Effects of long-term hydration leading to the development of polar channels in hairless mouse stratum corneum.

Dramatic increases in in vitro permeability coefficients have been observed following the long-term hydration (days) of hairless mouse skin. The effect is greatest for extremely polar or ionized solutes. Differential scanning calorimetry has been utilize in complementary studies in an attempt to assess if the altered permeability may be due to the irreversible alteration of lipid fluidity or keratin structure. The melting range of the lipid endotherms appears to be unaffected when stratum corneum is subjected to hydration conditions similar to those in the permeability experiments. Endotherms attributed to keratin appear to be altered by long-term hydration. However, no quantitative correlation was found between the keratin endotherm area and the permeabilities. The permeability data fit a model where the total permeability coefficient at a given time is the sum of the permeability coefficients associated with the lipid route and the polar route. Permeation increases with hydration time due primarily to alteration of the polar route. Based on molecular weight, no limitation of diffusion in the polar pathway was detected.