Stratum corneum pH and ceramides: Key regulators and biomarkers of skin barrier function in atopic dermatitis

The skin, as the outermost layer of the body, serves as a crucial protective barrier against environmental insults while maintaining homeostasis. Atopic dermatitis (AD), a chronic inflammatory skin disorder characterized by recurrent eczema and type 2 inflammation, affects a significant global population. The pathophysiology of AD is closely linked to skin barrier dysfunction, which contributes to increased permeability, immune dysregulation, and microbial imbalances. Historically, skin barrier research has centered on the stratum corneum (SC) and intercellular lipids within the epidermis, primarily conceptualized through the "brick-and-mortar" model. However, recent advancements have revealed a more intricate interplay among various barrier components. Two key determinants of skin barrier—SC pH and SC ceramides—have gained substantial attention. Elevated SC pH leads to enhanced serine protease activity, impaired lipid metabolism, and microbiome dysbiosis, all of which exacerbate barrier dysfunction and inflammation in AD. Concurrently, alterations in SC ceramide profiles and structures compromise skin barrier function. Emerging evidence underscores the potential of SC pH and ceramides as biomarkers for disease progression and as therapeutic targets for barrier restoration. Advances in lipid analyses and non-invasive pH assessment offer promising prospects for personalized dermatologic interventions. This review explores the complex interactions of SC pH and ceramides in AD pathogenesis, discussing their implications for predicting disease flares, guiding treatment strategies, and identifying novel drug targets. A deeper understanding of these mechanisms could pave the way for next-generation therapeutic approaches in AD and other skin barrier-related disorders.