Stratum corneum hydration regulates key epidermal function and serves as an indicator and contributor to other conditions

Abstract

Xerotic skin is a skin condition characterized by a reduced water content in the stratum corneum (SC). Measurements of skin impedance or capacitance are commonly used to assess SC water content (i.e. ‘SC hydration’). Levels of SC hydration are largely determined by the quality and quantity of endogenous moisturizers, including SC lipids, glycerol, amino acids and salts. Alterations in natural moisturizers in xerotic skin vary with age and/ or skin conditions. While reductions in epidermal lipids and filaggrin likely contribute to decreased SC hydration in both atopic dermatitis and aged skin (Fig. 1), ‘seasonal dry skin’ is clearly associated with reduced content of sodium and potassium in the SC. Moreover, reduction in SC glycerol levels also contributes to decreased SC hydration in both aged and aquaporin 3 knockout mice. Thus, a variety of different endogenous moisturizers can affect SC hydration. Certain systemic conditions can also reduce SC hydration. Both type II diabetes and haemodialysis patients display reduced SC hydration levels. Moreover, obese women also frequently exhibit reduced SC hydration, likely due to decreased SC ceramide content. Over the lifetime of normal humans, the SC hydration levels peak at 45 years of age, followed by a progressive decline. Therefore, systemic disorders and developmental age can alter SC hydration levels. In this issue of the Journal, Autustin et al. provide further evidence that the levels of SC hydration are associated with both systemic (ageing) and cutaneous disorders. Surprisingly, over 16% of all subjects ≥16 years of age had dry skin, and the prevalence of dry skin increased with age. Another important, confirmative finding in Autustin’s study is that dry skin is associated with certain inflammatory dermatoses. Previous studies have shown that the levels of SC hydration correlate negatively with severity of eczematous dermatitis, including ‘Scoring of Atopic Dermatitis (SCORAD)’, ‘Eczema Area and Severity Index (EASI)’ and ‘Atopic Dermatitis Severity Index (ADSI)’. Recently, reductions in SC hydration have been implicated in the pathogenesis of certain disorders. Dry skin is linked to the development of both seasonal and age-associated pruritus, and reduced SC hydration is also associated with pruritus, potentially leading to exacerbations of atopic dermatitis (Fig 1). Additionally, reduced SC hydration can also provoke cutaneous and possibly systemic inflammation. The benefits of emollients for both eczematous dermatitis and pruritus have been well demonstrated. For example, recent studies have shown that reduced SC hydration levels can predict relapses of psoriasis, while improvements in epidermal function, including SC hydration, can prevent such relapse. Our recent studies demonstrate that improving SC hydration can lower the circulating levels of pro-inflammatory cytokines in the chronically aged mice and humans (Elias PM and Man MQ, unpublished data), suggesting that decreased SC hydration could contribute to the downstream development of age-associated systemic inflammation. Therefore, measurements of SC hydration could provide an auxiliary approach to monitor/assess cutaneous and systemic conditions, and routine applications of emollients could provide an inexpensive and valuable alternative to improve human health.