Although the acidic surface pH of the skin has been known for over a century, the stratum corneum (SC) pH gradient has been discovered only recently. After removal of SC layers with sequential tape strips, in humans, surface pH starts at 4.5–5.3, increasing by about 2–3 units until it reaches 6.8 in the lower SC. Both exogenous [free fatty acids (FFA) from sebaceous lipid, microbial metabolites, lactic acid from eccrine glands] and endogenous epidermal (enzymatic; membrane antiporters/pumps) mechanisms have been hypothesized to contribute to SC acidification. Three endogenous mechanisms have been identified to date that not only could influence SC pH, but also regulate one or more key SC functions: (1) the histidine-to-urocanic-acid pathway; (2) the phospholipid-to-FFA pathway, and (3) the sodium proton antiporter (NHE1). Additional factors that influence human surface pH have been detected. Endogenous factors, unrelated to pathological features, such as racial differences, topographical variation, gender differences, developmental and age-related changes have been reported. In contrast to adults, human newborn SC displays a near-neutral surface pH, which declines rapidly over the first postnatal month. Furthermore, endogenous factors have been described, as in atopic dermatitis, seborrheic dermatitis, diabetes, renal insufficiency and ichthyosis associated with an increased pH. Exogenous factors, like the use of detergents and cleansing products as well as SC hydration, can modify the surface pH. The consequences of SC acidification for several key SC functions are becoming clear, including (1) the role of an acidic pH for SC permeability barrier homeostasis, (2) the pH dependence of extracellular lipid processing and (3) SC integrity/cohesion. Integrity is defined as a measure of resistance to dissociation of adjacent corneocytes by tape stripping (cohesion is a related index defined as the amount of protein removed per stripping). (4) Proteolytic processes leading to desquamation are pH dependent, and (5) an acidic pH provides important antimicrobial resistance.

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