Water diffusion characteristics of human stratum corneum at different anatomical sites in vivo.

Despite its heterogeneity, stratum corneum (SC) has been described as a homogeneous membrane for water diffusion. We measured water flux across the SC, transepidermal water loss (TEWL), in six women, in vivo. At four anatomical sites--back, abdomen, forearm, and thigh--we took measurements during sequential tape stripping. The inverse of TEWL (1/TEWL) and removed SC thickness yielded a highly linear correlation (Pearson's r ranging between 0.88 and 0.99). Applying Fick's law of diffusion, we calculated SC thickness (H), and SC water diffusion coefficient (D). Comparing the results, SC of all women was significantly thicker (p < 0.05) at the extremities (12.7 +/- 4.2 microm, mean +/- SD, n = 12) than the abdomen (7.7 +/- 1.8 microm, n = 6). The calculated diffusion coefficient approximated 2.16 +/- 1.14x10(-9) cm2/s. Compared with the diffusion constant found for SC depleted of lipids, our value was 100-fold lower. In agreement with previous findings that intercellular lipids are a rate determining component of the SC barrier, we suggest that water diffuses mainly through the intercellular space. The calculation of H and/or D, however, is based on several variables: SC density, the water concentration difference, and the partition coefficient of water between viable epidermis and SC. The literature values vary widely. It is desirable to determine these parameters more precisely, especially if discrete differences, such as between anatomical sites, are to be revealed.