PATTERNS OF NERVE GROWTH-FACTOR (NGF), PRONGF, AND P75NGF RECEPTOR EXPRESSION IN THE RAT INCISOR - COMPARISON WITH EXPRESSION IN THE MOLAR

Nerve growth factor (NGF), a target-derived neurotrophic substance, ma y have broader biological functions in various types of non-neuronal d ifferentiating cells. The effects of NGF are dependent on initial bind ing of NGF to specific cell-surface receptors (p75NGFR and p140protork ) on responsive cells. The continously growing rat incisor offers an e xcellent 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, wher eas NGF mRNA expression was visualized by in situ hybridization in the developing rat molar and incisor. Strictly similar patterns of NGF mR NA, proNGF and NGF expression were observed in differentiating cells r esponsible for the production of the main structural matrices of the t ooth. 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 wi th differentiation events only in dental mesenchyme: polarizing odonto blasts expressed p75NGFR whereas the molecule was absent in functional odontoblasts. In dental epithelium, the restricted expression of p75N GFR in ameloblast precursor cells was correlated with proliferative ph enomena. The patterns of proNGF, NGF and p75NGFR expression in epithel ium and mesenchyme implicate both an autocrine and paracrine mode of a ction of the NGF molecule in dental tissues. The findings reported her e are important for understanding NGF action in specific dental cell p opulations and suggest that this molecule is involved in the cascade o f events that directs tooth development.