A fine-structural investigation on the extent of perineurial investment of the nerve supply to the pulp in rat molar teeth.

The fine structure of the tissues immediately adjacent to nerve fibres supplying the rat molar pulp has been examined and compared with that of perineurium of other peripheral nerves. In the periapical region, almost all nerve fibres were invested with typical perineurium; only a few Schwann cell units (unmyelinated axons) lacked perineurium. In the pulp at the apex of the distal root, most fibres were invested by less typical perineurium but some were not invested at all. In the pulp at the coronal end of the root, nerve fibres were completely invested or incompletely invested with attenuated perineurium or lacked investment altogether. In the coronal pulp, there was virtually no perineurium at all. Thus rat molar pulp nerve fibres lack perineurium for distances of about 0.5-3.0 mm, distances greater than in situations such as skin or muscle. In normal peripheral nerves, the perineurium behaves as a barrier to the passage of substances from the immediate environment of the nerve fibres (endoneurium) to the surrounding environment (epineurium) and vice versa. Its absence around pulp nerve fibres raises questions about the nature of the pulp environment and may help to explain the sensitivity of pulp nerves to chemical, osmotic and other stimuli.