In situ precipitation: a novel cytochemical technique for visualization of permeability pathways in mammalian stratum corneum.

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Abstract
Although the stratum corneum was treated as a homogeneous film across which substances penetrate without regard to cellular substructure, recent evidence indicates that the stratum corneum comprises a heterogeneous layer of corneocytes embedded in expanded, neutral, lipid-rich intercellular domains. Lipophilic substances should preferentially traverse the stratum corneum between cells rather than through them. Transport pathways were visualized by perfusing neonatal mouse and human stratum corneum sheets with n-butanol for 5 min to 2 h, followed by in situ precipitation of n-butanol with Os vapor. Control samples were treated with Os alone or with rapidly penetrating agents (n-hexanol) that were nonreactive with Os. Ultrathin sections of tissues perfused with n-butanol but not with n-hexanol demonstrated expanded stratum corneum interstices filled with electron-dense precipitate, identified as Os by energy-dispersive X-ray analysis (EDAX). More Os (8-fold) was demonstrated in intercellular domains of n-butanol-treated samples than in control locales by EDAX. When randomly selected negatives were analyzed by scanning densitometry there was approximately 3-fold more Os precipitate in the interstices than in the cytoplasm in n-butanol perfused tissues. In situ precipitation represented a useful approach to the study of percutaneous transport and may be applicable as a technique for studying lipid vs. aqueous channels in other tissues. The stratum corneum intercellular spaces may serve as a preferential transport pathway for certain lipid-soluble substances.