depth-dependent profiles of water and of SC water-binding components were calculated from the respective Raman spectra.
Results The depth profile of the SC water content was observed in a pattern ranging from around 30% (water/wet tissue) at the outermost layer of the SC to about 70% at the deeper living layer. Although the water content at a depth of 10–30 μm in the forearm skin tended to be lower in older subjects than in younger subjects, no such difference was found in the much thinner SC of the cheek. Moreover, there was no seasonal difference in depth profile of water content from the mid part of the SC to an 80-μm depth from the skin surface both in the cheek and in the forearm. The water content of all the evaluated SC components showed a gradual decrease from the surface to deeper portions. The mean amounts of lactate in the forearm skin and cholesterol in the cheek skin were significantly higher in younger subjects than in older subjects. In contrast, the levels of free amino acids and trans-urocanic acid were higher in the forearm skin of older subjects than in younger subjects. The relative amounts of urea and lactate were the highest in summer, when that of trans-urocanic acid was the lowest. Prolonged water application on the forearm skin even for 90 min resulted in a remarkable increase in water content throughout the SC, even reaching the granular layer, which was only gradually released from the upper part of the SC after discontinuation of the hydration procedure.
Conclusions Our present findings suggest that changes in the concentration depth profiles of water, free amino acids and lipids in the skin depend on age, anatomical site and season. These findings indicate the important roles played by various water-holding substances in the SC in the regulation of SC water content.