Characterizing stratum corneum structure, barrier function, and chemical content of human skin with coherent Raman scattering imaging.

The most superficial layer of the epidermis, the stratum corneum, plays a crucial role in retaining hydration; if its structure or composition is compromised, dry skin may result as a consequence of poor water retention. Dry skin is typically treated with topical application of humectant agents that attract water into the skin. Corneometry, the industry standard for measuring skin hydration, works by assessing the bulk electrical properties of skin. However, this technique samples a large volume of tissue and thus does not resolve the biochemical changes that occur at the cellular level that may underlie mechanisms of dry skin. These limitations can be addressed using coherent Raman scattering (CRS) microscopy to probe the intrinsic vibrational modes of chemical groups such as lipids and water. In the present study, ex vivo human skin explants undergoing dehydration and humectant-induced rehydration were measured via CRS imaging and corneometry. Corneometry data and chemically specific images were obtained from the stratum corneum of each patient sample at each timepoint. The resulting data was statistically analyzed using linear mixed effect model regression analysis. The cellular imaging data revealed water loss in the stratum corneum during dehydration that was correlated with corneometer readings. Interestingly, the imaging data and corneometer readings show differences under the experimental rehydration conditions. The rehydration results suggest that hydration restored by the humectant agents may not be retained by the corneocytes in the ex vivo model system. Given the complementary nature of corneometry, a bulk assessment tool, and CRS microscopy, a modality with subcellular resolution implemented here in an en-face tissue imaging setup, these techniques can be used to measure uptake and efficacy of topical compounds in order to better understand their mode of action and improve therapeutic applications.