Barrier Skin
The skin is a major interface between the body and the environment. In most cases the skin serves as a perfect barrier; however it is dependent on several factors such as the chemical properties of a compound to which the skin is exposed topically, but al
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Barrier Skin Hans F. Merk
The skin is a major interface between the body and the environment. In most cases the skin serves as a perfect barrier; however it is dependent on several factors such as the chemical properties of a compound to which the skin is exposed topically, but also its concentration, the contact duration, frequency of exposure, and the exposed surface area influence the amount of penetration which may lead to local reactions including irritation, sensitization, and inflammation but also to penetration and entering the systemic circulation which may result in systemic effects [1, 48]. There are several lines of defenses by the barrier organ skin: –– The chemo-physical barrier of the stratum corneum –– The immunocompetent cells of the epidermis and corium –– The armamentarium of xenobiotica-metabolizing enzymes in epidermal cells such as the keratinocytes as well as antigen-presenting cells, e.g., the Langerhans cells –– The melanocyte-keratinocyte unit and its role in pigmentation and protection against UV radiation H.F. Merk Department of Dermatology and Allergology, RWTH Aachen University, Aachen, Germany Dohlenfeld 8, 45479 Muelheim adR, Auf’m Hennekamp 50, D-40225 Düsseldorf, Germany e-mail: [email protected] © Springer International Publishing Switzerland 2018 J. Krutmann, H.F. Merk (eds.), Environment and Skin, https://doi.org/10.1007/978-3-319-43102-4_1
The main chemo-physical barrier of the skin is located in the outermost layer of the skin, the stratum corneum, which consists out of corneocytes surrounded by lipid regions. The main protein are the keratins; however they need several other proteins for their formation in a way that they can function as a barrier. One of these proteins is filaggrin. Filaggrin is a protein which therefore is essential for a normal barrier formation in stratum corneum, and a diminished expression of this protein is associated with the precipitation of atopic dermatitis and ichthyosis vulgaris if the mutation of filaggrin is homozygous and no active filaggrin results [2]. The water content of the cells in the stratum corneum is with 10–15% much lower than in the keratinocytes of the basal area of the epidermis—the stratum basale—or in other cells of the body with about 75–85%. Therefore there is a gradient of the concentration of water in the epidermis resulting in a water loss which is much higher than the water loss by sweating at room temperature up to 29 °C. Under these conditions, the total amount is about 500 mL/day with a diffusion gradient of 0.5–1.0 mg cm−2 h−1. This gradient is highly controlled by the stratum corneum, and the transepidermal water loss (TEWL) can be severalfold enhanced if the stratum corneum is removed. Therefore the TEWL correlates well with the barrier function of the skin [3]. The pH value of stratum corneum in healthy skin is around 4.6–5.6; that means it is acidic. This acidity is necessary for a normal ceramide and lipid 3
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H.F. Merk
production, because it is a prerequisite for a nor- Table 1.1 Xenobiotic
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