Development of sensory innervation in dentin

We have labeled dental nerves of 3-week- to 1-year-old rats by axonal transport of radioactive protein in order to detect nerves in developing dentin by autoradiography. We found that, in addition to nerve growth, two processes determine adult dentinal nerve location: (1) enclosure of nerves within circumpulpal dentinal tubules during the last few weeks of dentinogenesis, beginning at the tip of the pulp horn and spreading to include most coronal dentin; and (2) gradual loss of the nerves near the tip of the cusp because of dentinal attrition and replacement by noninnervated, reparative dentin. Several days before a molar erupts, nerves at the tip of the cusp have already begun to be enclosed by dentin; 2-3 weeks later, when dentinogenesis at the cusp tip slows down, most of the innervation for that region has been established, nerves extend up to 160 micron into dentin, and the molar has reached the stage of functional occlusion. Soon after initial occlusion, the process of dentinal attrition and replacement by noninnervated reparative dentin begins; however, simultaneous dentinogenesis in more apical regions produces new innervated dentin. We studied nerve position in relation to dentin growth lines and found that there is a long-term association of nerve endings with specific sites in the dentinal tubules. The similarities between the development of dentinal innervation in rat molars and in human permanent teeth are discussed; it is found that as teeth mature and become more sensitive to painful stimuli, the density of dentinal nerves increases.