Stratum corneum pH: Formation and Function of the ‘Acid Mantle’

Abstract

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.

Reconstructed Human Epidermis Predicts Barrier-Improving Effects of <b><i>Lactococcus lactis</i></b> Emulsion in Humans

“…1g), which likely masks the effects of filaggrin degradation products [36]. The formulation should foster the formation of a healthy microbiological skin flora [37, 38]. A skin surface pH of 5.5 enables the stratum corneum lipid-forming enzymes β-glucocerebrosidase and acidic sphingomyelinase to operate close to their optimum pH, which implies enhanced barrier formation [39, 40] and nicely fits to our findings described above.…”

The physical chemistry of the stratum corneum lipids

“…Applying the same strategy to measure the pH within the SC lipid matrix, however, can be very misleading. The often-quoted profiles of pH within the SC obtained using flat pH electrodes and tape-stripping [51,52] uniformly increasing with depth from acidic at the surface to neutral in the deep SC layers -carry no information about the lipid matrix. As these electrodes contact the skin surface through a small water droplet, what they measure in fact is the average distribution of water-soluble species over the SC surface.…”

Water relations of tetrapod integument

“…Normal stratum corneum, as studied in mice and humans, demonstrates a markedly acidic pH, ranging from 4.5 to 5.5 at the surface to physiologic pH (Ϸ7.4) in hydrated, viable epidermis (Ohman and Vahlquist, 1994;Turner et al, 1998;Fluhr and Elias, 2002). The acidic pH is attributable to influence of key metabolites including urocanic acid, free fatty acids, and a non-energy dependent sodium-proton exchanger (Chuong et al, 2002).…”