Developmental changes in pulpal sensory innervation of rat incisors and molars shown on a single injection of the fluorescent dye AM1-43.

Developmental changes in pulpal innervation of rat incisors and molars were examined using the fluorescent styryl dye AM1-43, which labels sensory cells and nerves in vivo. From 2 to 40 days after birth, the animals were injected once with a small amount of AM1-43 solution (2 microg/g bodyweight). One day after the injection, the animals were killed and examined. In 3-day-old rats, neither incisors nor molars were innervated. In 7-day-old rats, the pulp of incisors and molars was innervated as indicated by fine intensely stained varicose sensory fibers and thicker intensely stained fibers running mostly along the blood vessels. In 15-, 27-, and 41-day-old rats, sensory nerve fibers neither passed through the odontoblast layer nor penetrated into the dentin in incisors, whereas the sensory nerve fibers penetrated into the coronal dentin through the odontoblast layers in molars. These results suggest that innervation of dental pulp is composed of two phases: (i) linear penetration of nerve fibers along blood vessels into the pulp space; and (ii) sprouting and extension of nerve fibers into coronal dentin. Innervation of the incisor pulp may stop at the first phase.