Stratum corneum, corneodesmosomes and ex vivo percutaneous penetration

The stratum corneum is composed of cornified keratinocytes, joined together with corneodesmosomes and embedded in the relatively hydrophobic intercellular substance. Formation of this horny layer and its constant desquamation are fundamental processes leading to the establishment of an efficient epidermal barrier. We examined structural changes occurring in the stratum corneum ex vivo, in order to better understand the mechanisms regulating corneocyte desquamation at the epidermal surface and influencing percutaneous penetration of exogenous substances. Morphologic modifications were induced by occlusion, increasing the tissue hydration, or by application of propylene glycol, a hydrophilic solvent of small molecular size. Distribution of the major epidermal antigens, markers of terminal differentiation, was studied immunohistochemically and showed no modification related to the tissue alteration. Skin samples were fixed in paraformaldehyde and either postfixed in OsO4 and embedded in Epon or postfixed in RuO4 and embedded in epoxy and acrylic resins. Structural composition and spatial organization of the elements present in the intercorneocyte spaces were evaluated ultrastructurally, with special attention paid to the fate of corneodesmosomes. Our results suggest that the spontaneous organization of lipids excreted into the intercellular spaces and constitution of the lamellar multilayers in the lower stratum corneum are at the origin of partition of the intercellular compartment to the hydrophobic and hydrophilic domains. The latter, compatible with the proteinic contents, seem to be displaced by the hydrophobic components undergoing reorganization, towards the invariable hydrophilic foci represented by corneodesmosomes. This mechanism may be involved in the delivery of proteolytic enzymes, thus contributing to the progressive degradation of corneodesmosomal proteins.