Abstract
Lanthanum salts were applied topically or injected subepithelially into keratinized and nonkeratinized oral mucosa from rabbits and rats; tissue was subsequently removed and examined in the electron microscope. The tracer was evident both on the surface and within the intercellular spaces of the deeper layers of the epithelium but it failed to penetrate the outer quarter of either type of epithelium, indicating the presence of an intercellular barrier. The outer limit of penetration in both tissues coincided with the appearance of “membrane coating granules” at the superficial cell margin, indicating that these organelles may be associated with the formation of the barrier. Identical results have been obtained using the tracer protein horseradish peroxidase, which suggests that the epithelial barrier is impermeable to two tracers differing in both their size and chemical properties.
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