Past attempts to relate mechanical properties of the stratum corneum to its state of hydration or disease condition have been hampered by the lack of adequate in vivo methodology. With the technique described herein, viscoelastic properties of the skin surface can be measured in vivo or on excised skin in vitro. This method is sensitive enough to detect changes in the properties of the stratum corneum induced by topically applied agents or by mechanical disruption of it. The viscoelastic data suggest that the outermost layers of the stratum corneum may become dry in vivo despite their proximity to the underlying wet tissue, and that the instrument is sensitive enough to measure the changes that occur. It also appears that the in vivo behavior of the stratum corneum is similar to that of isolated stratum corneum membranes which become brittle when they are dried and supple when they are hydrated.

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The role of stratum corneum (SC) constituents in SC barrier properties was assessed directly in vitro in a reaggregated system. Fragments of SC from the sole of the foot (hyperplastic tissue) and from the calf (normal tissue) were reduced, separately, to individual cells and these cells were depleted of lipids by solvent extraction. Lipids taken from the cells themselves or from SC from other areas of the body were then added to the cells. None of the fractions used allowed for reaggregation of plantar SC cells, or of trypsinized calf SC cells. In contrast, calf SC cells reaggregated equally well with all lipids and formed coherent SC sheets. At equal lipid:cell ratios, however, plantar lipid-calf cell SC sheets were about three time more permeable to water than calf lipid-calf cell SC sheets. The effectiveness of the barrier function of the SC sheets was directly proportional to the amount of added lipid. We conclude that in normal SC the extractable lipids are intimately involved in cellular adhesion/dyshesion and in barrier formation.

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