Influence of skin type, race, sex, and anatomic location on epidermal barrier function Razvigor Darlenski, MD a, ⁎ , Joachim W. Fluhr, MD b a Department of Dermatology and Venereology, Tokuda Hospital-Sofia, 51B Nikola Vaptsarov Blvd, 1407 Sofia, Bulgaria b Department of Dermatology, Charité University Clinic, Charitéplatz 1, 10117 Berlin, Germany Abstract The intact skin represents a barrier to the uncontrolled loss of water, proteins, and plasma components from the organism. Owing to its complex structure, the epidermal barrier with its major layer, the stratum corneum, is the rate-limiting unit for the penetration of exogenous substances through the skin. The epidermal barrier is not a static structure. The status of different functions of the epidermis can be monitored by assessing specific biophysical parameters such as transepidermal water loss, stratum corneum hydration, and skin surface pH. Variables originating from the individual as well as exogenous factors have an important influence on the epidermal barrier parameters. © 2012 Elsevier Inc. All rights reserved. Introduction Skin, being the largest and the outermost organ of the human body, accomplishes multiple defensive and regula- tory functions. 1 The skin barrier function resides almost entirely in the epidermis and, in particular, in its superficial layer, the stratum corneum. 2 The SC consists of a protein component, the corneocyte (bricks), which provides mechanical resistance to the SC and a platform for the second component, the extracellular matrix of the SC, consisting of multiple layers of lipids (mortar), which, in turn, mediate the permeability barrier against excess water and electrolyte loss. The epidermal barrier protects the human body against many external stressors, including physical stress (eg, mechanical, thermal, ultraviolet radiation), chemical stress (eg, tensides, solvents, topical xenobiotics), and environ- mental conditions. 3,4 In past decades, a number of in vitro, ex vivo, in silico, and mathematic models have been developed for studying and predicting skin barrier perme- ation and the penetration of exogenous agents 5-10 ; however, none of these methodologies can simulate thoroughly real- life conditions in humans. 11 Current methods applied in in vivo human studies, such as suction blister fluid, micro- dialysis, skin biopsy, and tape stripping, exhibit certain disadvantages, among them, the need for standardization and the invasiveness of the test procedures. 9,11 The assessment of the epidermal barrier routinely involves measurements of transepidermal water loss (TEWL), providing information on epidermal permeability barrier status under normal, experimentally perturbed, or diseased conditions. 12 The validity of TEWL as a parameter reflecting permeability barrier status was proved by correlating ex vivo gravimetric measurements to absolute rates of water loss and determination of TEWL. A low TEWL is generally a characteristic feature of an intact skin function in vivo. 12,13 Elevated TEWL values are observed in a number of diseases with skin barrier abnormalities, such as atopic dermatitis and ichthyosis vulgaris, and in experimental barrier perturbation, including application of detergents, solvents, physical stimuli, and cellophane tape stripping. 14-19 TEWL measurements can be used to assess the homeostasis of the permeability barrier and indirectly to ⁎ Corresponding author. Tel.: +359 988 2933 713. E-mail address: darlenski@abv.bg (R. Darlenski). 0738-081X/$ – see front matter © 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.clindermatol.2011.08.013 Clinics in Dermatology (2012) 30, 269–273