ABSTRACT
Neonatal skin must adapt rapidly from an aqueous to a terrestrial environment at birth. The development of an acidic stratum corneum (SC), required for the normal processing of SC lipid to form a competent epidermal barrier, is an essential feature of this adjustment. Babies develop an acidic SC, the so-called ‘‘acid mantle,’’ within 4 weeks of birth. Both the etiology of the SC acidity and the mechanisms through which it produces a normal epidermal barrier have been subjects of controversy. Proposed functions of the acid mantle include (a) the facilitation of enzymatic processing of lipids to form competent lipid bilayers that provide the epidermal permeability barrier, (b) regulation of enzymes that control desquamation and cohesion, and (c) antimicrobial defense. Proposed extracellular etiologies include (a) exogenous substances such as lactic acid from eccrine glands, breakdown products from sebaceous gland derived free fatty acids, and metabolic byproducts of bacteria, (b) endogenous metabolic products such as transurocanic acid or free fatty acids, and (c) H+ generated from cellular antiporters such as the Na+/H+. This article will review studies of stratum corneum pH in adults and infants. Although normal skin morphology coexists with an alkaline or neutral surface pH in newborn infants, alkaline skin pH is found in pathological states later in infancy, suggesting that, after the immediate postnatal period, infant skin resembles adult skin in its mechanisms of barrier maintenance and repair. The SC pH gradient probably is generated by multiple factors and probably fulfills multiple roles.
