Ultrastructure of the epidermal barrier after irritation

The stratum corneum (SC) controls the diffusion and penetration of chemical substances and drugs into and through the skin. Surprisingly, knowledge of the SC structure and reaction to the various irritants is still poorly understood. Routine transmission electron microscopy has not been effective in demonstrating the epidermal lipids (EL) of SC which are believed to morphologically represent the water permeability barrier. To gain a better understanding of the interaction of chemically different irritants with the SC, we investigated the ultrastructural changes of epidermal lipids resulting from the topical application of sodium dodecyl sulfate (SDS 0.5% and 1% w/v) and absolute acetone. The disturbance of barrier function by these irritants was determined by the increase of transepidermal water loss (TEWL). Punch biopsies from the treated sites showed a maximum increase of TEWL. To visualize the EL which derive from lamellar body (LB) lipids (sheets), we used a special fixation method utilizing 0.5% ruthenium tetroxide/0.25% KFe(CN)6 as the postfixative. The 0.5% SDS caused cell damage to the nucleated cells of the epidermis with disturbance of LB lipid extrusion and the transformation into the lipid bilayers. However, the upper portions of SC displayed intact intercellular lipid layers. With the acetone treatment, the EL lamellae showed disruption and loss of cohesion between the lamellae at all levels of the SC. The more polar LB lipids appeared more resistant to acetone. The results of this study suggest that different irritants induce distinct and characteristic alterations to reflect the specific interaction with the epidermal permeability barrier.