Interactions of elastic and rigid vesicles with human skin in vitro: electron microscopy and two-photon excitation microscopy

Interactions between vesicle formulations and human skin were studied, in v itro, in relation to their composition and elasticity. The skin ultrastruct ure was investigated using transmission electron microscopy (TEM), freeze-f racture electron microscopy (FFEM) and two-photon fluorescence microscopy ( TPE). The main difference between the vesicle formulations was their elasti city. Elastic vesicle formulations contained bilayer forming surfactants/li pids and single-chain surfactant octaoxyethylenelaurate-ester (PEG-8-L), wh ereas rigid vesicles contained bilayer surfactants in combination with chol esterol. TEM results showed three types of interactions after non-occlusive application of elastic PEG-8-L containing vesicle formulations on human sk in: (1) the presence of spherical lipid structures containing or surrounded by electron dense spots; (2) oligolamellar vesicles were observed between the corneocytes in the upper part of the stratum corneum; and (3) large are as containing lipids, surfactants and electron dense spots were observed de eper down into the stratum corneum. Furthermore, after treatment with vesic les containing PEG-8-L and a saturated C12-chain surfactant, small stacks o f bilayers were found in intercellular spaces of the stratum corneum. Rigid vesicles affected only the most apical corneocytes to some extent. FFEM ob servations supported the TEM findings. Major morphological changes in the i ntercellular lipid bilayer structure were only observed after treatment wit h PEG-8-L containing elastic vesicles. TPE showed a distinct difference in penetration pathways after non-occlusive application of elastic or rigid ve sicles. After treatment with elastic vesicles, threadlike channels were for med within the entire stratum corneum and the polygonal cell shape of corne ocytes could not be distinguished. Fluorescent label incorporated in rigid vesicles was confined to the intercellular spaces of the upper 2-5 mu m of the stratum corneum and the cell contours could still be distinguished. (C) 1999 Elsevier Science B.V. All rights reserved.