INSULIN-LIKE GROWTH-FACTOR-I RECEPTOR IN THE CELL BIOLOGY OF THE AMELOBLAST - AN IMMUNOHISTOCHEMICAL STUDY ON THE RAT INCISOR

The distribution of IGF-I receptor is reported in the odontogenic epit helium and mesenchyme of the continuously erupting mandibular incisor of the rat by immunohistochemistry using a polyclonal antibody specifi c to the IGF-I receptor. Odontogenic epithelium is a unique odontogeni c sequence in that all stages of the complex life cycle of the amelobl ast are represented along the length of the enamel-forming aspect of t he tooth. Pre-ameloblasts become post-mitotic before secreting enamel matrix. When the full thickness of the enamel has been formed, a remar kable transition in phenotype takes place in the ameloblast. It change s from a protein secretory cell to one active in maturation of enamel matrix by removal of water and protein from the increasingly mineraliz ed matrix. The distribution and intensity of IGF-I receptor expression varied with the phenotypic stages of the ameloblasts. Diffuse cellula r staining for IGF-I receptor was found during the active secretory ph ase of amelogenesis. However, towards the end of this phase, the stain ing was confirmed to granular or vesicular structures within the cytop lasm. These granular deposits gradually decreased as the ameloblasts m ade the transition towards enamel maturation. This transition is accom panied by programmed cell death (apoptosis) of approximately 25% of th e ameloblasts and cells in this zone did not stain for IGF-I receptor. With the onset of enamel maturation, diffuse staining of the amelobla st layer was re-established gradually and staining remained evident ri ght up to the reduced enamel epithelium, which joins with the oral epi thelium. Strong IGF-I receptor immunoreactivity was observed in the st ratum basale and stratum spino-sum of the adjacent labial gingival epi thelium. The presence of type 1 receptors in the ameloblast layer, at different stages of its development, implicates IGF-I involvement in c ell proliferation, differentiation and enamel formation throughout ame logenesis. The non-expression of IGF-I receptor in the transitional zo ne suggests that a decline in the expression of IGF-I receptor is acco mpanied by modulation of the ameloblasts to a different functional phe notype and by programmed cell death (apoptosis) in some cells of this population. In the dental mesenchyme, post-mitotic odontoblasts and pr edentine matrix were positive for IGF-I receptor, as were osteoblasts and osteoclasts.