Barriers to paracellular permeability in rabbit esophageal epithelium.

Morphological and electrophysiological techniques were used to define the location and nature of the barriers to diffusion across the intercellular space (paracellular pathway) of rabbit esophageal epithelium. Transmission electron microscopy and light microscopy coupled with histochemistry identified a series of tight junctions and an intercellular material staining positively for neutral and acidic glycoconjugates as likely barrier candidates. Additional studies with lanthanum and horseradish peroxidase showed that the barrier to diffusion of tracers was present throughout the stratum corneum and extended to the upper three to seven layers of stratum spinosum and that these findings were most compatible with the presence of the intercellular glycoconjugate material but not the tight junctions. Further positive staining for carbohydrate moieties at the electron microscopic level with periodic acid-thiocarbohydrazide-silver proteinate suggested that the glycoconjugate material was synthesized in the cells of the barrier layers and packaged in intracellular membrane-bound vesicles before secretion into the intercellular space. Although tight junctions were present in series within stratum corneum and, less commonly, extended to two to three cell layers of upper stratum spinosum, analysis of tracer studies, freeze-fracture replicas, electrophysiological data, and mannitol fluxes, while not conclusive, provided little to support a major role for these junctions in barrier function in this tissue.