INTRODUCTION
The skin is a complex barrier composed of tightly packed junctions consisting of 3 parts: stratified epidermis, dermis, and subcutaneous tissue.1 A primary function of the skin is to maintain a physical barrier against external exposures, preventing entry of harmful substances and maintaining physiologic water content.1 The permeability barrier function of the epidermis prevents excessive fluid gain or loss from the body occurring within the stratum corneum (SC), the outer portion of the epidermis, which is collectively active in maintaining multiple skin barrier functions necessary to sustain healthy skin.2 When the skin is compromised by exogenous and/or external environmental exposures, permeability barrier function is impaired. This leads to increased transepidermal water loss (TEWL), which predisposes the skin to adverse sequelae of xerosis; microfissuring; decreased resiliency; natural elastic recoil, scaling, and hyperkeratosis; and an increased risk of microbial infections.2-4 The water content inside the SC is essential to maintaining the structural and functional integrity of the skin and the multiple functions of the skin barrier.3,5
The SC usually is described as a brick-and-mortar complex filled with keratin cells and a lipid matrix, which sounds fixed in its structure, but is dynamic, responds to exposures, and corrects itself with innate repair mechanisms.2,6,7 Specific enzymes in the epidermis act on phospholipids to synthesize ceramides, free fatty acids, and cholesterol in a physiologic ratio of relative concentrations, thus regulating the permeability and structural functions of the SC.8,9 Ceramides are the predominant lipid of the SC, making up approximately 50% of the intracellular content by mass.10 However, the SC contains additional key elements associated with barrier functionality, including urea, lactic acid, and filaggrin-derived amino acids and amino acid metabolites (ie, pyrrolidone carboxylic acid [PCA] and urocanic acid [UCA]).11 These components, known collectively as the natural moisturizing factor (NMF), are important in regulating SC homeostasis. They act as "nature's humectant" to retain physiologic epidermal water content and maintain the structural integrity of keratin bundles in corneocytes essential for reducing TEWL.12 Importantly, other hydrolytic enzymes in the SC are responsible for desmolytic activity, which allows for
The SC usually is described as a brick-and-mortar complex filled with keratin cells and a lipid matrix, which sounds fixed in its structure, but is dynamic, responds to exposures, and corrects itself with innate repair mechanisms.2,6,7 Specific enzymes in the epidermis act on phospholipids to synthesize ceramides, free fatty acids, and cholesterol in a physiologic ratio of relative concentrations, thus regulating the permeability and structural functions of the SC.8,9 Ceramides are the predominant lipid of the SC, making up approximately 50% of the intracellular content by mass.10 However, the SC contains additional key elements associated with barrier functionality, including urea, lactic acid, and filaggrin-derived amino acids and amino acid metabolites (ie, pyrrolidone carboxylic acid [PCA] and urocanic acid [UCA]).11 These components, known collectively as the natural moisturizing factor (NMF), are important in regulating SC homeostasis. They act as "nature's humectant" to retain physiologic epidermal water content and maintain the structural integrity of keratin bundles in corneocytes essential for reducing TEWL.12 Importantly, other hydrolytic enzymes in the SC are responsible for desmolytic activity, which allows for
