Normal skin appearance, water balance, and continued barrier integrity necessitate an intact epidermal barrier with maintenance of the proper water content required for physiologic and enzymatic functions. As the epidermis is a living dynamic unit, several physiologic functions continue as an ongoing process, with perturbations of barrier integrity requiring necessary adjustments and repairs before the epidermal barrier can return to its normally functioning physiologic state. The epidermal barrier is comprised of two components which work in concert to assure barrier integrity through functions such as maintenance of proper epidermal water balance, physiologic stratum corneum water content (20-35%), optimal lipid synthesis, limitation of transepidermal water loss (TEWL), and orderly corneocyte desquamation (1-4). The first component of the epidermal barrier, the cellular matrix, is comprised of a staggered and layered lattice of keratinocytes, referred to as the “bricks.” In its uppermost layer, the flattened stratum corneum cells are referred to as corneocytes. The second component of the epidermal barrier, the intercellular lipid bilayer matrix, surrounds the keratinocytes, and is referred to as the “mortar” (1-3). Disturbances of these epidermal barrier components, associated with a variety of causes such as use of harsh soaps or underlying “sensitive skin” disorders such as atopic dermatitis or rosacea, enhance TEWL, which can lead to xerotic skin changes. When increased TEWL produces a reduction in stratum corneum water content to below 10%, this marked loss of epidermal barrier integrity is visibly expressed as dryness, scaling, roughness, and fine fissuring, the clinical features of xerosis (2,3,6-8).
The epidermis is in constant flux as keratinocytes traverse from the basal layer, later flattening as they pass upward into the stratum corneum, leading ultimately to surface shedding, or corneocyte desquamation. As referred to above, under normal circumstances, adequate water content allows for enzymatic degradation of the attachments between corneocytes (corneodesmosomes), allowing for the physiologic separation and shedding of superficial corneocytes. Corneocyte moisture content is maintained by a collection of diverse intracellular hygroscopic compounds which have been collectively termed “natural moisturizing factor” (NMF). The components of NMF include filaggrin-derived
amino acids, pyrrolidone carboxylic acid, lactate, sugars, and several electrolytes (1-3,5). Under abnormal conditions associated with xerosis, corneodesmosomes are not readily deagraded, leading to clumping of corneocytes. The visible expressions of clumped corneocytes with impaired desquamation are flaking and scaling (1-3,5,8,9).