Immunohistochemical analysis of pulpal innervation in developing rat molars.

The beginning of pulpal innervation was examined in these developing teeth. Mandibular first molars removed from newborn to 7-day-old rats were cryosectioned and nerve fibres were localized by the peroxidase-antiperoxidase technique using a neuropeptide-specific (200 kDa) antibody. Some pulps from 5- and 7-day-old rats were also examined by conventional electron microscopy. In newborn to 4-day-old rats, molars in the initial stages of the dentine deposition were innervated in the follicles but not in the pulps. In molars from 5-day-old rats, nerve fibres were found in the pulps of 11 out of 16 samples. The fibres were mainly located along the blood vessels in the basal part of the pulp, with some arborizations. In rats of 6 and 7 days old, nerve fibres were found in all the pulps examined. These had gradually extended deep into the cuspal area and were increasingly arborized with increasing age. Nerve fibres were also found along the basal laminae of blood vessels in some dental pulps from 5-day-old rats when examined by electron microscopy. At 7 days, nerve fibres were composed of bundles of axons, some of which were covered with Schwann cell processes and basal laminae. These observations indicate that the innervation of rat molar dental pulps begins after the start of the deposition of enamel, in animals of around 5 days of age, which is at the same stage as in mouse molars, as others have shown by a silver-staining method.