Expression of growth associated protein (GAP)-43 in mature and developing rat dental pulp

This study describes on GAP-43 expression in mature and immature rat molars using immunocytochemistry at the light and electron microscopic levels. The GAP-43 positive nerve fibers first entered the dental papilla at the bell stage though they were distributed external to the tooth germ at an earlier stage. The number of nerve fibers in the dental papilla increased daily, and a subodontoblast nerve plexus was gradually formed once root formation had started. A high density of GAP-43-positive nerve fibers was found in the dental pulp of adult rat molars. In these mature teeth, many of the nerve fibers penetrating into the dentin and predentin were GAP-43 positive. GAP-43 immunoreactive fibers had a pulpal and dentinal distribution similar to that demonstrated by immunocytochemistry for other neuronal markers. Ultrastructural immunocytochemistry showed that the reaction products filled all the cytoplasm around the intracellular organelles. A few Schwann cells also expressed GAP-43-immunoreactivity in the mature dental pulp, but no GAP-43 positive Schwann cells were observed in the immature dental pulp. Non-neural elements or extracellular matrix in the dental sac and lamina propria of the oral mucosa also showed GAP-43 immunoreactivity in immature molars. Pulpal and dentinal nerve fibers exhibit GAP-43-immunoreactivity during tooth development as well as in mature teeth. The retention of GAP-43 immunoreactivity may be indication of a potential neural plasticity.