Patterns of nerve growth factor (NGF), proNGF, and p75 NGF receptor expression in the rat incisor: comparison with expression in the molar.

Nerve growth factor (NGF), a target-derived neurotrophic substance, may have broader biological functions in various types of non-neuronal differentiating cells. The effects of NGF are dependent on initial binding of NGF to specific cell-surface receptors (p75NGFR and p140prototrk) on responsive cells. The continuously growing rat incisor offers an excellent model demonstrating defined territories of differentiation of specific cell populations. We used immunohistochemistry to determine sites of NGF, proNGF and p75NGFR accumulation in the rat incisor, whereas NGF mRNA expression was visualized by in situ hybridization in the developing rat molar and incisor. Strictly similar patterns of NGF mRNA, proNGF and NGF expression were observed in differentiating cells responsible for the production of the main structural matrices of the tooth. Thus, proNGF-like and NGF-like immunoreactivity, as well as the NGF mRNA signal were observed in preameloblasts and young ameloblasts of the dental epithelium and in polarizing odontoblasts of the dental mesenchyme. In contrast, the distribution of p75NGFR was correlated with differentiation event only in dental mesenchyme: polarizing odontoblasts expressed p75NGFR whereas the molecule was absent in functional odontoblasts. In dental epithelium, the restricted expression of p75NGFR in ameloblast precursor cells was correlated with proliferative phenomena. The patterns of proNGF, NGF and p75NGFR expression in epithelium and mesenchyme implicate both an autocrine and paracrine mode of action of the NGF molecule in dental tissues. The findings reported here are important for understanding NGF action in specific dental cell populations and suggest that this molecule is involved in the cascade of events that directs tooth development.