Gene expression and localization of insulin-like growth factors and their receptors throughout amelogenesis in rat incisors.

Insulin-like growth factors (IGFs) are expressed in many tissues and control cell differentiation, proliferation, and apoptosis. In teeth, the temporo-spatial pattern of expression IGFs and their receptors has not been fully characterized. The purpose of this study was to obtain a comprehensive profile of their expression throughout the life cycle of ameloblasts, using the continuously erupting rat incisor model. Upper incisors of young male rats were fixed by perfusion, decalcified, and embedded in paraffin. Sections were processed for in situ hybridization and immunohistochemistry. mRNA and protein expression profiles IGF-I, IGF-II, IGF-IR, and IGF-IIR mRNA were essentially identical. At the apical loop of the incisor, very strong signals were seen in the outer enamel epithelium while the inner enamel epithelium showed a moderate reaction. In the region of ameloblasts facing pulp, inner enamel epithelium cells were still moderately reactive while signals over the outer enamel epithelium were slightly reduced. In the region of ameloblasts facing dentin and the initial portion of the secretory zone, signals in ameloblasts were weak while those over the outer enamel epithelium were strong. In the region of postsecretory transition, signals in both ameloblasts and papillary layer cells gradually increased. In maturation proper, signals in ameloblasts appeared as alternating bands of strong and weak reactivities, which corresponded to the regions of ruffle-ended and smooth-ended ameloblasts, respectively. Papillary layer cells also showed alternations in signal intensity that matched those in ameloblasts. These results suggest that the IGF family may act as an autocrine/paracrine system that influences not only cell differentiation but also the physiological activity of ameloblasts.