Abstract
Confocal Raman microscopy has a number of advantages in investigating the human stratum corneum (SC) in vivo and ex vivo. The penetration profiles of xenobiotics in the SC, as well as depth profiles of the physiological parameters of the SC, such as the concentration of water depending on the strength of hydrogen bonds, total water concentration, the hydrogen bonding state of water molecules, concentration of intercellular lipids, the lamellar and lateral packing order of intercellular lipids, the concentration of natural moisturizing factor molecules, carotenoids, and the secondary and tertiary structure properties of keratin are well investigated. To consider the depth-dependent Raman signal attenuation, in most cases a normalization procedure is needed, which uses the main SC's protein keratin-related Raman peaks, based on the assumption that keratin is homogeneously distributed in the SC. We found that this assumption is not accurate for the bottom part of the SC, where the water concentration is considerably increased, thus, reducing the presence of keratin. Our results demonstrate that the bottom part of the SC depth profile should be multiplied by 0.94 in average in order to match this non-homogeneity, which result in a decrease of the uncorrected values in these depths. The correctly normalized depth profiles of the concentration of lipids, water, natural moisturizing factor and carotenoids are presented in this work. The obtained results should be taken into consideration in future skin research using confocal Raman microscopy.
Highlights
The stratum corneum (SC), the outermost continuously renewing skin layer, provides the barrier function
Confocal Raman microscopy has a number of advantages in investigating the human stratum corneum (SC) in vivo and ex vivo
The intercellular lipids (ICL) of the lipid matrix are organized in the form of a classical membrane and their orthorhombic lateral organization represents a highly ordered and very densely packed structure, which is notpermeable for the majority of the non-destructive substances [2,3,4]
Summary
The stratum corneum (SC), the outermost continuously renewing skin layer, provides the barrier function. A recent in vivo study using CRM confirmed the non-homogeneous distribution of lateral packing order of ICL, showing the prevalence of the orthorhombic phase in the depth of approx. These results correlate with further parameters sensitive to the skin barrier function that were determined using in vivo CRM, such as the NMF concentration [17], or the hydrogen bonding state of water [18] One perception from these studies was, that the maximal skin barrier function occurs at the depths 20–40% of the SC thickness. The analysis of barrier disrupted skin, as e.g. in patients with atopic dermatitis is a further focus using CRM [20]
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