It is well known that nerve growth factor (NGF) is expressed in the odontob
lasts of developing teeth, but not in completely grown teeth in rats. Howev
er, there have been no detailed studies of the expression of NGF in reactiv
ated primary odontoblasts or in newly differentiated odontoblast-like cells
during pulp repair. Therefore, to examine the involvement of NGF in odonto
blasts during pulp repair, we immunohistochemically observed the localizati
on of NGF in dental pulp after cavity preparation in rat molars. Based on t
he depth of the cavity, two types of pulp reaction were thus distinguished.
1) In shallow cavities, primary odontoblasts secreted a new dentin matrix,
reactionary dentin, and also expressed NGF. 2) In deep cavities, odontobla
st-like cells, differentiated from pulpal mesenchymal cells, formed a large
amount of reparative dentin or osteodentin. Additionally, intense NGF-like
immunoreactivity was observed in odontoblast-like cells. These results sug
gest that NGF not only acts as a neurotrophic factor of dental pulp, but is
also involved in the differentiation and functional change of odontoblasts
during pulp repair.