The permeability of mammalian nonkeratinized oral epithelia to horseradish peroxidase applied in vivo and in vitro.

Horseradish peroxidase, an intercellular tracer, was injected sub-epithelially beneath keratinized gingiva and a variety of nonkeratinized oral epithelia. Fresh tissue biopsies from the same regions were also incubated with this tracer for 1 h. In separate experiments, horseradish peroxidase was applied topically to biopsies of oral mucosa maintained in tissue culture for periods up to 12 h. All specimens were treated so as to visualize the site of peroxidase penetration and the tissues were examined with the light and electron microscopes. In all the epithelia, a surface barrier was evident from which the tracer had been excluded. This was often narrow in the thin, nonkeratinized epithelia and could only be identified unambiguously with the electron microscope. Membrane-coating granules of the so-called nonkeratinized type were invariably associated with superficial plasma membrane of epithelial cells at the level where the barrier was first seen. The results suggest that a permeability barrier exists to horseradish peroxidase in all nonkeratinized oral epithelia with a similar location to that seen in the keratinized oral tissues. However, results obtained with a water-soluble tracer such as horseradish peroxidase may not apply to many drugs, the ready absorption of which may reflect their high lipid solubility.