Intercellular junctions of oral epithelium: I. Studies with freeze-fracture and tracing methods of normal rat keratinized oral epithelium

Abstract

The morphology and distribution of the intercellular junctions have been investigated in normal oral epithelium of the rat. Desmosomes have been found to undergo profound modifications during keratinization as revealed by freeze fracture: a decrease of the number of intramembranous particles or both P and E faces, as well as the formation of an amorphous plaque on the E face in the cornified layer have been observed. Gap junctions have been found in the basal and intermediate layers; in contrast, they were never observed between cornified cells. Tight junctions have been observed only at upper granular layer; they were arranged to form small discrete maculae occludentes rather than beltlike zonulae. Numerous lamellar bodies have been found in the intercellular space at the boundary of the granular and cornified layers. These findings are discussed in relation to the existence of the physiological permeability barrier in the oral epithelium. The hypothesis is put forward that both tight junctions and lamellar bodies might be responsible for establishing the barrier, which would thus consist of multiple discrete sealing devices, able of establishing a number of successive slow down steps in the extracellular diffusion of solutes, rather than of a single, continuous, impermeable structure.