The stratum corneum is the most easily accessed part of the skin itself, and there are two actions targeted to this tissue, namely, emolliency, the softening of the horny tissue, which comes about through remoisturizing it, and keratolysis, the chemical digestion and removal of thickened or scaly horny tissue. Tissue needing such removal is found in calluses, corns, and psoriasis and as dandruff. Common agents as salicylic acid and, to a lesser extent, sulfur, cause lysis of the sulfhydral linkages holding the keratin of the horny structure together, leading to its disintegration and sloughing.
It has been mentioned that elasticity of the stratum corneum depends on its formation and on the presence of adequate natural lipids, hygroscopic substances, and moisture (19,20). Simply occluding the surface and blocking insensible perspiration can induce remoisturization (emolliency). However, it is best accomplished by lotions, creams, and/or waxy formulations (e.g., lip balms), which replenish lost lipid constituents of the stratum corneum. The fatty acids and fatty acid esters these contain in part fill the microscopic cracks and crevices in the horny layer, sealing it off, stabilizing its bilayer structures, allowing it to retain moisture. Many emollient products also contain hygroscopic glycols and polyols to replenish and augment natural moisturizing factors of this kind, also assisting the stratum corneum in retaining moisture.
The introduction of moisturizing substances into the stratum corneum is ordinarily a straightforward process. Deposition of keratolytics, on the other hand, is not as easily achieved, as these agents must penetrate into the horny mass itself. Some salicylic acid-containing corn removers are therefore made up as concentrated nonaqueous solutions in volatile solvents. As these volatile solvents evaporate, drug is concentrated in the remaining vehicle and thereby thermodynamically driven into the tissue. These many examples illustrate the fact that when the therapeutic target is at the skin's surface or is the stratum corneum, the therapeutic rationale behind the treatment usually involves enhancing or repairing or otherwise modulating barrier functions.
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