Autoradiographic investigation of the effect of 1-hydroxyethylidene-1, 1-bisphosphonate on matrix protein synthesis and secretion by secretory ameloblasts in rat incisors

Seven daily subcutaneous injections of 1-hydroxyethylidene-1, 1-bisphosphonate (HEBP) can induce enamel hypoplasia. Several enamel-free zones were observed along the crown-analogue side of rat incisors during the secretory stage of amelogenesis. Ameloblasts related to the enamel-free zones lay directly on the abnormally non-mineralized mantle dentine, whereas the adjacent ameloblasts, which were forming the enamel matrix layer, were associated with the region where mineralization of dentine was proceeding. The further purpose of this study was to investigate the synthetic and secretory activity of these two groups of ameloblasts and to trace the fate of the radioactively labelled proteins. [3H]-proline was administered to Wistar rats 12 h after the last injection of HEBP. Light-microscopic autoradiography was performed. Quantitative analysis indicated that the ameloblasts of the enamel-forming zones in the drug-treated group showed a distribution pattern of silver grains similar to that of the controls. The ameloblasts of the enamel-free zones also demonstrated incorporation of [3H]-proline at the same level. There was some labelling over the non-mineralized mantle dentine, which was supposed to indicate the penetration of ameloblast products. From these results, it is concluded that HEBP does not affect the ameloblast activity in protein synthesis. The complete failure of enamel-layer formation in some specific regions is probably due to the failure in protein secretion and protein deposition. This study provides additional evidence that the mineralization of dentine is an essential factor in successful enamel matrix secretion and deposition.