Extracellular fluid movement in the pulp; the pulp/dentin permeability barrier.

Fluid movement in the pulp depends largely upon the physiology of the blood vessels; normally there is a net efflux of fluid and proteins from the capillaries into the extracellular environment. Most pulp capillaries lie close to the odontoblast layer and in order to see whether fluid can pass between the odontoblasts into the predentin we have perfused the vessels of molar tooth germs in anesthetized piglets with the electron dense tracer lanthanum. The results show that the tracer permeates the capillaries but encounters a barrier to permeability at the apical (predentinal) ends of the odontoblasts. The completeness of the barrier to the tracer lanthanum is discussed together with structural evidence of tight junctions between odontoblasts in both pigs and humans and the presence of collagen fibers through the tight junctional zone. It is concluded that there is little or no evidence that pulp fluid is normally confluent with predentin. An advantage of this arrangement may be that by maintaining an enclosed microenvironment it permits regulation of the orderly process of matrix deposition and mineralization of predentin to dentin. In order to maintain constant vascular and extracellular fluid pressures the capillary efflux has to be balanced by fluid removal; recent work in cats has shown that lymphatic vessels are available to transport fluid out of the pulp. In this paper the differences in the intrapulpal distribution of these vessels have been extrapolated to human teeth in an attempt to explain certain variations in the symptoms and progress of pulpal inflammatory conditions.